Virtex- FPGA Configurable Logic Block Virtex- User Guide FPGA CLB [optional] [optional]
Xilinx is disclosing this user guide, manual, release note, and/or specification (the "ocumentation") to you solely for use in the development of designs to operate with Xilinx hardware devices. You may not reproduce, distribute, republish, download, display, post, or transmit the ocumentation in any form or by any means including, but not limited to, electronic, mechanical, photocopying, recording, or otherwise, without the prior written consent of Xilinx. Xilinx expressly disclaims any liability arising out of your use of the ocumentation. Xilinx reserves the right, at its sole discretion, to change the ocumentation without notice at any time. Xilinx assumes no obligation to correct any errors contained in the ocumentation, or to advise you of any corrections or updates. Xilinx expressly disclaims any liability in connection with technical support or assistance that may be provided to you in connection with the Information. THE OCUMENTATION IS ISCLOSE TO YOU AS-IS WITH NO WARRANTY OF ANY KIN. XILINX MAKES NO OTHER WARRANTIES, WHETHER EXPRESS, IMPLIE, OR STATUTORY, REGARING THE OCUMENTATION, INCLUING ANY WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, OR NONINFRINGEMENT OF THIR-PARTY RIGHTS. IN NO EVENT WILL XILINX BE LIABLE FOR ANY CONSEUENTIAL, INIRECT, EXEMPLARY, SPECIAL, OR INCIENTAL AMAGES, INCLUING ANY LOSS OF ATA OR LOST PROFITS, ARISING FROM YOUR USE OF THE OCUMENTATION. 2009 Xilinx, Inc. XILINX, the Xilinx logo, Virtex, Spartan, ISE, and other designated brands included herein are trademarks of Xilinx in the United States and other countries. All other trademarks are the property of their respective owners. Revision History The following table shows the revision history for this document. ate Version Revision 0/24/09 1.0 Initial Xilinx release. 09/1/09 1.1 Add Virtex- HXT devices to Table 2. Updated discussions at Look-Up Table (LUT), page 11, and Static Read Operation, page 29. CLB labeling change in figures throughout document (Figure through Figure 13, Figure 1, Figure 17, Figure 27 through Figure 29), including clarifying the T S /T H functions, descriptions, and notes in Table 7, page 39 and Table 8, page 42. Virtex- FPGA CLB User Guide www.xilinx.com
Table of Contents Revision History............................................................. 2 Preface: About This Guide Additional ocumentation................................................... Additional Support Resources................................................ Virtex- FPGA CLB CLB Overview............................................................... 7 Slice escription........................................................... 8 CLB / Slice Timing Models.................................................. 34 General Slice Timing Model and Parameters................................. 34 Slice istributed RAM Timing Model and Parameters (Available in SLIM only) 38 Slice L Timing Model and Parameters (Available in SLIM only)............ 41 Slice Carry-Chain Timing Model and Parameters............................. 44 CLB Primitives.............................................................. 4 istributed RAM Primitives............................................... 4 Shift Registers (Ls) Primitive............................................. 47 Other Shift Register Applications........................................... 48 Multiplexer Primitives.................................................... 49 Carry Chain Primitive..................................................... 49 Virtex- FPGA CLB User Guide www.xilinx.com 3 UG34 (v1.1) August 21, 2009
4 www.xilinx.com Virtex- FPGA CLB User Guide UG34 (v1.1) August 21, 2009
Preface About This Guide Additional ocumentation This guide serves as a technical reference describing the Virtex - FPGA configurable logic blocks (CLBs). Usually, the logic synthesis software assigns the CLB resources without system designer intervention. It can be advantageous for the designer to understand certain CLB details, including the varying capabilities of the look-up tables (LUTs), the physical direction of the carry propagation, the number and distribution of the available flip-flops, and the availability of the very efficient shift registers. This guide describes these and other features of the CLB in detail. The following documents are also available for download at http://www.xilinx.com/support/documentation/virtex-.htm. Virtex- Family Overview The features and product selection of the Virtex- family are outlined in this overview. Virtex- FPGA ata Sheet: C and Switching Characteristics This data sheet contains the C and Switching Characteristic specifications for the Virtex- family. Virtex- FPGA Packaging and Pinout Specifications This specification includes the tables for device/package combinations and maximum I/Os, pin definitions, pinout tables, pinout diagrams, mechanical drawings, and thermal specifications. Virtex- FPGA Configuration Guide This all-encompassing configuration guide includes chapters on configuration interfaces (serial and SelectMAP), bitstream encryption, boundary-scan and JTAG configuration, reconfiguration techniques, and readback through the SelectMAP and JTAG interfaces. Virtex- FPGA Clocking Resources User Guide This guide describes the clocking resources available in all Virtex- devices, including the MMCM and PLLs. Virtex- FPGA Memory Resources User Guide The functionality of the block RAM and FIFO are described in this user guide. Virtex- FPGA SelectIO Resources User Guide This guide describes the SelectIO resources available in all Virtex- devices. Virtex- FPGA CLB User Guide www.xilinx.com
Preface: About This Guide Virtex- FPGA GTH Transceivers User Guide This guide describes the GTH transceivers available in all Virtex- HXT FPGAs except the XCVHX20T and the XCVHX380T in the FF114 package.. Virtex- FPGA GTX Transceivers User Guide This guide describes the GTX transceivers available in all Virtex- FPGAs except the XCVLX70. Virtex- FPGA Embedded Tri-Mode Ethernet MAC User Guide This guide describes the dedicated Tri-Mode Ethernet Media Access Controller available in all Virtex- FPGAs except the XCVLX70. Virtex- FPGA SP48E1 Slice User Guide This guide describes the architecture of the SP48E1 slice in Virtex- FPGAs and provides configuration examples. Virtex- FPGA System Monitor User Guide The System Monitor functionality available in all Virtex- devices is outlined in this guide. Virtex- FPGA PCB esign Guide This guide provides information on PCB design for Virtex- devices, with a focus on strategies for making design decisions at the PCB and interface level. Additional Support Resources To search the database of silicon and software questions and answers or to create a technical support case in WebCase, see the Xilinx website at: http://www.xilinx.com/support. www.xilinx.com Virtex- FPGA CLB User Guide
Virtex- FPGA CLB CLB Overview The Configurable Logic Blocks (CLBs) are the main logic resources for implementing sequential as well as combinatorial circuits. Each CLB element is connected to a switch matrix for access to the general routing matrix (shown in Figure 1). A CLB element contains a pair of slices. These two slices do not have direct connections to each other, and each slice is organized as a column. Each slice in a column has an independent carry chain. For each CLB, slices in the bottom of the CLB are labeled as SLI(0), and slices in the top of the CLB are labeled as SLI(1). X-Ref Target - Figure 1 COUT COUT CLB Slice(1) Switch Matrix Slice(0) CIN CIN ug34_01_040209 Figure 1: Arrangement of Slices within the CLB The Xilinx tools designate slices with the following definitions. An X followed by a number identifies the position of each slice in a pair as well as the column position of the slice. The X number counts slices starting from the bottom in sequence 0, 1 (the first CLB column); 2, 3 (the second CLB column); etc. A Y followed by a number identifies a row of slices. The number remains the same within a CLB, but counts up in sequence from one CLB row to the next CLB row, starting from the bottom. Figure 2 shows four CLBs located in the bottom-left corner of the die. Virtex- FPGA CLB User Guide www.xilinx.com 7
X-Ref Target - Figure 2 COUT COUT COUT COUT CLB Slice X1Y1 CLB Slice X3Y1 Slice X0Y1 Slice X2Y1 CIN CIN CIN CIN CLB COUT Slice X1Y0 COUT CLB COUT Slice X3Y0 COUT Slice X0Y0 Slice X2Y0 ug34_02_040209 Figure 2: Row and Column Relationship between CLBs and Slices Slice escription Every slice contains four logic-function generators (or look-up tables), eight storage elements, wide-function multiplexers, and carry logic. These elements are used by all slices to provide logic, arithmetic, and ROM functions. In addition to this, some slices support two additional functions: storing data using distributed RAM and shifting data with 32-bit registers. Slices that support these additional functions are called SLIM; others are called SLIL. SLIM (shown in Figure 3) represents a superset of elements and connections found in all slices. SLIL is shown in Figure 4. Each CLB can contain zero or one SLIM. Every other CLB column contains a SLIMs. In addition, the two CLB columns to the left of the SP48E columns both contain a SLIL and a SLIM. 8 www.xilinx.com Virtex- FPGA CLB User Guide
X-Ref Target - Figure 3 COUT HI LO INIT1 INIT0 Reset Type Sync/Async FF/LAT X :1 I I2 A:A1 W:W1 O I1 N MC31 HI LO INIT1 INIT0 X FF/LAT INIT1 INIT0 HI LO MUX CX C:1 CI I2 A:A1 W:W1 O I1 N MC31 HI LO INIT1 INIT0 C CX FF/LAT INIT1 INIT0 HI LO CMUX C C BX B:1 BI I2 A:A1 W:W1 O I1 N MC31 HI LO INIT1 INIT0 B BX FF/LAT INIT1 INIT0 HI LO BMUX B B AX A:1 AI I2 A:A1 W:W1 O I1 N MC31 0/1 N CIN A AX FF/LAT INIT1 INIT0 HI LO AMUX A A ug34_03_040209 Figure 3: iagram of SLIM Virtex- FPGA CLB User Guide www.xilinx.com 9
X-Ref Target - Figure 4 X COUT HI LO INIT1 INIT0 Reset Type Sync/Async FF/LAT MUX :1 A:A1 O HI LO INIT1 INIT0 X FF/LAT INIT1 INIT0 HI LO CX CMUX C:1 A:A1 O HI LO INIT1 INIT0 C CX FF/LAT INIT1 INIT0 HI LO C C BX BMUX B:1 A:A1 O HI LO INIT1 INIT0 B BX FF/LAT INIT1 INIT0 HI LO B B AX AMUX A:1 A:A1 O A AX FF/LAT INIT1 INIT0 HI LO A A 0/1 CIN ug34_04_040209 Figure 4: iagram of SLIL 10 www.xilinx.com Virtex- FPGA CLB User Guide
CLB/Slice Configurations Table 1 summarizes the logic resources in one CLB. Each CLB or slice can be implemented in one of the configurations listed. Table 2 shows the available resources in all CLBs. Table 1: Logic Resources in One CLB Slices LUTs Flip-Flops Arithmetic and Carry Chains istributed RAM (1) Shift Registers (1) 2 8 1 2 2 bits 128 bits Notes: 1. SLIM only, SLIL does not have distributed RAM or shift registers. Table 2: Virtex- FPGA Logic Resources Available in All CLBs evice Total Slices SLILs SLIMs Number of -Input LUTs Maximum istributed RAM (Kb) Shift Register (Kb) Number of Flip-Flops XCVLX7T 11,40 7,40 4,180 4,0 1,04 22. 93,120 XCVLX130T 20,000 13,040,90 80,000 1,740 870 10,000 XCVLX19T 31,200 19,040 12,10 124,800 3,140 1,70 249,00 XCVLX240T 37,80 23,080 14,00 10,720 3,770 1,88 301,440 XCVLX3T,880 40,30 1,20 227,20 4,130 2,0 4,040 XCVLX0T 8,920 1,120 24,800 343,80,200 3,100 87,30 XCVLX70 118,0 8,440 33,120 474,240 8,280 4,140 948,480 XCVSX31T 49,200 28,840 20,30 19,800,090 2,4 393,00 XCVSX47T 74,400 48,840 30,0 297,00 7,40 3,820 9,200 XCVHX20T 39,30 27,200 12,10 17,440 3,040 1,20 314,880 XCVHX2T 39,00 27,400 12,200 18,400 3,00 1,2 31,800 XCVHX380T 9,70 41,20 18,240 239,040 4,70 2,28 478,080 XCVHXT 88,0 3,080 2,480 34,240,370 3,18 708,480 Look-Up Table (LUT) The function generators in Virtex- FPGAs are implemented as six-input look-up tables (LUTs). There are six independent inputs (A inputs - A1 to A) and two independent outputs (O and ) for each of the four function generators in a slice (A, B, C, and ). The function generators can implement any arbitrarily defined six-input Boolean function. Each function generator can also implement two arbitrarily defined five-input Boolean functions, as long as these two functions share common inputs. Only the output of the function generator is used when a six-input function is implemented. Both O and are used for each of the five-input function generators implemented. In this case, A is driven High by the software. The propagation delay through a LUT is independent of the function implemented, or whether one six-input or two five-input generators are implemented. Signals from the function generators can exit the slice (through A, B, C, output for or AMUX, BMUX, CMUX, MUX output for O), enter the XOR dedicated gate from an output (see Fast Lookahead Carry Logic), enter the carry-logic chain from an O output (see Fast Lookahead Carry Logic), enter the select line of the carry-logic multiplexer from Virtex- FPGA CLB User Guide www.xilinx.com 11
output (see Fast Lookahead Carry Logic), feed the input of the storage element, or go to F7AMUX/F7BMUX from output. In addition to the basic LUTs, slices contain three multiplexers (F7AMUX, F7BMUX, and F8MUX). These multiplexers are used to combine up to four function generators to provide any function of seven or eight inputs in a slice. F7AMUX and F7BMUX are used to generate seven input functions from LUTs A and B, or C and, while F8MUX is used to combine all LUTs to generate eight input functions. Functions with more than eight inputs can be implemented using multiple slices. There are no direct connections between slices to form function generators greater than eight inputs within a CLB. Storage Elements As in previous Virtex architectures, there are four (original) storage elements in a slice that can be configured as either edge-triggered -type flip-flops or level-sensitive latches. The input can be driven directly by a LUT output via AFFMUX, BFFMUX, CFFMUX or FFMUX, or by the BYPASS slice inputs bypassing the function generators via AX, BX, CX, or X input. When configured as a latch, the latch is transparent when the is Low. In Virtex- devices, there are now four additional storage elements that can only be configured as edge-triggered -type flip-flops. The input can be driven by the O output of the LUT or the BYPASS slice inputs via AX, BX, CX, or X input. When the original 4 storage elements are configured as latches, these 4 additional storage elements can not be used. The control signals clock (), clock enable (), and set/reset () are common to all storage elements in one slice. When one flip-flop in a slice has or enabled, the other flip-flops used in the slice will also have or enabled by the common signal. Only the signal has independent polarity. Any inverter placed on the clock signal is automatically absorbed. The and signals are active High. All flip-flop and latch primitives have and non- versions. The signal forces the storage element into the state specified by the attribute HIGH or LOW. HIGH forces a logic High at the storage element output when is asserted, while LOW forces a logic Low at the storage element output (see Table 3). Table 3: Truth Table when using LOW and HIGH VAL Function 0 LOW (default) No Logic Change 1 LOW (default) 0 0 HIGH No Logic Change 1 HIGH 1 Figure shows both the register only and the register/latch configuration in a slice. 12 www.xilinx.com Virtex- FPGA CLB User Guide
X-Ref Target - Figure X LUT O FF INIT1 INIT0 HIGH LOW X LUT FF/LATCH FF LATCH INIT1 INIT0 HIGH LOW CX LUT C O CFF INIT1 INIT0 HIGH LOW C CX LUT C CFF/LATCH FF LATCH INIT1 INIT0 HIGH LOW C BX LUT B O BFF INIT1 INIT0 HIGH LOW Reset Type Sync Async B BX LUT B BFF/LATCH FF LATCH INIT1 INIT0 HIGH LOW Reset Type Sync Async B AX LUT A O AFF INIT1 INIT0 HIGH LOW A AX LUT A AFF/LATCH FF LATCH INIT1 INIT0 HIGH LOW A ug34_0_040209 Figure : Two Versions of Configuration in a Slice: 4 Registers Only and 4 Register/Latch HIGH and LOW can be set individually for each storage element in a slice. The choice of synchronous (SYNC) or asynchronous (ASYNC) set/reset (TYPE) cannot be set individually for each storage element in a slice. The initial state after configuration or global initial state is defined by separate INIT0 and INIT1 attributes. By default, setting the LOW attribute sets INIT0, and setting the HIGH attribute sets INIT1. Virtex- devices can set INIT0 and INIT1 independent of HIGH and LOW. The configuration options for the set and reset functionality of a register or the four storage elements capable of functioning as a latch are as follows: No set or reset Synchronous set Synchronous reset Asynchronous set (preset) Asynchronous reset (clear) Virtex- FPGA CLB User Guide www.xilinx.com 13
istributed RAM and Memory (Available in SLIM only) Multiple LUTs in a SLIM can be combined in various ways to store larger amount of data. The function generators (LUTs) in SLIMs can be implemented as a synchronous RAM resource called a distributed RAM element. RAM elements are configurable within a SLIM to implement the following: Single-Port 32 x 1-bit RAM ual-port 32 x 1-bit RAM uad-port 32 x 2-bit RAM Simple ual-port 32 x -bit RAM Single-Port 4 x 1-bit RAM ual-port 4 x 1-bit RAM uad-port 4 x 1-bit RAM Simple ual-port 4 x 3-bit RAM Single-Port 128 x 1-bit RAM ual-port 128 x 1-bit RAM Single-Port 2 x 1-bit RAM istributed RAM modules are synchronous (write) resources. A synchronous read can be implemented with a storage element or a flip-flop in the same slice. By placing this flip-flop, the distributed RAM performance is improved by decreasing the delay into the clock-to-out value of the flip-flop. However, an additional clock latency is added. The distributed elements share the same clock input. For a write operation, the Write Enable () input, driven by either the or pin of a SLIM, must be set High. Table 4 shows the number of LUTs (four per slice) occupied by each distributed RAM configuration. Table 4: istributed RAM Configuration RAM Number of LUTs 32 x 1S 1 32 x 1 2 32 x 2 (2) 4 32 x SP (2) 4 4 x 1S 1 4 x 1 2 4 x 1 (3) 4 4 x 3SP (3) 4 128 x 1S 2 128 x 1 4 14 www.xilinx.com Virtex- FPGA CLB User Guide
Table 4: istributed RAM Configuration (Cont d) RAM Number of LUTs Notes: 2 x 1S 4 1. S = single-port configuration; = dual-port configuration; = quad-port configuration; SP = simple dual-port configuration. 2. RAM32M is the associated primitive for this configuration. 3. RAM4M is the associated primitive for this configuration. For single-port configurations, distributed RAM has a common address port for synchronous writes and asynchronous reads. For dual-port configurations, distributed RAM has one port for synchronous writes and asynchronous reads, and another port for asynchronous reads. In simple dual-port configuration, there is no data out (read port) from the write port. For quad-port configurations, distributed RAM has one port for synchronous writes and asynchronous reads, and three additional ports for asynchronous reads. In single-port mode, read and write addresses share the same address bus. In dual-port mode, one function generator is connected with the shared read and write port address. The second function generator has the A inputs connected to a second read-only port address and the WA inputs shared with the first read/write port address. Figure through Figure 14 illustrate various example distributed RAM configurations occupying one SLIM. When using x2 configuration (RAM32X2), A and WA are driven High by the software to keep O and independent. Virtex- FPGA CLB User Guide www.xilinx.com 1
X-Ref Target - Figure RAM 32X2 I[1] I[0] AR[4:0] W (I) (AX/BX/CX/X) [:1] () () PRAM32 I1 I2 A[:1] WA[:1] O O[0] O[1] PRAM32 ARC[4:0] C[:1] I1 I2 A[:1] WA[:1] O OC[0] OC[1] ARB[4:0] B[:1] PRAM32 I1 I2 A[:1] WA[:1] O OB[0] OB[1] ARA[4:0] A[:1] PRAM32 I1 I2 A[:1] WA[:1] O OA[0] OA[1] ug34_0_08009 Figure : istributed RAM (RAM32X2) 1 www.xilinx.com Virtex- FPGA CLB User Guide
X-Ref Target - Figure 7 RAM 32XSP unused unused WAR[:1] WAR[] = 1 W [:1] () () I1 I2 A[:1] WA[:1] PRAM32 ATA[1] ATA[2] RAR[:1] RAR[] = 1 C[:1] PRAM32 I1 I2 A[:1] WA[:1] O O[2] O[1] ATA[3] ATA[4] B[:1] PRAM32 I1 I2 A[:1] WA[:1] O O[4] O[3] ATA[] ATA[] A[:1] PRAM32 I1 I2 A[:1] WA[:1] O O[] O[] ug34_07_08009 Figure 7: istributed RAM (RAM32XSP) Virtex- FPGA CLB User Guide www.xilinx.com 17
X-Ref Target - Figure 8 RAM4X1S (I) I1 SPRAM4 O A[:0] W ([:1]) () (/) A[:1] WA[:1] Registered ug34_08_08009 Figure 8: istributed RAM (RAM4X1S) If four single-port 4 x 1-bit modules are built, the four RAM4X1S primitives can occupy a SLIM, as long as they share the same clock, write enable, and shared read and write port address inputs. This configuration equates to 4 x 4-bit single-port distributed RAM. X-Ref Target - Figure 9 RAM4X1 (I) I1 PRAM4 SPO A[:0] W ([:1]) () (/) A[:1] WA[:1] Registered I1 PRAM4 PO PRA[:0] (C[:1]) A[:1] WA[:1] Registered ug34_09_08009 Figure 9: istributed RAM (RAM4X1) If two dual-port 4 x 1-bit modules are built, the two RAM4X1 primitives can occupy a SLIM, as long as they share the same clock, write enable, and shared read and write port address inputs. This configuration equates to 4 x 2-bit dual-port distributed RAM. 18 www.xilinx.com Virtex- FPGA CLB User Guide
X-Ref Target - Figure 10 RAM4X1 I (I) I1 PRAM4 O AR W ([:1]) () () A[:1] WA[:1] Registered I1 PRAM4 OC ARC (C[:1]) A[:1] WA[:1] Registered I1 PRAM4 OB ARB (B[:1]) A[:1] WA[:1] Registered I1 PRAM4 OA ARA (A[:1]) A[:1] WA[:1] Registered ug34_10_08009 Figure 10: istributed RAM (RAM4X1) Virtex- FPGA CLB User Guide www.xilinx.com 19
X-Ref Target - Figure 11 RAM 4X3SP unused unused WAR[:1] W [:1] () () I1 I2 A[:1] WA[:1] PRAM4 ATA[1] RAR[:1] C[:1] PRAM4 I1 I2 A[:1] WA[:1] O O[1] ATA[2] B[:1] PRAM4 I1 I2 A[:1] WA[:1] O O[2] ATA[3] A[:1] PRAM4 I1 I2 A[:1] WA[:1] O O[3] ug34_11_08009 Figure 11: istributed RAM (RAM4X3SP) Implementation of distributed RAM configurations with depth greater than 4 requires the usage of wide-function multiplexers (F7AMUX, F7BMUX, and F8MUX). 20 www.xilinx.com Virtex- FPGA CLB User Guide
X-Ref Target - Figure 12 A (CX) RAM128X1S (I) I1 SPRAM4 A[:0] W [:0] 7 () (/) A[:1] WA[7:1] 0 [:0] 7 SPRAM4 I1 A[:1] WA[7:1] F7BMUX Registered ug34_12_08009 Figure 12: istributed RAM (RAM128X1S) If two single-port 128 x 1-bit modules are built, the two RAM128X1S primitives can occupy a SLIM, as long as they share the same clock, write enable, and shared read and write port address inputs. This configuration equates to 128 x 2-bit single-port distributed RAM. Virtex- FPGA CLB User Guide www.xilinx.com 21
X-Ref Target - Figure 13 RAM128X1 A (CX) I I1 PRAM4 A[:0] W 7 () () A[:1] WA[7:1] SPO 7 PRAM4 I1 A[:1] WA[7:1] F7BMUX Registered I1 PRAM4 PRA[:0] 7 A[:1] WA[7:1] PO AX 7 PRAM4 I1 A[:1] WA[7:1] F7AMUX Registered ug34_13_08009 Figure 13: istributed RAM (RAM128X1) 22 www.xilinx.com Virtex- FPGA CLB User Guide
X-Ref Target - Figure 14 RAM2X1S I1 SPRAM4 A[7:0] W 8 () (/) A[:1] WA[8:1] A (CX) I1 SPRAM4 F7BMUX 8 A[:1] WA[8:1] A7 (BX) O I1 SPRAM4 F8MUX Registered 8 A[:1] WA[8:1] A (AX) I1 SPRAM4 F7AMUX 8 A[:1] WA[8:1] ug34_14_040209 Figure 14: istributed RAM (RAM2X1S) istributed RAM configurations greater than the provided examples require more than one SLIM. There are no direct connections between slices to form larger distributed RAM configurations within a CLB or between slices. istributed RAM ata Flow Synchronous Write Operation The synchronous write operation is a single clock-edge operation with an active-high write-enable () feature. When is High, the input () is loaded into the memory location at address A. Asynchronous Read Operation Virtex- FPGA CLB User Guide www.xilinx.com 23
The output is determined by the address A (for single-port mode output/spo output of dual-port mode), or address PRA (PO output of dual-port mode). Each time a new address is applied to the address pins, the data value in the memory location of that address is available on the output after the time delay to access the LUT. This operation is asynchronous and independent of the clock signal. istributed RAM Summary Single-port and dual-port modes are available in SLIMs. A write operation requires one clock edge. Read operations are asynchronous ( output). The data input has a setup-to-clock timing specification. Read Only Memory (ROM) Each function generator in SLIMs and SLILs can implement a 4 x 1-bit ROM. Three configurations are available: ROM4x1, ROM128x1, and ROM2x1. ROM contents are loaded at each device configuration. Table shows the number of LUTs occupied by each ROM configuration. Table : ROM Configuration ROM Number of LUTs 4 x 1 1 128 x 1 2 2 x 1 4 Shift Registers (Available in SLIM only) A SLIM function generator can also be configured as a 32-bit shift register without using the flip-flops available in a slice. Used in this way, each LUT can delay serial data anywhere from one to 32 clock cycles. The shiftin (I1 LUT pin) and shiftout 31 (MC31 LUT pin) lines cascade LUTs to form larger shift registers. The four LUTs in a SLIM are thus cascaded to produce delays up to 128 clock cycles. It is also possible to combine shift registers across more than one SLIM. Note that there are no direct connections between slices to form longer shift registers, nor is the MC31 output at LUT B/C/ available. The resulting programmable delays can be used to balance the timing of data pipelines. Applications requiring delay or latency compensation use these shift registers to develop efficient designs. Shift registers are also useful in synchronous FIFO and content addressable memory (CAM) designs. The write operation is synchronous with a clock input () and an optional clock enable (). A dynamic read access is performed through the -bit address bus, A[4:0]. The LSB of the LUT is unused and the software automatically ties it to a logic High. The configurable shift registers cannot be set or reset. The read is asynchronous; however, a storage element or flip-flop is available to implement a synchronous read. In this case, the clock-to-out of the flip-flop determines the overall delay and improves performance. However, one additional cycle of clock latency is added. Any of the 32 bits can be read out asynchronously (at the LUT outputs) by varying the -bit address. This capability is useful in creating smaller shift registers (less than 32 bits). For example, when building a 13-bit shift register, simply set the address to the 13 th bit. Figure 1 is a logic block diagram of a 32-bit shift register. 24 www.xilinx.com Virtex- FPGA CLB User Guide
X-Ref Target - Figure 1 SHIFTIN (MC31 of Previous LUT) LC32E SHIFTIN () A[4:0] (AI) (A[:2]) L32 I1 MC31 A[:2] SHIFTOUT (31) () (/) () (A) Registered ug34_1_08009 Figure 1: 32-bit Shift Register Configuration Figure 1 illustrates an example shift register configuration occupying one function generator. X-Ref Target - Figure 1 SHIFTIN () 32-bit Shift Register SHIFTOUT(31) Address (A[4:0]) MUX ug34_1_040209 Figure 1: Representation of a Shift Register Figure 17 shows two 1-bit shift registers. The example shown can be implemented in a single LUT. Virtex- FPGA CLB User Guide www.xilinx.com 2
X-Ref Target - Figure 17 L1 SHIFTIN1 (AI) I1 O A[3:0] 4 A[:2] L1 SHIFTIN2 (AX) I2 4 A[:2] MC31 ug34_17_08009 Figure 17: ual 1-bit Shift Register Configuration As mentioned earlier, an additional output (MC31) and a dedicated connection between shift registers allows connecting the last bit of one shift register to the first bit of the next, without using the LUT output. Longer shift registers can be built with dynamic access to any bit in the chain. The shift register chaining and the F7AMUX, F7BMUX, and F8MUX multiplexers allow up to a 128-bit shift register with addressable access to be implemented in one SLIM. Figure 18 through Figure 20 illustrate various example shift register configurations that can occupy one SLIM. X-Ref Target - Figure 18 L32 SHIFTIN () I1 A[:0] () (/) A[:2] MC31 A (AX) () I1 L32 F7AMUX (A) Registered A[:2] MC31 (MC31) SHIFTOUT (3) ug34_18_040209 Figure 18: 4-bit Shift Register Configuration 2 www.xilinx.com Virtex- FPGA CLB User Guide
X-Ref Target - Figure 19 CX (A) L32 SHIFTIN () I1 A[:0] () (/) A[:2] MC31 F7BMUX BX (A) (BMUX) () F8MUX (B) Registered L32 I1 A[:2] MC31 AX (A) I1 A[:2] L32 Not Used F7AMUX UG34_19_01209 Figure 19: 9-bit Shift Register Configuration Virtex- FPGA CLB User Guide www.xilinx.com 27
X-Ref Target - Figure 20 L32 SHIFTIN () I1 A[:0] () (/) A[:2] MC31 CX (A) I1 L32 F7BMUX A[:2] I1 MC31 L32 BX (A) F8MUX (BMUX) (B) () Registered A[:2] MC31 AX (A) I1 L32 F7AMUX A[:2] MC31 (MC31) SHIFTOUT (127) ug34_20_040209 Figure 20: 128-bit Shift Register Configuration It is possible to create shift registers longer than 128 bits across more than one SLIM. However, there are no direct connections between slices to form these shift registers. Shift Register ata Flow Shift Operation The shift operation is a single clock-edge operation, with an active-high clock enable feature. When enable is High, the input () is loaded into the first bit of the shift register. Each bit is also shifted to the next highest bit position. In a cascadable shift register configuration, the last bit is shifted out on the M31 output. The bit selected by the -bit address port (A[4:0]) appears on the output. ynamic Read Operation The output is determined by the -bit address. Each time a new address is applied to the -input address pins, the new bit position value is available on the output after the time 28 www.xilinx.com Virtex- FPGA CLB User Guide
delay to access the LUT. This operation is asynchronous and independent of the clock and clock-enable signals. Static Read Operation If the -bit address is fixed, the output always uses the same bit position. This mode implements any shift-register length from 1 to 32 bits in one LUT. The shift register length is (N+1), where N is the input address (0 31). The output changes synchronously with each shift operation. The previous bit is shifted to the next position and appears on the output. Shift Register Summary A shift operation requires one clock edge. ynamic-length read operations are asynchronous ( output). Static-length read operations are synchronous ( output). The data input has a setup-to-clock timing specification. In a cascadable configuration, the 31 output always contains the last bit value. The 31 output changes synchronously after each shift operation. Multiplexers Function generators and associated multiplexers in Virtex- FPGAs can implement the following: 4:1 multiplexers using one LUT 8:1 multiplexers using two LUTs 1:1 multiplexers using four LUTs These wide input multiplexers are implemented in one level or logic (or LUT) using the dedicated F7AMUX, F7BMUX, and F8MUX multiplexers. These multiplexers allow LUT combinations of up to four LUTs in a slice. esigning Large Multiplexers 4:1 Multiplexer Each LUT can be configured into a 4:1 MUX. The 4:1 MUX can be implemented with a flipflop in the same slice. Up to four 4:1 MUXes can be implemented in a slice, as shown in Figure 21. Virtex- FPGA CLB User Guide www.xilinx.com 29
X-Ref Target - Figure 21 SLI LUT () 4:1 MUX SEL [1:0], ATA [3:0] Input ([:1]) A[:1] () Registered LUT (C) 4:1 MUX SEL C [1:0], ATA C [3:0] Input (C[:1]) A[:1] (C) Registered LUT (B) 4:1 MUX SEL B [1:0], ATA B [3:0] Input (B[:1]) A[:1] (B) Registered LUT (A) 4:1 MUX SEL A [1:0], ATA A [3:0] Input (A[:1]) A[:1] (A) Registered () ug34_21_040209 8:1 Multiplexer Figure 21: Four 4:1 Multiplexers in a Slice Each slice has an F7AMUX and an F7BMUX. These two muxes combine the output of two LUTs to form a combinatorial function up to 13 inputs (or an 8:1 MUX). Up to two 8:1 MUXes can be implemented in a slice, as shown in Figure 22. 30 www.xilinx.com Virtex- FPGA CLB User Guide
X-Ref Target - Figure 22 SLI LUT SEL [1:0], ATA [3:0] Input (1) ([:1]) A[:1] F7BMUX LUT (CMUX) 8:1 MUX (1) SEL C [1:0], ATA C [3:0] Input (1) (C[:1]) A[:1] (C) Registered SELF7(1) (CX) () LUT SEL B [1:0], ATA B [3:0] Input (2) (B[:1]) A[:1] F7AMUX LUT (AMUX) 8:1 MUX (2) SEL A [1:0], ATA A [3:0] Input (2) (A[:1]) A[:1] (A) Registered SELF7(2) (AX) ug34_22_040209 Figure 22: Two 8:1 Multiplexers in a Slice 1:1 Multiplexer Each slice has an F8MUX. F8MUX combines the outputs of F7AMUX and F7BMUX to form a combinatorial function up to 27 inputs (or a 1:1 MUX). Only one 1:1 MUX can be implemented in a slice, as shown in Figure 23. Virtex- FPGA CLB User Guide www.xilinx.com 31
X-Ref Target - Figure 23 SLI LUT SEL [1:0], ATA [3:0] Input ([:1]) A[:1] F7BMUX LUT SEL C [1:0], ATA C [3:0] Input (C[:1]) A[:1] F8MUX SELF7 (CX) LUT (BMUX) (B) 1:1 MUX Registered SEL B [1:0], ATA B [3:0] Input (B[:1]) A[:1] F7AMUX LUT SEL A [1:0], ATA A [3:0] Input (A[:1]) A[:1] SELF7 SELF8 (AX) (BX) () ug34_23_040209 Figure 23: 1:1 Multiplexer in a Slice It is possible to create multiplexers wider than 1:1 across more than one SLIM. However, there are no direct connections between slices to form these wide multiplexers. Fast Lookahead Carry Logic In addition to function generators, dedicated carry logic is provided to perform fast arithmetic addition and subtraction in a slice. A Virtex- FPGA CLB has two separate carry chains, as shown in Figure 1. The carry chains are cascadable to form wider add/subtract logic, as shown in Figure 2. The carry chain in the Virtex- device is running upward and has a height of four bits per slice. For each bit, there is a carry multiplexer (MUXCY) and a dedicated XOR gate for adding/subtracting the operands with a selected carry bits. The dedicated carry path and carry multiplexer (MUXCY) can also be used to cascade function generators for implementing wide logic functions. Figure 24 illustrates the carry chain with associated logic elements in a slice. 32 www.xilinx.com Virtex- FPGA CLB User Guide
X-Ref Target - Figure 24 COUT (To Next Slice) Carry Chain Block (CARRY4) From LUT S3 MUXCY CO3 MUX/* O From LUT X I3 O3 MUX From LUTC S2 MUXCY CO2 CMUX/C* O From LUTC CX I2 O2 CMUX C From LUTB S1 MUXCY CO1 BMUX/B* O From LUTB BX I1 O1 BMUX B From LUTA O From LUTA AX S0 I0 MUXCY CO0 O0 AMUX/A* ug34_09_040209 AMUX A CYINIT CIN 0 1 CIN (From Previous Slice) *Can be used if unregistered/registered outputs are free. ug34_24_040209 Figure 24: Fast Carry Logic Path and Associated Elements The carry chains carry lookahead logic along with the function generators. There are ten independent inputs (S inputs S0 to S3, I inputs I1 to I4, CYINIT and CIN) and eight independent outputs (O outputs O0 to O3, and CO outputs CO0 to CO3). The S inputs are used for the propagate signals of the carry lookahead logic. The propagate signals are sourced from the output of a function generator. The I inputs are used for the generate signals of the carry lookahead logic. The generate signals are sourced from either the O output of a function generator or the BYPASS input (AX, BX, CX, or X) of a slice. The former input is used to create a multiplier, while the latter is used to create an adder/accumulator. CYINIT is the CIN of the first bit in a carry chain. The CYINIT value can be 0 (for add), 1 (for subtract), or AX input (for the dynamic first carry bit). The CIN input is used to cascade slices to form a longer carry chain. The O outputs contain the sum of the addition/subtraction. The CO outputs compute the carry out for Virtex- FPGA CLB User Guide www.xilinx.com 33
CLB / Slice Timing Models CLB / Slice Timing Models each bit. CO3 is connected to COUT output of a slice to form a longer carry chain by cascading multiple slices. The propagation delay for an adder increases linearly with the number of bits in the operand, as more carry chains are cascaded. The carry chain can be implemented with a storage element or a flip-flop in the same slice. ue to the large size and complexity of Virtex- FPGAs, understanding the timing associated with the various paths and functional elements is a difficult and important task. Although it is not necessary to understand the various timing parameters to implement most designs using Xilinx software, a thorough timing model can assist advanced users in analyzing critical paths or planning speed-sensitive designs. Three timing model sections are described: Functional element diagram basic architectural schematic illustrating pins and connections Timing parameters definitions of Virtex- FPGA ata Sheet timing parameters Timing iagram - illustrates functional element timing parameters relative to each other Use the models in this chapter in conjunction with both the Xilinx Timing Analyzer software (TR) and the section on switching characteristics in the Virtex- FPGA ata Sheet. All pin names, parameter names, and paths are consistent with the post-route timing and pre-route static timing reports. Most of the timing parameters found in the section on switching characteristics are described in this chapter. All timing parameters reported in the Virtex- FPGA ata Sheet are associated with slices and CLBs. The following sections correspond to specific switching characteristics sections in the Virtex- FPGA ata Sheet: General Slice Timing Model and Parameters (CLB Switching Characteristics) Slice istributed RAM Timing Model and Parameters (Available in SLIM only) (CLB istributed RAM Switching Characteristics) Slice L Timing Model and Parameters (Available in SLIM only) (CLB L Switching Characteristics) Slice Carry-Chain Timing Model and Parameters (CLB Application Switching Characteristics) General Slice Timing Model and Parameters A simplified Virtex- FPGA slice is shown in Figure 2. Some elements of the slice are omitted for clarity. Only the elements relevant to the timing paths described in this section are shown. 34 www.xilinx.com Virtex- FPGA CLB User Guide
CLB / Slice Timing Models X-Ref Target - Figure 2 LUT Inputs O FF/LAT MUX X C Inputs LUT F7BMUX C O FF/LAT CMUX CX LUT F8MUX C B Inputs B O FF/LAT BMUX BX A Inputs LUT F7AMUX B A O AMUX AX FF/LAT A ug34_2_040209 Figure 2: Simplified Virtex- FPGA Slice Virtex- FPGA CLB User Guide www.xilinx.com 3
CLB / Slice Timing Models Timing Parameters Table shows the general slice timing parameters for a majority of the paths in Figure 2. Table : General Slice Timing Parameters Parameter Function escription Combinatorial elays T (1) ILO T ILO_2 T ILO_3 Sequential elays T O Flip-Flop/ Latch element T O Flip-Flop only element A/B/C/ inputs to A/B/C/ outputs A/B/C/ inputs to AMUX/CMUX outputs A/B/C/ inputs to BMUX output FF Clock () to A/B/C/ outputs FF Clock () to A/B/C/ outputs Propagation delay from the A/B/C/ inputs of the slice, through the look-up tables (LUTs), to the A/B/C/ outputs of the slice (six-input function). Propagation delay from the A/B/C/ inputs of the slice, through the LUTs and F7AMUX/F7BMUX to the AMUX/CMUX outputs (seven-input function). Propagation delay from the A/B/C/ inputs of the slice, through the LUTs, F7AMUX/F7BMUX, and F8MUX to the BMUX output (eight-input function). Time after the clock that data is stable at the A/B/C/ outputs of the slice sequential elements (configured as a flip-flop). Time after the clock that data is stable at the A/B/C/ outputs of the slice sequential elements. T LO Latch Clock () to A/B/C/ outputs Setup and Hold Times for Slice Sequential Elements (2) Time after the clock that data is stable at the A/B/C/ outputs of the slice sequential elements (configured as a latch). T I /T I Flip-Flop/ Latch element T I /T I Flip-Flop only element T /T Flip-Flop/ Latch element T /T Flip-Flop only element T /T Flip-Flop/ Latch element AX/BX/CX/X inputs AX/BX/CX/X inputs input input input Time before/after the that data from the AX/BX/CX/X inputs of the slice must be stable at the input of the slice sequential elements (configured as a flip-flop). Time before/after the that data from the AX/BX/CX/X inputs of the slice must be stable at the input of the slice sequential elements. Time before/after the that the input of the slice must be stable at the input of the slice sequential elements (configured as a flip-flop). Time before/after the that the input of the slice must be stable at the input of the slice sequential elements. Time before/after the that the (Set/Reset) of the slice must be stable at the inputs of the slice sequential elements (configured as a flipflop). 3 www.xilinx.com Virtex- FPGA CLB User Guide
CLB / Slice Timing Models Table : General Slice Timing Parameters (Cont d) Parameter Function escription T /T Flip-Flop only element Set/Reset T RPW T R F TOG input Time before/after the that the (Set/Reset) inputs of the slice must be stable at the inputs of the slice sequential elements Minimum Pulse Width for the (Set/Reset). Propagation delay for an asynchronous Set/Reset of the slice sequential elements. From the inputs to the A/B/C/ outputs. Toggle Frequency Maximum frequency that a CLB flip-flop can be clocked: 1/(T CH + T CL ). Notes: 1. This parameter includes a LUT configured as two five-input functions. 2. T XX = Setup Time (before clock edge), and T XX = Hold Time (after clock edge). Timing Characteristics Figure 2 illustrates the general timing characteristics of a Virtex- FPGA slice. X-Ref Target - Figure 2 1 2 3 AX/BX/CX/X (ATA) (RESET) A/B/C/ (OUT) T O T I T O T T O ug34_2_040209 Figure 2: General Slice Timing Characteristics At time T O before clock event (1), the clock-enable signal becomes valid-high at the input of the slice register. At time T I before clock event (1), data from either AX, BX, CX, or X inputs become valid-high at the input of the slice register and is reflected on either the A, B, C, or pin at time T O after clock event (1). At time T before clock event (3), the signal (configured as synchronous reset) becomes valid-high, resetting the slice register. This is reflected on the A, B, C, or pin at time T O after clock event (3). Virtex- FPGA CLB User Guide www.xilinx.com 37
CLB / Slice Timing Models Slice istributed RAM Timing Model and Parameters (Available in SLIM only) Figure 27 illustrates the details of distributed RAM implemented in a Virtex- FPGA slice. Some elements of the slice are omitted for clarity. Only the elements relevant to the timing paths described in this section are shown. X-Ref Target - Figure 27 RAM I X input I1 I2 A[:0] WA[:0] O MUX RAM CI CX C input I1 I2 A[:0] WA[:0] O C CMUX RAM BI BX B input I1 I2 A[:0] WA[:0] O B BMUX RAM AI AX A input I1 I2 A[:0] WA[:0] O A AMUX ug34_27_08009 Figure 27: Simplified Virtex- FPGA SLIM istributed RAM 38 www.xilinx.com Virtex- FPGA CLB User Guide
CLB / Slice Timing Models istributed RAM Timing Parameters Table 7 shows the timing parameters for the distributed RAM in SLIM for a majority of the paths in Figure 27. Table 7: istributed RAM Timing Parameters Parameter Function escription Sequential elays for a Slice LUT Configured as RAM (istributed RAM) T (1) SHO to A/B/C/ outputs Time after the of a write operation that the data written to the distributed RAM is stable on the A/B/C/ output of the slice. Setup and Hold Times for a Slice LUT Configured as RAM (istributed RAM) (2) T S /T H (3) AI/BI/CI/I configured as data input (I1) Time before/after the clock that data must be stable at the AI/BI/CI/I input of the slice. T A /T A A/B/C/ address inputs Time before/after the clock that address signals must be stable at the A/B/C/ inputs of the slice LUT (configured as RAM). T WS /T WH input Time before/after the clock that the write enable signal must be stable at the input of the slice LUT (configured as RAM). Clock T WPH T WPL T WC Minimum Pulse Width, High Minimum Pulse Width, Low Minimum clock period to meet address write cycle time. Notes: 1. This parameters includes a LUT configured as a two-bit distributed RAM. 2. T XX = Setup Time (before clock edge), and T XX = Hold Time (after clock edge). 3. Parameter includes AX/BX/CX/X configured as a data input (I2). Virtex- FPGA CLB User Guide www.xilinx.com 39
CLB / Slice Timing Models istributed RAM Timing Characteristics The timing characteristics of a 1-bit distributed RAM implemented in a Virtex- FPGA slice (LUT configured as RAM) are shown in Figure 28. X-Ref Target - Figure 28 1 2 3 4 7 T WC T WPH T WPL A/B/C/ (AR) T AS 2 F 3 4 E AI/BI/CI/I (I) 1 T S X 0 1 0 X T WS TILO T ILO ATA_OUT A/B/C/ T SHO 1 MEM(F) 0 1 0 WRITE REA WRITE WRITE WRITE REA MEM(E) ug34_28_08009 Figure 28: Clock Event 1: Write Operation uring a Write operation, the contents of the memory at the address on the AR inputs are changed. The data written to this memory location is reflected on the A/B/C/ outputs synchronously. At time T WS before clock event 1, the write-enable signal () becomes valid-high, enabling the RAM for a Write operation. At time T AS before clock event 1, the address (2) becomes valid at the A/B/C/ inputs of the RAM. At time T S before clock event 1, the ATA becomes valid (1) at the I input of the RAM and is reflected on the A/B/C/ output at time T SHO after clock event 1. This is also applicable to the AMUX, BMUX, CMUX, MUX, and COUT outputs at time T SHO and T WOSCO after clock event 1. Clock Event 2: Read Operation Slice istributed RAM Timing Characteristics All Read operations are asynchronous in distributed RAM. As long as is Low, the address bus can be asserted at any time. The contents of the RAM on the address bus are reflected on the A/B/C/ outputs after a delay of length T ILO (propagation delay through a LUT). The address (F) is asserted after clock event 2, and the contents of the RAM at address (F) are reflected at the output after a delay of length T ILO. 40 www.xilinx.com Virtex- FPGA CLB User Guide
CLB / Slice Timing Models Slice L Timing Model and Parameters (Available in SLIM only) Figure 29 illustrates shift register implementation in a Virtex- FPGA slice. Some elements of the slice have been omitted for clarity. Only the elements relevant to the timing paths described in this section are shown. X-Ref Target - Figure 29 L I I1 address A MC31 W L CI I1 C C address A MC31 L BI I1 B B address A MC31 L AI A address I1 A MC31 A MUX ug34_29_08009 Figure 29: Simplified Virtex- FPGA Slice L Virtex- FPGA CLB User Guide www.xilinx.com 41
CLB / Slice Timing Models Slice L Timing Parameters Table 8 shows the SLIM L timing parameters for a majority of the paths in Figure 29. Table 8: Slice L Timing Parameters Parameter Function escription Sequential elays for a Slice LUT Configured as an L T (1) REG to A/B/C/ outputs Time after the of a write operation that the data written to the L is stable on the A/B/C/ outputs of the slice. T (1) REG_MUX to AMUX - MUX output Time after the of a write operation that the data written to the L is stable on the MUX output of the slice. T REG_M31 to MUX output via MC31 output Setup and Hold Times for a Slice LUT Configured L (2) Time after the of a write operation that the data written to the L is stable on the MUX output via MC31 output. T WS /T WH input () Time before/after the clock that the write enable signal must be stable at the input of the slice LUT (configured as an L). T S /T H (3) AI/BI/CI/I configured as data input (I) Time before the clock that the data must be stable at the AI/BI/CI/I input of the slice (configured as an L). Notes: 1. This parameter includes a LUT configured as a two-bit shift register. 2. T XX = Setup Time (before clock edge), and T XX = Hold Time (after clock edge). 3. Parameter includes AX/BX/CX/X configured as a data input (I2) or two bits with a common shift. Slice L Timing Characteristics Figure 30 illustrates the timing characteristics of a 1-bit shift register implemented in a Virtex- FPGA slice (a LUT configured as an L). X-Ref Target - Figure 30 1 2 3 4 32 Write Enable () Shift_In (I) Address (A/B/C/) ata Out (A/B/C/) MSB (MC31/MUX) T WS T S 0 1 1 0 1 0 0 2 1 T REG T ILO T ILO X 0 1 1 0 1 1 0 1 T REG X X X X X X X 0 ug34_30_040209 Figure 30: Slice L Timing Characteristics 42 www.xilinx.com Virtex- FPGA CLB User Guide
CLB / Slice Timing Models Clock Event 1: Shift In uring a write (Shift In) operation, the single-bit content of the register at the address on the A/B/C/ inputs is changed, as data is shifted through the L. The data written to this register is reflected on the A/B/C/ outputs synchronously, if the address is unchanged during the clock event. If the A/B/C/ inputs are changed during a clock event, the value of the data at the addressable output (A/B/C/ outputs) is invalid. At time T WS before clock event 1, the write-enable signal () becomes valid-high, enabling the L for the Write operation that follows. At time T S before clock event 1 the data becomes valid (0) at the I input of the L and is reflected on the A/B/C/ output after a delay of length T REG after clock event 1. Since the address 0 is specified at clock event 1, the data on the I input is reflected at A/B/C/ output, because it is written to register 0. Clock Event 2: Shift In At time T S before clock event 2, the data becomes valid (1) at the I input of the L and is reflected on the A/B/C/ output after a delay of length T REG after clock event 2. Since the address 0 is still specified at clock event 2, the data on the I input is reflected at the output, because it is written to register 0. Clock Event 3: Shift In/Addressable (Asynchronous) REA All Read operations are asynchronous to the signal. If the address is changed (between clock events), the contents of the register at that address are reflected at the addressable output (A/B/C/ outputs) after a delay of length T ILO (propagation delay through a LUT). At time T S before clock event 3, the data becomes valid (1) at the I input of the L and is reflected on the A/B/C/ output T REG time after clock event 3. The address is changed (from 0 to 2). The value stored in register 2 at this time is a 0 (in this example, this was the first data shifted in), and it is reflected on the A/B/C/ output after a delay of length T ILO. Clock Event 32: MSB (Most Significant Bit) Changes At time T REG after clock event 32, the first bit shifted into the L becomes valid (logical 0 in this case) on the MUX output of the slice via the MC31 output of LUT A (L). This is also applicable to the AMUX, BMUX, CMUX, MUX, and COUT outputs at time T REG and T WOSCO after clock event 1. Virtex- FPGA CLB User Guide www.xilinx.com 43