EECS150 - Digital Design Lecture 3 - Timing January 29, 2002 John Wawrzynek Spring 2002 EECS150 - Lec03-Timing Page 1
Outline General Model of Synchronous Systems Performance Limits Announcements Delay in logic gates Delay in wires Delay in flip-flops Spring 2002 EECS150 - Lec03-Timing Page 2
General Model of Synchronous Circuit clock input input CL reg CL reg output option feedback All wires, except clock, may be multiple bits wide. Registers (reg) collections of flip-flops clock distributed to all flip-flops typical rate? output Combinational Logic Blocks (CL) no internal state output only a function of inputs Particular inputs/outputs are optional Optional Feedback Spring 2002 EECS150 - Lec03-Timing Page 3
Example Circuit Parallel to Serial Converter All signal paths single bit wide Registers are single flip-flops Combinational Logic blocks are simple multiplexors No feedback. Spring 2002 EECS150 - Lec03-Timing Page 4
General Model of Synchronous Circuit clock input input CL reg CL reg output option feedback output How do we measure performance? operations/sec? cycles/sec? What limits the clock rate? What happens as we increase the clock rate? Spring 2002 EECS150 - Lec03-Timing Page 5
Limitations on Clock Rate Logic Gate Delay Delays in flip-flops input output What are typical delay values? t D clk Q setup time clock to Q delay Both times contribute to limiting the clock period. What must happen in one clock cycle for correct operation? Assuming perfect clock distribution (all flip-flops see the clock at the same time): All signals must be ready and setup before rising edge of clock. Spring 2002 EECS150 - Lec03-Timing Page 6
Example Parallel to serial converter: T > time(clk->q) + time(mux) + time(setup) a b Spring 2002 EECS150 - Lec03-Timing Page 7
Announcements Lectures now being web-cast and recorded online. URL: Look at notes online before class. Suggestion: print out bring copy to class and annotate when necessary. My notes are intentionally incomplete. Homework #1 online. Turn in before 12 noon Friday. Discussions, TA office hours, and labs this week. Quiz Friday at lab lecture. Spring 2002 EECS150 - Lec03-Timing Page 8
Qualitative Analysis of Logic Delay Improved Transistor Model: nfet We refer to transistor "strength" as the amount of current that flows for a given Vds and Vgs. The strength is linearly proportional to the ratio of W/L. pfet Spring 2002 EECS150 - Lec03-Timing Page 9
Gate Switching Behavior Inverter: NAND gate: Spring 2002 EECS150 - Lec03-Timing Page 10
Gate Delay Cascaded gates: Vout Vin Spring 2002 EECS150 - Lec03-Timing Page 11
Gate Delay Fan-out: Fan-in What is the delay in this circuit? Critical Delay Spring 2002 EECS150 - Lec03-Timing Page 12
Wire Delay t In general wire behave as transmission lines : signal wave-front moves close to the speed of light ~1ft/ns In ICs most wires are short, therefore the transit times are relatively short compared to the clock period and can be ignored. Not so on PC boards. x Spring 2002 EECS150 - Lec03-Timing Page 13
Wire Delay Even in those cases where the transmission line effect is negligible: Wires posses distributed resistance and capacitance For long wires on ICs: busses, clock lines, global control signal, etc. distributed RC (and therefore long delay) significant signals are rebuffered to reduce delay: Time constant associated with distributed RC is proportional to the square of the length For short wires on ICs, resistance is insignificant (relative to effective R of transistors), but C is important. Typically around half of C of gate load is in the wires. Spring 2002 EECS150 - Lec03-Timing Page 14
Delay in Flip-flops Setup time results delay through first latch. Clock to Q delay results from delay through second latch. Spring 2002 EECS150 - Lec03-Timing Page 15