Sequential Logic ounters and Registers ounters Introduction: ounters Asynchronous (Ripple) ounters Asynchronous ounters with MOD number < 2 n Asynchronous Down ounters ascading Asynchronous ounters svbitec.wordpress.com
Sequential Logic ounters and Registers Synchronous (Parallel) ounters Up/Down Synchronous ounters Designing Synchronous ounters Decoding A ounter ounters with Parallel Load svbitec.wordpress.com 2
Sequential Logic ounters and Registers Registers Introduction: Registers Simple Registers Registers with Parallel Load Using Registers to implement Sequential ircuits Shift Registers Serial In/Serial Out Shift Registers Serial In/Parallel Out Shift Registers Parallel In/Serial Out Shift Registers Parallel In/Parallel Out Shift Registers svbitec.wordpress.com 3
Sequential Logic ounters and Registers Bidirectional Shift Registers An Application Serial Addition Shift Register ounters Ring ounters ohnson ounters Random-Access Memory (RAM) svbitec.wordpress.com 4
Introduction: ounters ounters are circuits that cycle through a specified number of states. Two types of counters: synchronous (parallel) counters asynchronous (ripple) counters Ripple counters allow some flip-flop outputs to be used as a source of clock for other flip-flops. Synchronous counters apply the same clock to all flip-flops. svbitec.wordpress.com 5
Asynchronous (Ripple) ounters Asynchronous counters: the flip-flops do not change states at exactly the same time as they do not have a common clock pulse. Also known as ripple counters, as the input clock pulse ripples through the counter cumulative delay is a drawback. n flip-flops a MOD (modulus) 2 n counter. (Note: A MOD-x counter cycles through x states.) Output of the last flip-flop (MSB) divides the input clock frequency by the MOD number of the counter, hence a counter is also a frequency divider. svbitec.wordpress.com 6
Asynchronous (Ripple) ounters Example: 2-bit ripple binary counter. Output of one flip-flop is connected to the clock input of the next more-significant flip-flop. HIGH L FF FF L 2 3 4 Timing diagram... svbitec.wordpress.com 7
Asynchronous (Ripple) ounters Example: 3-bit ripple binary counter. HIGH 2 L FF FF FF2 L 2 3 4 5 6 7 8 2 Recycles back to svbitec.wordpress.com 8
Asynchronous (Ripple) ounters Propagation delays in an asynchronous (rippleclocked) binary counter. If the accumulated delay is greater than the clock pulse, some counter states may be misrepresented! L 2 3 4 2 t PLH (L to ) t PHL (L to ) t PLH ( to ) t PHL (L to ) t PHL ( to ) t PLH ( to 2 ) svbitec.wordpress.com 9
Asynchronous (Ripple) ounters Example: 4-bit ripple binary counter (negative-edge triggered). HIGH 2 3 L FF FF FF2 FF3 L 2 3 4 5 6 7 8 9 2 3 4 5 6 2 3 svbitec.wordpress.com
Asyn. ounters with MOD no. < 2 n States may be skipped resulting in a truncated sequence. Technique: force counter to recycle before going through all of the states in the binary sequence. Example: Given the following circuit, determine the counting sequence (and hence the modulus no.) All, inputs are (HIGH). B A L LR B L LR L LR svbitec.wordpress.com
Asyn. ounters with MOD no. < 2 n Example (cont d): All, inputs are (HIGH). B A L LR B L LR L LR lock NAND Output A B 2 3 4 5 6 7 8 9 2 MOD-6 counter produced by clearing (a MOD-8 binary counter) when count of six () occurs. svbitec.wordpress.com 2
Asyn. ounters with MOD no. < 2 n Example (cont d): ounting sequence of circuit (in BA order). lock A B NAND Output 2 3 4 5 6 7 8 9 2 Temporary state ounter is a MOD-6 counter. svbitec.wordpress.com 3
Asyn. ounters with MOD no. < 2 n Exercise: How to construct an asynchronous MOD-5 counter? MOD-7 counter? MOD-2 counter? uestion: The following is a MOD-? counter? F E D B A LR LR LR LR LR LR D E F All = =. svbitec.wordpress.com 4
Asyn. ounters with MOD no. < 2 n Decade counters (or BD counters) are counters with states (modulus-) in their sequence. They are commonly used in daily life (e.g.: utility meters, odometers, etc.). Design an asynchronous decade counter. (A.)' HIGH D B A L LR LR LR LR svbitec.wordpress.com 5
Asyn. ounters with MOD no. < 2 n Asynchronous decade/bd counter (cont d). HIGH D B A (A.)' L LR LR LR LR lock 2 3 4 5 6 7 8 9 D B A NAND output svbitec.wordpress.com 6
Asynchronous Down ounters So far we are dealing with up counters. Down counters, on the other hand, count downward from a maximum value to zero, and repeat. Example: A 3-bit binary (MOD-2 3 ) down counter. L ' ' ' 2 3-bit binary up counter L ' ' ' 2 3-bit binary down counter svbitec.wordpress.com 7
Asynchronous Down ounters Example: A 3-bit binary (MOD-8) down counter. L ' ' ' 2 L 2 3 4 5 6 7 8 2 svbitec.wordpress.com 8