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Counters

Counters. Introduction: Counters. Counters 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.

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Counters

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  1. Counters

  2. Introduction: Counters • Counters 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. Introduction: Counters

  3. Asynchronous (Ripple) Counters • 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) 2n 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. Asynchronous (Ripple) Counters

  4. J J HIGH K K Q0 Q1 C C CLK Q0 FF1 FF0 CLK 1 2 3 4 Q0 Q0 0 1 0 1 0 Q1 0 0 1 1 0 Asynchronous (Ripple) Counters • Example: 2-bit ripple binary counter. • Output of one flip-flop is connected to the clock input of the next more-significant flip-flop. Timing diagram 00  01  10  11  00 ... Asynchronous (Ripple) Counters

  5. HIGH J J J Q0 Q1 Q2 CLK C C C Q0 Q1 K K K FF0 FF2 FF1 CLK 1 2 3 4 5 6 7 8 Q0 0 1 0 1 0 1 0 1 0 Q1 0 0 1 1 0 0 1 1 0 Q2 0 0 0 0 1 1 1 1 0 Recycles back to 0 Asynchronous (Ripple) Counters • Example: 3-bit ripple binary counter. Asynchronous (Ripple) Counters

  6. CLK 1 2 3 4 Q0 Q1 Q2 tPHL (CLK to Q0) tPHL (CLK to Q0) tPHL (Q0 toQ1) tPLH (Q0 toQ1) tPLH (CLK to Q0) tPLH (Q1 toQ2) Asynchronous (Ripple) Counters • Propagation delays in an asynchronous (ripple-clocked) binary counter. • If the accumulated delay is greater than the clock pulse, some counter states may be misrepresented! Asynchronous (Ripple) Counters

  7. J J J J HIGH Q0 Q1 Q2 Q3 CLK C C C C K K K K FF0 FF1 FF2 FF3 CLK 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Q0 Q1 Q2 Q3 Asynchronous (Ripple) Counters • Example: 4-bit ripple binary counter (negative-edge triggered). Asynchronous (Ripple) Counters

  8. C B A Q Q Q J J J CLK CLK CLK All J, K inputs are 1 (HIGH). K K K Q Q Q CLR CLR CLR B C Asyn. Counters with MOD no. < 2n • 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.) Asynchronous Counters with MOD number < 2^n

  9. C B A All J, K inputs are 1 (HIGH). B C 1 2 3 4 5 6 7 8 9 10 11 12 MOD-6 counter produced by clearing (a MOD-8 binary counter) when count of six (110) occurs. Clock Q Q Q J J J A CLK CLK CLK K K K Q Q Q B CLR CLR CLR C NAND Output 1 0 Asyn. Counters with MOD no. < 2n • Example (cont’d): Asynchronous Counters with MOD number < 2^n

  10. 1 2 3 4 5 6 7 8 9 10 11 12 Clock A B C NAND Output 1 0 111 Temporary state 001 000 010 101 011 100 110 Asyn. Counters with MOD no. < 2n • Example (cont’d): Counting sequence of circuit (in CBA order). 0 0 0 1 0 0 0 1 0 1 1 0 0 0 1 1 0 1 0 0 0 1 0 0 Counter is a MOD-6 counter. Asynchronous Counters with MOD number < 2^n

  11. F E D C B A Q Q Q Q Q Q J J J J J J C D EF Q Q Q Q Q Q K K K K K K All J = K = 1. CLR CLR CLR CLR CLR CLR Asyn. Counters with MOD no. < 2n • Exercise: How to construct an asynchronous MOD-5 counter? MOD-7 counter? MOD-12 counter? • Question: The following is a MOD-? counter? Asynchronous Counters with MOD number < 2^n

  12. (A.C)' HIGH D C B A CLK J J J J Q Q Q Q C C C C K K K K CLR CLR CLR CLR Asyn. Counters with MOD no. < 2n • Decade counters (or BCD counters) are counters with 10 states (modulus-10) in their sequence. They are commonly used in daily life (e.g.: utility meters, odometers, etc.). • Design an asynchronous decade counter. Asynchronous Counters with MOD number < 2^n

  13. HIGH D C B A (A.C)' J Q J Q J Q J Q C C C C CLK K K K K CLR CLR CLR CLR 1 2 3 4 5 6 7 8 9 10 11 Clock D 0 0 0 0 1 0 0 0 0 1 0 0 1 1 0 0 0 0 1 0 1 0 1 0 0 1 1 0 1 1 1 0 0 0 0 1 1 0 0 1 C B A NAND output Asyn. Counters with MOD no. < 2n • Asynchronous decade/BCD counter (cont’d). 0 0 0 0 Asynchronous Counters with MOD number < 2^n

  14. J J J J J J K K K K K K 1 Q2 Q0 Q1 Q Q Q Q Q Q Q C C CLK C Q' Q' Q' Q' Q' Q' Q' 1 Q2 Q0 Q1 Q C C CLK C Q' Asynchronous Down Counters • 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-23) down counter. 3-bit binary up counter 3-bit binary down counter Asynchronous Down Counters

  15. J J J K K K CLK 1 Q Q Q 2 3 4 5 6 7 8 Q' Q' Q' Q0 0 1 0 1 0 1 0 1 0 101 011 010 110 100 001 000 111 Q1 0 1 1 0 0 1 1 0 0 1 Q2 Q2 Q0 Q1 0 1 1 1 1 0 0 0 0 Q C C CLK C Q' Asynchronous Down Counters • Example: A 3-bit binary (MOD-8) down counter. Asynchronous Down Counters

  16. J J J J J K K K K K Q Q Q Q Q Q' Q' Q' Q' Q' Q0 Q1 Q2 Q3 Q4 Q Q C C C CLK C C Q' Q' Modulus-4 counter Modulus-8 counter Cascading Asynchronous Counters • Larger asynchronous (ripple) counter can be constructed by cascading smaller ripple counters. • Connect last-stage output of one counter to the clock input of next counter so as to achieve higher-modulus operation. • Example: A modulus-32 ripple counter constructed from a modulus-4 counter and a modulus-8 counter. Cascading Asynchronous Counters

  17. A0A1A2 A3A4A5 Count pulse 3-bit binary counter 3-bit binary counter Cascading Asynchronous Counters • Example: A 6-bit binary counter (counts from 0 to 63) constructed from two 3-bit counters. Cascading Asynchronous Counters

  18. freq/10 1 CTEN CTEN freq/100 Decade counter Decade counter TC TC CLK C C Q3 Q2 Q1 Q0 Q3 Q2 Q1 Q0 freq Cascading Asynchronous Counters • If counter is a not a binary counter, requires additional output. • Example: A modulus-100 counter using two decade counters. TC = 1 when counter recycles to 0000 Cascading Asynchronous Counters

  19. 01 00 10 11 Synchronous (Parallel) Counters • Synchronous (parallel) counters: the flip-flops are clocked at the same time by a common clock pulse. • We can design these counters using the sequential logic design process (covered in Lecture #12). • Example: 2-bit synchronous binary counter (using T flip-flops, or JK flip-flops with identical J,K inputs). Synchronous (Parallel) Counters

  20. J J K K Q Q Q' Q' 1 A0 A1 Q C C Q' CLK Synchronous (Parallel) Counters • Example: 2-bit synchronous binary counter (using T flip-flops, or JK flip-flops with identical J,K inputs). TA1 = A0 TA0 = 1 Synchronous (Parallel) Counters

  21. A1 1 1 1 1 A2 1 1 1 1 A1 A0 1 A1 A2 1 1 1 A0 A2 1 1 A0 Synchronous (Parallel) Counters • Example: 3-bit synchronous binary counter (using T flip-flops, or JK flip-flops with identical J, K inputs). TA2 = A1.A0 TA1 = A0 TA0 = 1 Synchronous (Parallel) Counters

  22. A2 A1 A0 Q Q Q J J J K K K CP 1 Synchronous (Parallel) Counters • Example: 3-bit synchronous binary counter (cont’d). TA2 = A1.A0TA1 = A0TA0 = 1 Synchronous (Parallel) Counters

  23. Synchronous (Parallel) Counters • Note that in a binary counter, the nth bit (shown underlined) is always complemented whenever 011…11 100…00 or 111…11 000…00 • Hence, Xn is complemented whenever Xn-1Xn-2 ... X1X0 = 11…11. • As a result, if T flip-flops are used, then TXn = Xn-1 . Xn-2 . ... . X1 .X0 Synchronous (Parallel) Counters

  24. J J K K A1.A0 1 A2.A1.A0 Q Q Q Q A0 A1 A2 A3 J J Q Q' Q' Q' Q' C C C C Q' K K CLK Synchronous (Parallel) Counters • Example: 4-bit synchronous binary counter. TA3 = A2 . A1. A0 TA2 = A1. A0 TA1 = A0 TA0 = 1 Synchronous (Parallel) Counters

  25. Synchronous (Parallel) Counters • Example: Synchronous decade/BCD counter. T0 = 1 T1 = Q3'.Q0 T2 = Q1.Q0 T3 = Q2.Q1.Q0 + Q3.Q0 Synchronous (Parallel) Counters

  26. T T T T Q Q Q Q Q' Q' Q' Q' Q0 Q1 Q2 Q3 1 Q Q Q Q C C C C Q' Q' Q' Q' CLK Synchronous (Parallel) Counters • Example: Synchronous decade/BCD counter (cont’d). T0 = 1 T1 = Q3'.Q0 T2 = Q1.Q0 T3 = Q2.Q1.Q0 + Q3.Q0 Synchronous (Parallel) Counters

  27. Up/Down Synchronous Counters • Up/down synchronous counter: a bidirectional counter that is capable of counting either up or down. • An input (control) line Up/Down (or simply Up) specifies the direction of counting. • Up/Down = 1  Count upward • Up/Down = 0  Count downward Up/Down Synchronous Counters

  28. Up/Down Synchronous Counters • Example: A 3-bit up/down synchronous binary counter. TQ0 = 1 TQ1 = (Q0.Up) + (Q0'.Up' ) TQ2 = ( Q0.Q1.Up )+ (Q0'. Q1'. Up' ) Up counter TQ0 = 1 TQ1 = Q0 TQ2 = Q0.Q1 Down counter TQ0 = 1 TQ1 = Q0’ TQ2 = Q0’.Q1’ Up/Down Synchronous Counters

  29. T T T Q Q Q Q0 Q1 Q' Q' Q' Q2 1 Up Q Q Q C C C Q' Q' Q' CLK Up/Down Synchronous Counters TQ0 = 1 TQ1 = (Q0.Up) + (Q0'.Up' ) TQ2 = ( Q0.Q1.Up )+ (Q0'. Q1'. Up' ) • Example: A 3-bit up/down synchronous binary counter (cont’d). Up/Down Synchronous Counters

  30. 000 100 001 101 111 110 011 010 Designing Synchronous Counters • Covered in Lecture #12. • Example: A 3-bit Gray code counter (using JK flip-flops). Designing Synchronous Counters

  31. Q1Q0 Q1Q0 Q1Q0 Q2 Q2 Q2 00 01 11 10 00 01 11 10 00 01 11 10 0 1 1 0 1 1 X X 0 1 1 X X X X X X X X X X 1 JQ2 = Q1.Q0' JQ1 = Q2'.Q0 JQ0 = Q2.Q1 + Q2'.Q1' = (Q2Q1)' Q1Q0 Q1Q0 Q1Q0 Q2 Q2 00 01 11 10 00 01 11 10 Q2 00 01 11 10 0 1 X X 0 1 X 1 X 0 1 X X X X X X 1 X 1 X 1 KQ1 = Q2.Q0 KQ0 = Q2.Q1' + Q2'.Q1 = Q2Q1 KQ2 = Q1'.Q0' Designing Synchronous Counters • 3-bit Gray code counter: flip-flop inputs. Designing Synchronous Counters

  32. Q0 Q1 Q2 J J J Q Q Q C C C Q1' Q2' K K K Q' Q' Q' Q0' CLK Designing Synchronous Counters • 3-bit Gray code counter: logic diagram. JQ2 = Q1.Q0' JQ1 = Q2'.Q0 JQ0 = (Q2Q1)' KQ2 = Q1'.Q0' KQ1 = Q2.Q0 KQ0 = Q2Q1 Designing Synchronous Counters

  33. Shift Register Counters • Ring Counters • A ring counter is basically a circulating shift register in which the output of the most significant stage is fed back to the input of the least significant stage.  The following is a 4-bit ring counter constructed from D flip-flops.  The output of each stage is shifted into the next stage on the positive edge of a clock pulse.  If the CLEAR signal is high, all the flip-flops except the first one FF0 are reset to 0.  FF0 is preset to 1 instead.

  34. Shift Register Counters • Since the count sequence has 4 distinct states, the counter can be considered as a mod-4 counter.  Only 4 of the maximum 16 states are used, making ring counters very inefficient in terms of state usage.  But the major advantage of a ring counter over a binary counter is that it is self-decoding.  No extra decoding circuit is needed to determine what state the counter is in.

  35. Shift Register Counters • Johnson Counters

  36. Shift Register Counters • Again, the apparent disadvantage of this counter is that the maximum available states are not fully utilized.  Only eight of the sixteen states are being used. • Beware that for both the Ring and the Johnson counter must initially be forced into a valid state in the count sequence because they operate on a subset of the available number of states.  Otherwise, the ideal sequence will not be followed.

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