Counters

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Counters

  1. 1. EET 3350 Digital Systems Design Textbook: John Wakerly Chapter 8: 8.4 Counters 1
  2. 2. Counters• Counters – Definition – Types Count – Characteristics Clock Counter• Asynchronous Counters - 7490, 7492, 7493 optional inputs• Synchronous Counters• MSI Counters S1 S2 S3 – Especially the 74LS163 Sm S4• Counters in VHDL S5• Other Counter Types 2
  3. 3. Counters • A counter is a circuit that produces a numeric count each time an input clock pulse makes an active transition Clock Counter CountLoad an initial value, reset May also enable count,to starting count, etc. select direction, etc. optional inputs 3
  4. 4. Counter• From another viewpoint, a counter is any sequential circuit whose state diagram is a single cycle – in other words, counters are a special case of a finite state machine• Output is usually the state value, Moore machine EN EN RESET EN EN S2 EN EN EN S1 S3 EN EN Sm EN S4 EN EN S5 EN 4
  5. 5. Counters• Counters differ by a number of basic characteristics, including: Characteristic Description Modulus Length of sequence Coding Count sequence Direction Up or down Resetable Reset to zero Loadable Load a specific value 5
  6. 6. Counters• Applications include: Present State Next State – system clock A B A B 0 0 0 1 – timer, delays 0 1 1 0 – watches, clocks, alarms 1 0 1 1 1 1 0 0 – counting events – memory addressing – frequency division 00 01 – sequence control – cycle control – protocols 11 10 6
  7. 7. Counter Types• Asynchronous • Modulus – Ripple – Binary• Synchronous – Decade – Clocked – etc. • Ring 000 • Johnson – Twisted ring 101 001 • Up/Down • Linear Feedback Shift- 100 010 Register Counter 011 (LFSR) 7
  8. 8. Counters• Some examples of modulus and coding sequence for counters 8
  9. 9. Counters• Modulus – number of states in a counter’s cycle• Given m states – modulo-m counter or divide-by-m counter• Power-of-2 counters use all states• Non-power-of-2 counters have extra, unused states S1 S2 S3 Sm S4 S5 9
  10. 10. Example 4-bit Counters• 4-bit Binary / Hex / Mod-16 Counter – 0000, 0001, 0010, … 1110, 1111, 0000, 0001, … all states used• 4-bit BCD / Decade / Mod-10 Counter – 0000, 0001, 0010, … 1000, 1001, 0000, 0001, … six unused states• 4-bit Ring Counter – 1000, 0100, 0010, 0001, 1000, 0100, … twelve unused states 10
  11. 11. Counters• Ripple counters – asynchronous – an n-state counter that is formed from n cascaded flip-flops – the clock input to each of the individual flip-flops, with the exception of the first, is taken from the output of the preceding one – the count thus ripples along the counters length due to the propagation delay associated with each stage of counting 11
  12. 12. Asynchronous Ripple Counter Q3 Q2 Q1 Q0 Q0 0 0 0 0 0 0 0 1 Q1 0 0 1 0 0 0 1 1 0 1 0 0 Q2 0 1 0 1 0 1 1 0 Q3 0 1 1 1 1 0 0 0 . . . 12
  13. 13. Ripple Counter Timing• The ideal count sequence for the ripple counter yields the timing diagram below Q0 Q1 Q2 Q3 CLOCK Q0 Q1 Q2 Q3 13
  14. 14. Ripple Counter Timing• But there is delay ( ∆ ) as shown below:CLKQ0 1∆Q1 2∆Q2 3∆ 0 1 2 3 4 14
  15. 15. Asynchronous Ripple Counter Q0 divide-by-2 Q1 divide-by-4 a T flip-flop is a natural frequency divider … Q2 divide-by-8 Q3 divide-by-16 15
  16. 16. Decade and Binary CountersDM7490A• The monolithic counter contains four masterslave flip-flops• Gating to provide a divide-by-two counter and a three-stage binary counter for which the count cycle length is divide-by-five.• The counter has a gated zero reset and also has gated set-to-nine inputs for use in BCD nine’s complement applications.• To use the maximum count length (decade), the B input is connectedto the QA output.• The input count pulses are applied to input A and the outputs are as described in the appropriate Function Table.• A symmetrical divide-by-ten count can be obtained from the counters by connecting the QD output to the A input and applying the input count to the B input which gives a divide-by-ten square wave at output QA.
  17. 17. Connection Diagram
  18. 18. Function TablesBCD Count Sequence (Note 1) BCD Bi-Quinary (Note 2)H = HIGH LevelL = LOW LevelX = Don’t CareNote 1: Output QA is connected to input B for BCD count.Note 2: Output QD is connected to input A for bi-quinary count
  19. 19. BCD Bi-Quinary sequenceCLKQAQDQCQB 0 1 2 3 4 8 9 10 11 12 0
  20. 20. Reset/Count Function Table H = HIGH Level L = LOW Level X = Don’t Care
  21. 21. Logic DiagramThe J and K inputs shown withoutconnection are for reference only andare functionally at a HIGH level.
  22. 22. SN5490A, SN5492A, SN5493A, SN54LS90, SN54LS92, SN54LS93 SN7490A, SN7492A, SN7493A, SN74LS90, SN74LS92, SN74LS93 DECADE, DIVIDE-BY-TWELVE AND BINARY COUNTERSThe three-stage binary counter has the count cycle length ofdivide-by-five for the ’90, divide-by-six for the ’92, and divide-by-eight for the ’93.Logic Symbols
  23. 23. Function TablesCount Sequence for ’92 Count Sequence for ’93 H = HIGH Level, L = LOW Level, X = Don’t Care Note: Output QA is connected to input CKB.
  24. 24. Reset/Count Function Table H = HIGH Level L = LOW Level X = Don’t Care
  25. 25. Logic Diagrams
  26. 26. Mod 11 counter using 7493Clock CLK A QA CLK B QB 7493 QC R0(1) QD R0(2)
  27. 27. Synchronous Counters• Asynchronous counters are easy to understand, but avoid their use – slow, limited by propagation delays – error prone• Characteristics of synchronous counters – use a common clock pulse to trigger all flip-flops simultaneously – have a higher clock speed – hardware is more complex but more reliable 27
  28. 28. 4-Bit Counter LSBSynchronous counter serial enable logic MSB 28
  29. 29. 4-Bit Counter LSBSynchronous counter parallel enable logic MSB 29
  30. 30. MSI Counters• Counters can be built from individual SSI Flip-Flops, e.g., – 7470 D1 D2 – 7474 and many others … – 7479• Counters may also be built using MSI components – 74x90, 74x92, 74x93 – 74x160, 74x161, 74x162, 74x163 – 74x168, 74x169 – 74x190, 74x191 we’ll look at this one – 74x196, 74x197 30
  31. 31. MSI Counter• 4-bit synchronous counter – edge-triggered – synchronously presettable – cascadable• Typical Count Rate of 35 MHz• ‘160 and ‘162, Mod-10• ‘161 and ‘163, Mod-16 31
  32. 32. MSI Counter• 74LS163 4-bit synchronous counter 16-pin DIP 32
  33. 33. MSI Counter• 74LS163 characteristics – edge-triggered – synchronously presettable – cascadable – count modulo 16 (binary) 74x163• Synchronous Reset (Clear) input that overrides all other control inputs – active only during the rising clock edge 33
  34. 34. MSI Counter• 74LS163 logic symbols datasheet text 74x163 34
  35. 35. MSI Counter• 74LS163 state diagram and logic equations 35
  36. 36. MSI Counter• 74LS163 mode select table• All signals must be high ( H ) to enable the count sequence to begin 36
  37. 37. MSI Counter• 74x163 is a synchronous 4-bit binary counter• RCO=1 when all count bits are 1 and ENT is asserted 37
  38. 38. MSI Counter • The control inputs for the 74x163 have the following effects:clear load holdhold 38
  39. 39. 74x163Internal Logic Diagram

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