1. Introduction to Chapter 7
FFs and logic gates are combined to form various counters and registers.
Part 1 covers counter principles, various counter circuits, and IC counters.
Part 2 covers several types of IC registers and shift register counter troubleshooting.
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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2. 7-1 Asynchronous (Ripple) Counters
Review of four bit counter operation (refer to Figure 7-1)
Clock is applied only to FF A. J and K are high in all FFs.
Output of FF A is CLK of FF B and so forth.
FF outputs D, C, B, and A are a 4 bit binary number with D as the MSB.
After the NT of the 15th clock pulse the counter recycles to 0000.
This is an asynchronous counter because state is not changed in exact synchronism with the clock.
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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3. 7-1 Asynchronous (Ripple) Counters
Schematics are normally drawn from left to right, but counters will be drawn from right to left so that the MSB and LSB appear in the appropriate positions.
MOD number is equal to the number of states that the counter goes through before recycling. Adding FFs will increase the MOD number.
Frequency division – each FF will have an output frequency of ½ the input. The output frequency of the last FF of any counter will be the clock frequency divided by the MOD of the counter.
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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4. 7-2 Propagation Delay in Ripple Counters
Ripple counters are simple, but the cumulative propagation delay can cause problems at high frequencies.
For proper operation the following apply:
TclockN x tpd
Fmax=1/N x tpd
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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5. 7-2 Counters with MOD Number <2N
The MOD of a counter can be changed by designing the counter to normal parts of the counting sequence.
Refer to figure When outputs B and C are high the counter will be reset.
Notice the “glitch” in the waveform when the count reaches 6. This represents the brief time required to reset the counter.
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 9e
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6. 7-4 Counters with MOD Number < 2N
Changing the MOD number.
Find the smallest MOD required so that 2N is less than or equal to the requirement.
Connect a NAND gate to the asynchronous CLEAR inputs of all FFs.
Determine which FFs are HIGH at the desired count and connect the outputs of these FFs to the NAND gate inputs.
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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7. 7-4 Counters with MOD Number < 2N
Decade counters/BCD counters
A decade counter is any counter with 10 distinct states, regardless of the sequence. Any MOD-10 counter is a decade counter.
A BCD counter is a decade counter that counts from binary 0000 to 1001.
Decade counters are widely used for counting events and displaying results in decimal form.
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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8. 7-3 Synchronous (Parallel) Counters
All FFs are triggered by CPs simultaneously
Figure 7-5 illustrates operation of a synchronous counter.
Each FF has J and K inputs connected so they are HIGH only when the outputs of all lower-order FFs are HIGH.
The total propagation delay will be the same for any number of FFs.
Synchronous counters can operate at much higher frequencies than asynchronous counters.
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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9. 7-5 Synchronous Down and Up/Down Counters
The synchronous counter can be converted to a down counter by using the inverted FF outputs to drive the JK inputs.
A synchronous counter can be made an up/down counter by connecting as illustrated in Figure 7-11.
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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10. 7-6 Presettable Counters
A presettable counter can be set to any desired starting point either asynchronously or synchronously.
The preset operation is also called parallel loading the counter.
Figure 7-12 illustrates an asynchronous preset.
There are several TTL and CMOS devices that provide both synchronous and asynchronous presetting.
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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11. 7-7 IC Asynchronous Counters
TTL 74ALS160
The counter contains four FFs.
The FFs are triggered by a PGT at the CLK input.
The IC has an active-low asynchronous CLEAR input.
The counter can be preset to any value (applied to the A, B, C, and D inputs) by applying an active-low LOAD input.
The counter is controlled using the various input combinations shown in Figure 7-13c.
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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12. 7-8 Decoding a Counter
Decoding is the conversion of a binary output to a decimal value.
The active high decoder shown in Figure 7-20 could be used to light an LED representing each decimal number 0 to 7.
Active low decoding is obtained by replacing the AND gates with NAND gates.
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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13. 7-9 Analyzing Synchronous Counters
Figure 7-23 is a synchronous up counter.
The control inputs are as follows: JC = A  B, KC = C, JB = KB = A, JA = KA =
The count sequence is illustrated by Table 7-1 and the waveforms shown in Figure 7-24.
A synchronous counter built using D-type FFs is shown in Figure 7-25.
The count sequence is shown in Table 7-2.
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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14. 7-10 Synchronous Counter Design
Determine desired number of bits and desired counting sequence
Draw the state transition diagram showing all possible states
Use the diagram to create a table listing all PRESENT states and their NEXT states
Add a column for each JK input. Indicate the level required at each J and K in order to produce transition to the NEXT state.
Design the logic circuits to generate levels required at each JK input.
Implement the final expressions.
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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15. 7-11 Basic Counters Using HDL
Overview
Synchronous counter design with D FF
State Transition Description Methods
State descriptions in AHDL
State descriptions in VHDL
Behavioral Descriptions
AHDL
VHDL
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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16. 7-12 Full Featured Counters in HDL
Overview
How to make it count up and down
How to clear it asynchronously
How to load it synchronously
How to include synchronous cascade controls
AHDL full-featured counter
VHDL full-featured counter
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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17. 7-13 Wiring HDL Modules Together
Decoding the AHDL MOD-5 counter
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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18. 7-13 Wiring HDL Modules Together
AHDL
MOD-5
counter
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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19. 7-13 Wiring HDL Modules Together
VHDL
MOD-5
counter
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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20. 7-14 State Machines
Counter circuits used to generate a sequence of states without regard to their binary value are often called state machines. The washing machine states of idle, fill, agitate, and spin will be used in the examples.
AHDL state machines
VHDL state machines
Each method of describing logic circuits has advantages.
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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21. 7-14 State Machines AHDL state machine
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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22. 7-14 State Machines VHDL state machine
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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23. 7-15 Integrated-Circuit Registers
Registers can be classified by the way data is entered for storage, and by the way data is outputted from the register.
Parallel in/parallel out (PIPO)
Serial in/serial out (SISO)
Parallel in/serial out (PISO)
Serial in/parallel out (SIPO)
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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24. 7-16 PIPO – The 74ALS174/74HC174 Refer to Figure 7-62
Six bit register
Parallel inputs D5 through D0
Parallel outputs Q5 through Q0
Parallel data loaded to the register on the PGT of CP
Master reset can reset all FFs asynchronously
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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25. 7-17 SISO – The 74HC166 Refer to Figure 7-64
The chip contains an 8-bit shift register
The serial input is labeled SER
Only the QH output is accessible.
Clock input responds to PGT
A buffer is used to provide greater output current.
Inputs A-H provide the means for parallel data entry into register FFs
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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26. 7-18 PISO – The 74ALS165/74HC165 Refer to figure 7-66
8 bit register
Serial data entry via DS
Asynchronous parallel data entry P0 through P7
Only the outputs of Q7 are accessible
CP is clock input for shifting
Clock inhibit input
Shift load input
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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27. 7-19 SIPO – The 74ALS164/74HC164 Refer to Figure 7-68
8 bit shift register
Each FF output is externally accessible
A and B inputs are combined in an AND gate for serial input.
Shift occurs on NGT of the clock input.
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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28. 7-19 SIPO – The 74ALS164/74HC164 Other similar devices
74194/ASL194/HC194
4 bit bi-directional universal shift register
Performs shift left, shift right, parallel in and parallel out.
74373/ALS373/HC373/HCT373
8 bit PIPO with 8 D latches
Tristate outputs
74374/ALS374/HC374
8 bit PIPO with 8 edge triggered D FFs
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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29. 7-20 Shift Register Counters
Ring Counter
Refer to Figure 7-70
Last FF shifts its value to first FF
Uses D-type FFs (JK FFs can also be used)
Must start with only one FF in the 1 state and all others in the 0 state.
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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30. 7-20 Shift Register Counters
Johnson counter
Refer to Figure 7-72
Also called a twisted ring counter
Same as ring counter but the inverted output of the last FF is connected to input of the first FF
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
Copyright ©2007 by Pearson Education, Inc. Columbus, OH All rights reserved..

31. 7-21 Troubleshooting
Sequential logic systems are more complex, but basic troubleshooting procedures still apply.
Observe system operation
Use analytical reasoning to determine possible causes
Use test equipment to isolate the exact fault
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
Copyright ©2007 by Pearson Education, Inc. Columbus, OH All rights reserved..

32. 7-22 HDL Registers Using bit arrays to describe register data
Example 7-24 involves the design of a universal 4 bit shit register. Requirements are:
4 synchronous modes of operation: hold, shift left, shift right, and parallel load
2 input bits select operation to be performed on PGT of the clock
AHDL solution
VHDL solution
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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33. 7-23 HDL Ring Counters
Recall that a ring counter is a shift register that circulates a single active logic level through all its FFs.
AHDL solution (Figure 7-85)
VHDL solution (Figure 7-86)
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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34. 7-24 HDL One-Shots Non-retriggerable, level sensitive one-shot
AHDL solution
VHDL solution
Retriggerable edge-triggered one-shot
Ronald Tocci/Neal Widmer/Gregory Moss Digital Systems: Principles and Applications, 10e
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