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Digital Logic Design

CS302 - Digital Logic & Design

Lesson No. 28

TIMING DIAGRAM OF A SYNCHRONOUS DECADE COUNTER

CLOCK

Input

Output

t10

Figure 28.1

Timing diagram of a Synchronous Decade Counter

Mod-n Synchronous Counter

A Mod-n Synchronous can be implemented using appropriate number of J-K flip-flops

connected together with their clocks triggered simultaneously. A synchronous counter which

counts a truncated sequence of n unique states can be similarly implemented. The Modulus

number represents the unique number of states which the counter counts in a sequence. The

Modulus number determines the number of flip-flops required based on the relation n = 2m

where m is the number of flip-flops.

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Integrated Circuit Synchronous Counters

Instead of connecting a large number of flip-flops together to form large Synchronous

counters, counter circuits available in Integrated Circuit form can be quickly connected to form

large counters. The 74HC163 is a 4-bit Synchronous Counter. Figure 28.2. The counter has

the following pins.

Parallel data inputs D0, D1, D2 and D3

Data outputs Q0, Q1, Q2 and Q3

Positive edge-triggered CLOCK signal

Active-low CLR input which resets the Counter output to 0000

Active-low LOAD input which loads the 4-bit data applied at the counter inputs

Active-high ENT and ENP enable inputs. For the counter to operate both the enable inputs

have to be high

7. The Ripple Clock Output RCO goes high when the Counter reaches the terminal count

1111. The RCO output along with ENT and ENP enable input pins are used to cascade

multiple counter ICs for implementing larger counters

D0 D1 D2 D3

CLR

LOAD

RCO

74HC163

ENT

ENP

CLK

Q0 Q1 Q2 Q3

Figure 28.2a 74HC163 4-bit Synchronous Counter

Referring to the timing diagram, the CLR signal is activated between interval t0 and t1.

The counter output is reset synchronously at interval t1 as the CLR signal is active at interval

t1. If the CLR signal is deactivated before interval t1 then the counter output is not reset. The

LOAD signal is activated between interval t1 and t2. At the clock transition at t2, the counter is

loaded with the 4-bit data applied at the inputs D0, D1, D2 and D3. The ENP and ENT enable

signals are activated before interval t3 and the counter increments to the higher count at clock

transition at intervals t3 and t4. When the counter reaches the count 15 at interval t4, the RCO

(Ripple Clock Output) is set to high indicating that terminal count has been reached. At

intervals t5, t6, t7 and t8 the counter successively counts to 0, 1, 2 and 3. The counter enable

signal ENP is deactivated after interval t8, which inhibits the counter from counting any further.

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CLR

LOAD

Figure 28.2b Timing diagram of the 74HC163 Synchronous counter

The 74HC160 is a 4-bit Synchronous Decade counter with the same input and output

pins as the 74HC163. The RCO output of the decade counter is activated when the counter

reaches its terminal count 1001.

Cascading Counters

It is very convenient to cascade Integrated Circuit counters together to form larger

counters instead of connecting together flip-flops to implement a large counter. The enable

inputs and Ripple Clock Outputs of the Integrated Circuit counters allow cascading of multiple

counters together. Two, 74HC160 decade counters are shown connected together to divide

the input frequency by 10 and 100. Figure 28.3. The 74HC163 can also be similarly cascaded

together.

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CLR

LOAD

Figure 28.3a Cascaded Decade Counters

Figure 28.3b Timing diagram of a Cascaded Decade Counter

In the timing diagram, at interval t9 the first decade counter reaches the terminal count 1001.

The RCO output of the counter is set to logic 1. The RCO of the first counter is connected to

the ENP and ENT enable pins of the second counter, therefore the counter is enabled. At

interval t10 on a positive clock transition the first counter increments to count 0000. Since the

second counter is also enabled, it is incremented to 0001. As soon as the first counter is

incremented to 0000, the ECO signal is deactivated which in-turn also inhibits the second

counter. The first counter counts from 0001 to 1000 in the intervals t11 to t19. At interval t19 the

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first counter again reaches its terminal count 1001, the RCO output of counter once again

becomes active thereby activating the second counter. At interval t20 on a positive clock

transition the first and second counters increment to count 0000 and 0010 respectively. The

RCO signal is again deactivated inhibiting the second counter from counting. This sequence

continues after the first counter reaches its terminal count.

Integrated Circuit Counters with Truncated Sequences

Earlier, a decade counter was implemented by truncating the counting sequence of a

MOD-16 counter. The Integrated Circuit Counters can also be configured as MOD-n counters

where n represents the truncated sequence and is less than 16. Figure 28.4 shows the circuit

diagram of the 74HC163 counter configured as Mod-7 counter. The counter is preset with the

count value 1001 by setting the LOAD/NORMAL input to logic 1 at the NOR gate input. At the

positive clock transition t1, the count value is loaded. The counter increments and at interval t7

it reaches the terminal count. The RCO output is set to logic 1 which sets the LOAD input to

logic 0. At the positive clock transition at interval t8 the preset value 1001 is reloaded and the

counter continues its counting sequence.

D0 D1 D2 D3

LOAD/NORMAL 1/0

1001

CLR

LOAD

RCO

74HC163

ENT

ENP

CLK

Q0 Q1 Q2 Q3

Figure 28.4a 74HC163 configured as Mod-7 counter

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LOAD

Figure 28.4b The timing diagram of a truncated Mod-7 Counter

Another method to configure an Integrated Circuit counter is to reset the counter when

it reaches the maximum count value of its truncated sequence. This requires extra logic in the

form of logic gates that determine the terminating state and reset the counter. The circuit

diagram of the counter is shown. Figure 28.5

D0 D1 D2 D3

LOAD/NORMAL 1/0

CLR

LOAD

RCO

74HC161

ENT

ENP

CLK

Q0 Q1 Q2 Q3

Figure 28.5a 74HC161 configured as Mod-9 counter

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CLR

Figure 28.5b Timing diagram of a 74HC161 configured as Mod-9 counter

The counter used is 74HC161 instead of 74HC163. The 74HC161 has an

Asynchronous Clear input, where as the 74HC163 counter has a synchronous Clear input. At

time interval t9 the counter increments to 1001 which sets the output of the AND gate to logic

1. The NOR gate output is set to logic low which activates the clear input and resets the

counter to 0000. The 74HC163 counter which has a synchronous clear input, will reset counter

resets at interval t10 when there is a transition at the clock input. It is clear from the timing

diagram that to implement a Mod-9 counter the 74HC161 instead of 74HC163 counter has to

be used.

Cascaded Counters with Truncated Sequences

Cascaded counters can also be configured to count in a truncated sequence. The

circuit diagram of three cascaded 74HC163 is shown. Figure 28.6. The 12-bit cascaded

counter is loaded with initial count value 1000 0000 0000. When the counter counts to 1111

1111 1111, the RCO output set to logic 1 by the third counter reloads the initial count values

0000, 0000 and 1000 in all the three counters respectively. The 12-bit counter can be

configured for maximum count sequence as Modulus 4096 counter. The counter has been

configured to count from 1000 0000 0000 to 1111 1111 1111 that is 2048 states or Modulus

2048 counter.

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CLR

LOAD

Figure 28.6

74HC163 counters connected for cascaded truncated count sequence

Up-Down Counter

An up-down counter can increment its output count value at each clock transition or

decrement its count value at each clock transition, depending upon the count mode it is

configured in. The counter can be reconfigured to count in the opposite direction during its

count sequence. The circuit of an up-down 3-bit counter can be developed by studying the up-

down count sequence of the counter. Table 28.1.

Clock Pulse

Table 28.1a

Up-counting sequence of a 3-bit Synchronous Counter

Clock Pulse

Table 28.1b Down-counting sequence of a 3-bit Synchronous Counter

A 3-bit Synchronous up-counter has been discussed earlier. Consider the implementation of

down-counter, the up and down counter can be combined to form a single configurable up-

down counter. For the down-counting sequence the output Q0 of the first flip-flop toggles

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between 0 and 1, therefore the J-K inputs are connected to logic 1. The output Q1 of the

second flip-flop toggles between logic 0 and 1 when the Q0 output is logic 0 or Q 0 is logic 1.

The output Q2 of the third flip-flop toggles when Q0 and Q1 outputs are both logic 0 or Q 0 and

Q1 are both logic 1.

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