Digital Counting Circuits 0

*04 Electronics (160-187).qxd

2/2/07

12:23 pm

Page 177

Counting circuits 1 Key words

1 Binary up-counter D Q

S

Q

S

Clock

R

C

Clock

R

Q

Q

R

Q

S

Clock

Q

A

B S

Clock

Counting circuits

R

Q

D 0 0 0 0 0 0 0 0

C 0 0 0 0 1 1 1 1

B 0 0 1 1 0 0 1 1

A 0 1 0 1 0 1 0 1

3 Binary-down counter D Q

S Clock

R

C Q

S Clock

R

Q

Pulse number D 8 1 9 1 10 1 11 1 12 1 13 1 14 1 15 1

B Q

S Clock

Q

C 0 0 0 0 1 1 1 1

R

B 0 0 1 1 0 0 1 1

counters consist of bistables connected so that they toggle when pulses to be counted are applied to their clock input. ● Counting is done in binary code using only 0 and 1. 0 and 1 correspond to the low and high states of binary output. ● Electronic

Q

2 Counting table for binary-up counter Pulse number 0 1 2 3 4 5 6 7

binary code bistable clocked logic

A 0 1 0 1 0 1 0 1

1 Binary up-counter ● In

an up counter the count increases by one for each clock pulse. ● The Q output of each clocked bistable becomes the clock input of the next.

2 Counting table for binary-up counter ● The

total count at any time is given by the states of: D 23 8

A S

Q

C 22 4

B 21 2

A 20 1

●A

counter with four clocked bistables can count up to 1111 in binary (15 in base 10).

Clock

Q

R

Q

3 Binary-down counter ● In

a down counter the count decreases by one for each clock pulse.

4 Dual decade counter binary counter

A B Clock C

A B C

D

D

Reset

seven segment decoder and display

A B Clock C

A B C

D

D

Reset

The output of a binary counter is a sequence of 0s and 1s representing a number. This is decoded and different combinations of segments are turned on and off to represent each number.

© Diagram Visual Information Ltd.

●

4 Dual decade counter