Lecture-6

1.

Digital Logic Design
Lecture – 6:
Counters

2. Lecture base

Digital Electronics: Principles & Applications, 9th edition by Roger
Tokheim & Patrick E. Hoppe:
• Chapter 8
2

3.

Introduction 1
• Overview of Counters
• Characteristics of Counters
• Ripple Counters
• Mod-10 Ripple Counters
• Synchronous Counters
• Down Counters
3

4.

Overview of Counters
• Counter
• One input (clock)
• Outputs follow a defined sequence
• Common tasks of a counter
• Count up or down
• Sequence events
• Divide frequency
• Address memory
4

5.

Characteristics of Counters
• Combinational Circuits
• Logic gates
• Sequential Circuits
• Timing (Clocks)
• Memory devices
5

6.

Ripple Counters 1
• Counts in binary
• Asynchronous, all the flip-flops do
not trigger at the same time.
• A 4-bit binary counter will
count from 00002 to 11112.
MSB
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
LSB
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
6

7.

Ripple Counters 2
• The modulus of the counter is the
number of different states the
counter must go through to
complete its counting cycle.
• A modulo-16 (mod-16) counter
has 16 unique states.
MSB
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
LSB
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
7

8.

Ripple Counters 3
• Mod-16 counter and associated waveforms
8

9.

Ripple Counters 4
Mod # 2# of flip-flops
• A binary counter with four J-K flip-flops can count from
0000 to 1111 in binary.
Mod # 2# of flip-flops
Mod # 24
Mod # 16
9

10.

Ripple Counters 5
• The modulus number can be used to determine the
highest possible number in the count.
• Highest # in the count = Mod #-1
• Let’s determine the highest number a ripple counter
made up of four J-K flip-flops can count to.
Highest # in the count Mod # 1
Highest # in the count 2
# of flip-flops
1
Highest # in the count 24 1 15
Output frequency of the MSB
can be calculated
using
10

11.

Mod-10 Ripple Counters
• The counting sequence for a mod-10 counter is from
0000 to 1001 (0 to 9 in decimal).
• When the counter goes to 10102 it resets.
11

12.

Synchronous Counters
• All stages of the counter trigger together.
12

13.

Down Counters
• Mod-8 asynchronous down counter
13

14.

Self-Stopping Counter
• Up or down counters can be stopped after any
sequence of counts by using a logic gate or a
combination of gates.
This down counter will stop when the count reaches 0002
because the output of the OR gate will be LOW.
14

15.

Thank you!
15