Truncated

counters

Design

Truncated counters

Applications may require n-bit binary counters that do not count its full sequence (mod no < 2n)
–achieved by modifying the basic ripple counter to skip certain states
For example, a 3-bit binary ripple counter can be modified to a mod-6 counter by skipping states 6 & 7 to count states 0 to 5 (000, 001, ...101, 000)

–by decoding state 6 and using the decoder output to immediately reset the counter to state 0 via CLR inputs

–State 6 exists only for a short duration and NOT treated as part of the count sequence

Truncated counters

Start count is the first state in the count sequence

Last or end count is the last state in the count sequence before it recycles back to start count

Reset count is the count state immediately after the last count. It is used to “force” or “truncate” the counter sequence back to the start state.

–It is “temporary” or “momentary” remaining only for a few nano-seconds before recycling back to start state.

–Hence it is not counted as part of the count sequence

Example

Example:

For a DOWN counter with a truncated count sequence 1012 to 0012,

Mod number = 5 , Start Count = _1012____

Last Count = __ 0012 __ , Reset Count = __ 0002 ___

In this case, reset count 0002 is used to force the counter back to start count 1012 to achieve the desired MOD-5 count operation

Designing Truncated counters

In designing asynchronous truncated counter, the guidelines are:

1. Get the count sequence

2. From the count sequence, identify the start, last or end and reset states

3. Decode the reset state to give an active LOW output

4. Connect the decoder active LOW output to the asynchronous PRE and CLR inputs of the FFs to set the counter to the start state. The reset decoder is typically a NAND gate that gives a LOW output when the reset state connected to its inputs occurs

Example

Design a MOD - 5 asynchronous counter using negative edge triggered JK flip-flops. Assume that the start count is 001.

Write down the count sequence, start count, last count, reset count.

Draw the counter circuit on your answer booklet. Show all connections clearly and label completely.