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.
truncated counters
By nectar07
truncated counters
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