Up-down-counter

  
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Inside Circuits
The discrete-gate up/down-counter design has the unusual property of freezing, or saturating, when it reaches its lowest count in the down-count mode or its highest count in the count-up mode instead of rolling over and resetting as do most counters. This property proves especial!y useful in position-control systems. For example, you wouldn`t want a robot`s arm to slowly move to full extension as the counter counts up and then have it suddenly slam back to its rest position when the counter resets to zero.

You can cascade as many of the A cells as you need because the counter"s outputs are synchronous. The B cell accepts the carry bit from the most sigoificant bit. s A cell and provides the clock control that stops the counter. Make sure that the freeze input to the B cell doesn"t get asserted when the clock input is low; otherwise, the counter might make an extra count.





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