RZara

 Profile Answers by RZara 

• Nov 24th, 2009

 

If in the ripple counter, there are 3 flip flops, it is a mod-8 counter which counts from 0-7(binary). If additional circuitry is used, then it is a counter where few states have been trapped. Usually this is done by connecting a Nand gate output to the clear inputs of all the flip flops in a ripple counter. The clear signal is a active low signal and hence clears the flip flops to 000 when the output of the Nand gate goes low. The inputs to the Nand gates come from the flip flop outputs when they represent the count which we need to eliminate.

For ex: If a mod-8 counter is to be altered to count from 000-011 (0-6), i.e., we need to build a mod-7 counter from a mod-8 counter. To do this, we need to clear the counter to 000 once the count reaches 7. In effect, the counter needs to count from 0-6 and recycle back to 0. To do this connect the outputs of the flip flops which go high on a count of 7. This makes the output of the Nand gate high when a count of 7 is attempted, and hence clears the count back to 000.