UP_DOWN_COUNTER_WITH_XOR_GATES

Published:2009/7/12 23:02:00 Author:May | From:SeekIC

This circuit shows how a regular 4-bit binary counter can be extended with an up/down function just by adding four XOR (exclusive-OR) gates. The principle is simple: The level at the common in-puts of the XOR gates determines whether the gates invert the counter's QA to QB output levels or not. In this way, the outputs of the XOR gates can be made to cycle from 1111 dowtt to 0000 instead of from 0000 to 1111. The disadvantage of this circuit over a real up/down counter is the jump, which occurs when the level on the U/D control input is changed. The sum of the old state and the new state is always 15. For example, if the counter is at state 3 in count-up mode, the state becomes 12 when the U/D line is made logic high to initiate down counting.



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