Friday, August 28, 2009

74hc163 counter has a bug?

I have been playing around with some 74hc163 chips that I have. These are 4-bit synchronous counters with a parallel load capability. When both counter-enable inputs are logic high, and a positive clock edge arrives, the counter value increases by 1.

Oddly, the 74hc163 chips that I have seem to have a bug! When the counter value is at 1111 (i.e., 15, the highest possible count), and a positive clock edge arrives, the count changes to 0001 rather than 0000. From all of the 74hc163 datasheets I have seen, the count should wrap-around back to 0.

I'm pretty sure I have wired the circuit correctly. There are no floating inputs, for example.

The chips are labeled "SP74HC163N", and the manufacturer seems to be "SPI".

Here's a picture of the circuit with the 74hc163:


The following two pictures show LEDs which follow the '163 output. The 4 LEDs to the left are bits 0 through 3 of the count. The two LEDs on the right are showing the values at the -LOAD and enable inputs. (Note that they are both high, meaning that the counter is in counting mode.)

Here's how things look when the count is at 1111:


And here's how things look after one positive clock edge:


That's clearly not zero!

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