Not able to use 74HC164

[abhijitdutta2005]

I have a circuit (+5V) where a 74HC164 input pin-2 is driven by the output of a 74ACT74/74ls74.
Pin-1 of 'HC164 is high. Pin-9 (clear) is high. Initially all outputs are zero and also clock pin-8 zero.
When I give a clock (from a monoshot) low-high, all the outputs of 'HC 164 are changing to high if the input pin-2 is high, or changing to low if pin-2 is low, whereas, I'm supposed to see a serial shift in the outputs. There's no setup time violation issue as I'm proving the inputs (data pin-2 and clock pin-9) manually. Also there's no clock jitters.
Can someone help explaining what's wrong? I don't think HC/LS interfacing is a problem- or is it? I tried 3-4 samples of 'HC164. All behave the same way.

 

[betwixt]

The symptoms suggest there is more than one clock pulse and the 'B' input is propagating throughout the whole shft register. The logic levels shouldn't be a problem but noise on signal edges might be, particularly the one shot signal. Just temporarily, try connecting a capacitor (say 100pF) between pins 7 and 8 to see if it improves matters.

Can you confirm something though. You say the CLR input is high but the schematic shows it is floating. Is it actually tied/driven high or are you measuring a logic high with a testmeter? It should be tied high.

Brian.

 

[abhijitdutta2005]

After trying with 100pF unsuccessfully, I wired up a shift register with 74LS175 (4 bit) and tried with a larger decoupling cap on the clock pin and now the circuit behaves properly. I will now go back to HC164 and try with a larger decap on the clock. Hope it works. Seems that was the problem. Will update again soon. Thanks Brian for your suggestion!!