Two daisy-chained 74hc165 outputs are noisy as if they were floating

Other Hardware LEDs and Multiplexing
1

Hi,
I have this circuit

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The gpio's are all connected to 10k pullups.

I have tried every available code online, and the same result occurs: I have unreliable input values, as if the inputs were floating. For example, the arduino will read 0 on all 16 bits at bootup, and then I put my finger somewhere on the PCB and it reads FFFF, and then I press a button, nothing happens, then I press a button again but now the arduino reads F000 as if I had pressed 12 buttons, etc.

I am convinced it is not a sw problem, I already made these chips work in the past. I am out of ideas of what to test next... Ive checked all the connections from my arduino to the circuit, and the resistance between every gpio and vcc (4.5V from a arduino micro's vcc pin), to confirm that the pullups are well soldered. I also assembled a second pcb to see if I had made a mistake in assembly, in vain.

What could make these chips so unreliable??

Thanks!

2

Do you have the bypass caps installed? They are in the datasheet but not on your schematic. Also, you're powering them directly from your microcontroller?

1 Like
3

Looking at your incomplete schematic (on the right track) I will take a SWAG and say you are missing a resistor for each button. If connected as drawn the button wires are antennas and I would guess they are also over 10" in length. You mention pull up resistors but I cannot see them.

4

Are you building the circuit on a breadboard? Breadboard contacts are not reliable.

Measure the voltage on the input pins - are they really at Vcc level?

5

So what is different between what you did before and what you are doing now?

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