74HC595A Shift Register Outputs Low Level?

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Markkim
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74HC595A Shift Register Outputs Low Level?

Post by Markkim » Wed Apr 07, 2021 2:03 am

Hi
I'm working on a project, prototype everything on a breadboard and after testing soldered everything on protoboard.
I used the TI 74HC595 chip (component data source reference: https://www.apogeeweb.net/productdetail ... 8-scb80erz) in my prototype, I wanted to have everything on a PCB so I built schematic and ordered PCB with SMD assembly from JLCPCB.

Now I got the PCB and couldn't figure out why my leds are not working, tried to measured the voltage that the shift registers output and got very small voltage (less then 1 volt I think).

Is there anyone have ideas of it? I cannot get high voltage (>3v) in order to drive a led? Many thanks.

dyarker
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Joined: Fri Aug 22, 2003 1:01 am
Location: Izmir, Turkiye; from Rochester, NY
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Re: 74HC595A Shift Register Outputs Low Level?

Post by dyarker » Wed Apr 07, 2021 8:21 am

You need a better spec sheet, like -
https://www.onsemi.com/pdf/datasheet/mc74hc595a-d.pdf
(not from TI, but is for the "A" part (74HC585A))

Also needed -
A schematic diagram of your circuit.

My first EWAG based on limited info provided is OUTPUT ENABLE pin needs to be pulled up.

side comment - This is more a hardware question than computer programming. How did you pick here to post it?

Cheers,
Dale Y

dyarker
Posts: 1768
Joined: Fri Aug 22, 2003 1:01 am
Location: Izmir, Turkiye; from Rochester, NY
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Re: 74HC595A Shift Register Outputs Low Level?

Post by dyarker » Sun Apr 11, 2021 5:07 am

4 days. It was output enable?
Dale Y

Markkim
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Re: 74HC595A Shift Register Outputs Low Level?

Post by Markkim » Sat May 08, 2021 2:54 am

Good day, all
Such a busy day, what I usually face when such voltage is measured is short circuited pins, one trying to go to 1 state other 0 state, some intermediate voltage is measured. But if this is the condition for all pins then check VDD connection and VDD voltage, should be stable and there should be decoupling capacitor nearby. Also there is probability to have clk and data inputs are open, noise there causing some frequency at outputs causing DC measurement intermediate. Same is possible with OE input. I couldn't see open drain versions of this chip.

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