Ask Electronics @discuss.tchncs.de quiescentcurrent @discuss.tchncs.de 1y ago

Shift register missing bits

Hey friends,

I have a two daisy chained shift registers (74AHC595) which are controlled via an ESP32. I want to set one output to high at a time before switching to the next.

The code seems to work, but the outputs O_9 and O_10 are not staying high (zoom) after setting them, whereas all the other ones are working fine. This is the used code snipped:

pinMode(SHIFT_OUT_DATA, OUTPUT);
pinMode(SHIFT_OUT_CLK, OUTPUT);
pinMode(SHIFT_OUT_N_EN, OUTPUT);
pinMode(SHIFT_OUT_LATCH, OUTPUT);

digitalWrite(SHIFT_OUT_N_EN, LOW);

uint16_t input_bin = 0b1000000000000000;

for(int i=0; i&lt;17; i++){

    byte upper_byte = input_bin >> 8;
    byte lower_byte = input_bin &amp; 0x00FF;

    digitalWrite(SHIFT_OUT_LATCH, LOW);
    shiftDataOut(SHIFT_OUT_DATA, SHIFT_OUT_CLK, MSBFIRST, lower_byte);
    shiftDataOut(SHIFT_OUT_DATA, SHIFT_OUT_CLK, MSBFIRST, upper_byte);
    usleep(10);
    digitalWrite(SHIFT_OUT_LATCH, HIGH);

    delay(10)
    input_bin = input_bin>>1;
}

Is there anything I'm doing wrong, or any idea on where the problem may lie? I've already tried looking for shorts and other error sources, but the design was manufactured on a PCB and no assembly issues are noticeable.

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15 comments

Would it work if you made that delay 1000?
- Also try upping the usleep call?
  
  Hmmmm do you want to write to both shift register at the same time? I say this because you're looping 16 times, but seem to be sending the high and low bytes out 16 times over rather than one bit each time, although you are shifting the input.
  
  Maybe I'm getting ahead of myself, but maybe try using digitalWrite for a single bit instead of shiftDataOut?
  
  Good idea, I've tried usleep after all lines, but no change..

15 comments