Noise on OPTO_BIAS, not from VDD

We have issues with blinkup on 10 IMP002’s bought in June 2014. On IMP002’s bought in April we don’t have blinkup problems. I found noise on OPTIO_BIAS and it is also found in OPTO_IN since they are connected through the phototransistor. The noise does not come from the VDD power supply. On VDD I see only the switched mode power supply (few hundred kHz) but not the pulses like on OPTO_BIAS. I added a 10uF and 100pF to OPTO_BIAS and then blinkup worked. Is this a known problem? Can you suggest possible solutions?

OPTO_BIAS is just a GPIO output from the SoC. Do your other GPIOs show the same noise… what’s the imp doing at the time? There have been no design changes in IMP002 ever (ie, the first production revision is still the only one being made) so it’s a bit strange that you see differences here. The SoC and WiFi are also the same silicon revision.

What boards are they mounted on?


They are mounted on our custom designed boards. We have two models of boards. On one model I have two vias in OPTO_BIAS and two vias in OPTO_IN. On the other model (which has blinkup problems) there are no via’s in OPTO_BIAS.

I will do additional testing to find out:

  • if I need to adjust the sensitivity with the resistor
  • if noise on opto_bias is actually the problem
  • if the right phototransistor is placed
  • if different PCB layout cause the problem
  • if it is a noisy power supply