Dear Parallax team,
I recently found a very unusual behaviour which i am trying to root cause and would appreciate any advice. I was trying to activate a simple relay using output pin P12 (pin 24) to turn on a relay. This part of my circuit is quite stable in the past - see picture (Pump to CPU Schematic). What i found is that when i activate P12 (active high) the relay turns on as expected, but then the output signal coming from P12 begin to show a step function - see attached (periodic power cycle of P12) with a time of about 2 seconds. The step function drops from the expected 5VDC to near zero and last about 16ms - see attached picture (P12 pump activation). I checked the four +5VDC inputs and the +5VDC does have a small A/C spike of about 150mV @ 15MHz - see attached picture (Pin 4 Vdd BS2P40 OEM).
Things I have done so far:
1. I do have an external brownout circuit and have disconnected the external reset. which connects to Pin1 of the Bs2P40 OEM
2. I did hardwire the relay with +5VDC and it is stable (no power cycling) which suggests the relay circuitry and hardware is ok.
3. I placed the activation on signal for P12 at the very beginning of the software and then ended the program and the relay cycling continues - hoping other code was interfering.
I would like to figure out where this periodic pulses are originating from and why they seem to be affecting what is supposed to be a stable high signal (P12 should remain at +5VDC).
I am guessing that a clock or A/C harmonic is getting into the CPU and causing some unusual behaviour. All 4 Vdd do have decoupling caps in place.
Thanks very much,