Monday, December 15, 2025

Checking out the low ESR cause for the 1130 MRAM ringing

QUICK VERIFICATION OF THIS THEORY OF A ROOT CAUSE

I soldered a  ohm resistor in series with the 4.7 uF ceramic capacitor. I then tried the board on the 1130 again and observed the operation. 

It failed in the same way as before. Perhaps the regulator itself is bad or I still don't have the Equivalent Series Resistance (ESR) in the stable range. 

I used a quick and dirty hack was to use an external 3.3V power supply after lifting the LD1117 from the circuit. I used heavy stranded wires between the supply and the board. This would let me know immediately if oscillation of the regulator is the root cause of my problems. 

And . . . it is not. The exact same failure mode occurred using a bench supply to power the logic. Arrgh. 

Time for a plan C. I will think on this a while and figure out something else to do that might resolve this problem. I think it is down to three possibilities:

  1. The timer chips are either bizarre or defective, but that isn't likely since I see spurious repeats on the write timer chain as well as the read timer chain. 
  2. Something in the 1130 side is causing this, some way that I am not understanding. 
  3. Something on the PCB is causing this, maybe a combination of the decoupling capacitors and traces, again in a way I am not understanding.
That means my plan C has to be more of a desperation move where I change parts of the design just to see if a different set of parts or different approach avoids the problem. 

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