More work on bench testing for the 1130 MRAM core memory board

IMPLEMENTED BACKFEED PROTECTION CIRCUIT

I put a through hole IRF540N MOSFET on the prototyping tool breadboard, isolating the ground of my PCB until a voltage divider from the +12V bench supply feed is active. This low side isolation ensures no back-feed current will flow from the Arduino or any other input to the board unless we have our power turned on. 

REPLACED ONE OF THE BUFFER CHIPS ON THE BOARD

I had lifted a 74LV240 inverting buffer chip from the PCB while investigating the observed steady 1 voltage on the bit 14 net that had been producing a pulse on sense bit 14 for any and all addresses. I put a spare chip on the board so that I can write zero and one values to verify the memory chip is working.

I RESUMED BENCH TESTING BUT STILL HAVE POWER ANOMALIES

I checked to see if there was any back-feed voltage on the PCB with the Arduino active while I was not feeding voltage from the bench supply to the board. I used an ammeter on the ground line that passes through the MOSFET. After this was validated, with no current until I triggered the MOSFET, I thought I was ready for function testing. 

However, strange things are still happening. When I power the board from the bench supply, there is no indicated amps or watts being consumed when the MOSFET is connected. When I disconnected the MOSFET, I saw about 7ma of draw on the supply! At first I thought it might be my pullup resistors for the scope probes when monitoring the sense output pulses, but with those disconnected I still had 7ma flowing somewhere. 

I pulled the USB connection for the Arduino Due and my current disappeared! The Arduino is pulling current which is very odd since I have nothing but output pins defined on the Arduino. It isn't backfeed - it occurs when the Arduino is powered, not when it is getting power from my board. In fact, my board's outputs are open collector, with pullup done on the prototype breadboard (disconnected) not by my PCB components, so there is no voltage that should flow from my board to the Arduino configured in output mode - at least I don't expect there is any. 

I am going to disconnect groups of pins from the Arduino and monitor the current draw while my MOSFET is isolating my circuitry, until I get more of a clue to where this power consumption is going. The 12V from the bench supply flows through a regulator module to create 3.3V first, which means the draw is larger on the 3.3V rail than what I see on the bench supply. 

IMPROVED BENCH SETUP TO MONITOR PINS OF SMD CHIPS ON MY BOARD

I found it cumbersome to tack tiny wires on the pins of various chips in order to observe the signals during read and writes, so I pulled out my PCBite and put my board on that. This has probes with teeny pins suitable for measuring a surface mount chip pin, held in place with magnets onto the base plate. With my board anchored and the four probes set up, I could watch different pins as I study the function of the board in more detail.