RevA PCB (troubleshooting)

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manuel-tosoneManuel Tosone 04/04/2021 at 19:062 Comments

Hi everybody, PCBs finally arrived.
I ordered them from JLCPCB, this was the first time that I had PCBs assembled.

Visual inspection and I have nothing to say, quality is pretty good, a quick check with the multimeter to make sure there are no shorts on the power rails and I'm ready to solder the connectors.

Time to program the microcontroller, as a programmer I'm using the same STM32 Discovery board that I was using for testing. By removing the two jumpers on CN4, the integrated ST-LINK programmer is disconnected from the board and the programming signals are exposed on CN2.

So I applied power with an external power supply, connected to the programmer, and... the micro wasn't detected.

Ok, troubleshooting time. The first step is to check voltages, I measured the VDD on one of the bypass caps and it was 3V, power is good. Next, I measured the VDD on the programming connector and it was 1.6V. The only thing in series with the connector is a 100 Ohm resistor, maybe I connected something wrong and the programmer is loading it down? Disconnected the programmer and measured again, no change, still 1.6V. Ok, this doesn't make any sense, there's a voltage drop across a resistor without any current. Let me check the resistor value just to make sure, switch the meter into resistance mode, take a measure and... 10 MOhm.

Ok, so what's happening is that what was supposed to be a 100 Ohm resistor is instead 10 MOhm, now it makes more sense, when I measure the voltage the meter is loading down the circuit. Next question is, how has this happened, I double-checked the BOM before placing the order, this can't be my fault, right? Let me check on the schematic and... no It's my fault, I had specified the wrong part number for all the 100 Ohm resistors.

Now that I know what's wrong, it's time to fix it. The only problem is that I don't have a stock of 100 Ohm 0603 resistors, luckily I have salvaged plenty of boards, time to disorder some resistors.

Does anybody need 10 MOhm resistors?

I've replaced all the resistors, let's try again. This time I connected the programmer, the micro was detected correctly and I was able to program it.

This is my current setup. In the middle is my new board connected to the display. the STM32 Discovery on the right is used as a programmer, the USB to Serial converter is used to send commands to the micro. Power comes from 6 NiMH AA cells.


Ahron Wayne wrote 04/12/2021 at 03:01 point

You got that already assembled? Can I ask how much that was?

Don't forget to label each resistor with its own sticky note before you put it away :)

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Manuel Tosone wrote 04/12/2021 at 11:19 point

€59.40 for the PCBs and assembling plus €27.42 for shipping, and I've got five of them assembled.

Don't worry, I made sure to lose them all so I don't have to label them :)

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