DIYBMS modules + external clock issues

I’ve been doing a lot of testing since I first reported here. The controller board appears to work fine. When connecting RX to TX, the led blinks blue and I see the ‘ignored requests’ counter go up.

For the modules, it looks like there’s a problem with the external clock. When I flash the default image, then the module fail immediately and I can’t get them to reprogram even. Only when I reset the fuses (using high voltage) to use the internal clock I can reprogram them again. I’ve installed platformio and uploaded the v4.4 firmware but using default internal clock and then the module does communicate with the controller board. However, comms is very unreliable and fails most of the time. It’s unusable.

I’m not sure what next step to take… I desoldered an oscillator and tested it using a signal generator, and it does amplify 8Mhz just fine. So somehow the attiny841 isn’t working well with the oscillator. I’ve also ordered a new set of attiny’s thinking maybe I got a bad batch, but replacing them doesn’t help either.

Any suggestions are welcome.

Hey Tom,
I’m sorry for hijacking your post. All being well you get some assistance shortly.


Hi @Tom_Deckers, I missed the message you posted.

The problem you have seems very unusual. There are lots of these modules out there and this is the first I’ve heard of with clock issues when using the external osc.

Typically, as long as the fuses are set properly, then that part of the system works fine.

Did you swap any parts on the boards due to chip/part shortages?

Thanks @stuart for picking up this thread. I checked the osc and capacitors around it… those are the same as your blueprint. Turns out I did change D2 with LCSC C113974, could that have any effect on the overall behaviour? Can I just remove it to just test?

Thats a TVS diode, shouldn’t have any effect on the ATTINY. Safe to remove if you want to try.