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enter image description hereHi,

I'm designing a custom board with the ATMEGA32U4, with a GPS receiver and LoRa radio and it is powered from a LiPo battery. Since I'll need to make hundreds of this board later on, I'd like to program them over USB. I started out from the Arduino Pro Micro schematic and I use an 8MHz crystal. I'm able to make use of the factory default USB bootloader and upload Arduino sketches using the dfu-programmer in Linux. My first problem was that the blink example didn't really blink at 0.5Hz rate(delay(1000) wasn't 1sec), but much-much slower. No worries, reduce the delay a little bit. However, when I want to communicate with the GPS, the from the transmitted signal, I can see that the baud rate is not correct. I suspect that the clock settings are not correct. I read that these can be changed with the help of fuses, but as far as I know, they cannot be modified using only USB , I need a programmer for that, which I'd like to avoid. (Correct me if I'm wrong here)

So my question is, how do I set up the MCU correctly, so that it runs from the 8MHz external or 8MHz internal clock using only USB? Is it even possible?

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    Off topic: You can measure Vbatt by using it as AREF and measure internal bandgap voltage reference (= switch to the correct adc channel and measure it) Commented Oct 8, 2020 at 14:39
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    BTW, have you selected correct speed for pro micro board? (eg. 3V3/8MHz?). Also by default there is CKDIV8 fuse set, so if you didn't changed anytning in fuses, it'll be even slower than just half speed Commented Oct 8, 2020 at 14:44
  • Thanks for the tip regarding the battery voltage! Yes, I switched to 3v3/8MHz. In the meantime, I found this: avrfreaks.net/forum/clock-select-register-instead-fuse-bits I was able to switch the clock source back to 8MHz and the UART baudrate seems okay now. Commented Oct 8, 2020 at 15:28
  • Yes, it's possible to change prescaler, but you have to configure it in each program you create. Commented Oct 8, 2020 at 15:39
  • I'd still try to set the fuses. Using software to select the correct clock source and prescaler seems to me like a workaround, instead of proper solution. Though I can't give a good reason why. It just feels wrong. Commented Oct 8, 2020 at 15:47

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