Jon's Place

Sunday, February 2, 2014

MicroPython Boards, Crystal Trouble, Flashing LEDs

So, its a long and sordid tale. I got my boards back on Friday:

They sure look pretty. I soldered up the first board, and found out that I had switched the polarity of my battery connector between the Teensy and this board, so I let out a bunch of magic smoke when I first plugged it in. Fortunately, it was just the 5 volt regulator that smoked, and not the ARM chip, so I replaced the regulator and fixed my battery plug. I hooked everything back up, and...

Nothing. No response over USB. I did a lot of troubleshooting, and determined that it was probably an oscillator issue.

That picture shows the BOOT0 line (third row), the RESET line (second row), and the oscillator (top row) hooked up to my logic analyzer. So you pull BOOT0 high, and then, while its high, pull RESET low. At that point, the chip is supposed to start up the external oscillator, which clearly isn't happening.

My brother suggested I could use one of the serial interfaces to the bootloader, and he went so far as to put together a blinking LED "Hello World" style application I could flash, along with the serial bootloader programming code from the Espruino project. I re-purposed my IMU port (which uses I2C normally, but the SCL/SDA lines are also Tx/Rx from USART3 on the chip). I removed the I2C pullup resistors, removed the crystal, grounded the USB D+ and D- lines, and plugged in an FT232 USB to TTL converter to my Ubuntu laptop. I was able to program the chip successfully using that setup, so now I know the other issue is a crystal issue, and not something else fundamental wrong with the ARM chip or my board.

So, I'm not sure where to go from here. The math says I should be using 6 pF capacitors with the crystal, but something clearly isn't working.

I'm going to code up an 8 MHz signal generator using my Teensy 3.1 board, and see if I can get USB working with an external clock source like that. If that works, then I need to try and figure out why this crystal isn't working.


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