HardwareNotes.txt

- U2 - STM32F103 read from package (lasered off but still just readable).

- Read out - STM32F103 Device ID - 0x414 - Flash size 512kB
That makes it a STM32F103RET

- On USB power, VCC is 3.45V measured at P3/
* P3 Pinout * Same as standard nucleo board except for pins 4 & 5 are swapped.
1 - VCC
2 - SWCLK
3 - GND
4 - NRST
5 - SWDIO

- 12.0MHz crystal attached as main clock
- LEDs connect to VCC with 100R resistor.
- Switches require pullup, and the switch action grounds them.  They also have a small cap to reduce noise.

* I2C *
Clock speed about 360kHz
I2C EEPROM - 24C08 - Address is 0x5X where the X is the addressing nibble.
EEPROM A2 line is tied low.

I2C Display
Address is 0x3C
Is a ssd1306 - seems to work fine with this library:
https://github.com/afiskon/stm32-ssd1306


For the EEPROM, only the first 128 bytes appear to have anything in them.  This leaves 896 bytes free, starting at the 128 byte offset.