September 17, 2016
By my count, there are 30 GPIO pins you are free to use. 32 if you avoid using USB. But you say, "hey, there are three 16 bit GPIO gadgets, that should be 48 pins!". Well, a bunch (PC0 to PC12) just never made it to the outside world in the 48 pin package chip used on this board. That gets the count down to 35.
Some notes about "missing pins" (that are actually used for other things).
PA13 is SWDIO PA14 is SWCLK PB2 is BOOT1 PA11 is USB PA12 is USBThis leaves nothing unexplained in my book.
The pins along the sides:
Gnd Gnd 3.3 Reset B11 B10 B1 B0 A7 A6 A5 A4 A3 A2 A1 A0 C15 C14 C13 V-battery (goes to the chip, see data sheet)
B12 B13 B14 B15 A8 A9 A10 A11 A12 A15 B3 B4 B5 B6 B7 B8 B9 5 volt Gnd 3.3
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