Allocation of PIOs to functions
===============================

Test logic
----------
-       out TX_ON               PC13

u-blox NEO-M8N GPS module connection
------------------------------------
Yellow  out  UART TX to GPS RX  PD8
Orange  in   UART RX to GPS TX  PD9

Debug USART
-----------
-       out  UART TX to PC RX   PA9
-       in   UART RX to PC TX   PA10

For the little test board with the 3 LEDs and switches
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Blue    in   QRP_n              PC1
Violet  out  LED red            PC2
Grey    in   1750               PC4
White   out  LED yel            PC5
Black   -    GND                GND
Brown   in   RX_n               PC6
Red     in   U_n                PC8
Orange  out  LED grn            PC9
Green   in   D_n                PC11

I2C to Audio Codec
------------------
-       i/o  SDA                PB9
-       i/o  SCL                PB6