UART TX/RX label is misleading on schematic

[microbit-mark]

The schematic labels for the target diagram identify the UART pins as:

P0.06     UART_INTERNAL_RX 
P1.08     UART_INTERNAL_TX

As identified in microbit-foundation/dev-docs#353 and microbit-foundation/dev-docs#282 this is the opposite way around.

A note on the interface diagram is designed to clarify the direction of flow, but is also incorrect

but this is still causing confusion for people.

Suggest we correct the note and replicate/move it to clarify the flow on the target diagram.

Further info:

• serial constructor signature is MicroBitSerial(Pin& tx, Pin& rx, …)
  • https://github.com/lancaster-university/codal-microbit-v2/blob/111e4cf829fdfea25e301375643d1bad71fb7070/inc/compat/MicroBitSerial.h#L60
• uBit object instantiated with these arguments -> serial(io.usbTx, io.usbRx, …)
  • https://github.com/lancaster-university/codal-microbit-v2/blob/8d17105316628b0d69f0d0b123d13c5f13f7743a/model/MicroBit.cpp#L67
• io.usbTx -> MICROBIT_PIN_UART_TX
  • https://github.com/lancaster-university/codal-microbit-v2/blob/e7236cd69c01a5e803fe3697fd2ef994808794e9/model/MicroBitIO.cpp#L81
• MICROBIT_PIN_UART_TX -> P0_06
  • https://github.com/lancaster-university/codal-microbit-v2/blob/e7236cd69c01a5e803fe3697fd2ef994808794e9/model/MicroBitIO.h#L282

[paul-jewell]

Ha ha! I came here to raise exactly this issue! I have just spent two days debugging my UART program, and came to the same conclusion as above! I wish I had read this before hand.

I really think the schematic should be fixed, or at least clarified.

[paul-jewell]

One additional thought on this - the majority of users of the board don't do anything with the interface MCU, but instead work on programming the target. With this in mind, it would be clearer to reference the signals from the target perspective rather than from the interface MCU perspective. Developers working on the latter are much more familiar with the hardware, and would not be caught out by this convention.
Looking at the schematic again (version V2.2.1_nRF52820), it does indeed seem that the labelling by the interface MCU indicates that the signals are relative to the target MCU - e.g. "UART_INT_TX is serial data from nRF52833 to interface MCU". So the signal directions are correct on the target MCU and the interface MCU, the comments on the interface MCU are in the right place to clarify the signal reference - the only issue is the pin references on the target MCU. In my opinion, if these are swapped, this issue will be closed.