Unify SWD header on TR-9 with HMI

[tkuester]

As of 9ac1f05, the HMI board and the TR-9 share a different header layout for the SWD interface. Per a conversation with @sp5wwp, it would be nice to unify the headers to a common standard.

Some considerations I think are important:

• Having the same ordering, and number of pins. If pin 1 (square pad) is GND on one board, it should be the same on the other
• Having a common interface with common STM32 programmers. (Regrettably, I am new to the STM32 ecosystem, I'm not sure what's common.) SP5WWP indicated that the 1.27mm pin headers are frustrating to work with
• A silk screen indicating to the user which pin does what
• Consideration for accessibility. It would be nice to be able to access the headers without having to entirely disassemble the module!

[sp5wwp]

• Pin 1 can be GND on both connectors. Both should have the same ordering.
• SWD uses SWCLK, SWDIO, /RST (and GND) signals. I'm not sure if there's any "common connector".
• Info on the silk screen is OK if one can manage to put it there :-)
• We can use anything as the connector. It's worth noticing that HMI board won't receive frequent firmware updates.

[tkuester]

I have a proposal for the SWD connector.

Programmers like this are relatively cheap and easy to come by. (~$6 USD) Do you think we could use the header for this?

Alternatively, I have an STM32 Nucleo-64 development board on my desk, and the pin order is:

1 - VDD_TARGET 2 - SWCLK 3 - GND 4 - SWDIO 5 - nRST

I'm still new to the STM32 platform, I'm just looking for "standards" that are pre-existing.

[sp5wwp]

We can use that mapping. Also: it is not necessary to use VDD_TARGET line.