Unique serial numbers for picoprobe

[newbrain]

This PR (hinted at in the comments for #5) makes the USB serial number for each Picoprobe unique.
It will come handy for multi probe support in pyocd.

Code is very simple, a direct conversion of the flash unique ID to a C string.

As far as I can see, there's no need to bracket the call to pico_get_unique_board_id(); with disable/enable interrupt, as they seem not to be enabled (yet) when usb_serial_init() is called.

[lurch]

Or you could just use sprintf ?

[newbrain]

With a cast to uint64_t?
Yes, type punning would in this case be 100% safe (and even guaranteed by the standard), and printf is already brought in so that won't make much of a difference.
But, it will display the flash ID in reverse with respect to the "wire order". Not that it makes that much of a difference, I just find it weird.

Otherwise, it would be sprintf in a loop on the id[] array (quite an expensive thing) or a monster call (ugly), and not really any simpler in both cases - as you can see it's four simple lines.

[lurch]

sprintf in a loop on the id[] array (quite an expensive thing)

With the Pico running at 133MHz, I guess I was just being lazy 😉

[lurch]

there's no need to bracket the call to pico_get_unique_board_id(); with disable/enable interrupt

I believe fetching the flash-chip UID is only done once at startup? So any calls to pico_get_unique_board_id() just return the already-read value.

[newbrain]

I believe fetching the flash-chip UID is only done once at startup? So any calls to pico_get_unique_board_id() just return the already-read value.

Yes you are right! I was thinking of the underlying call.