machine/atsamd21,atsamd51: send zero length packet when USB transfer …

[deadprogram]

This PR should correct the last issues for atsamd21 and atsamd51 USB CDC by sending a zero length packet when USB transfer is complete on endpoint 0.

This fix is 100% due to awesome research by @sago35 thank you very much!

[aykevl]

I can confirm that this PR works for me (Linux kernel version 5.4) on both the Arduino Nano 33 IoT and the Adafruit Circuit Playground Bluefruit.

Unrelated: I noticed that for the CPB it gives an error:

error: failed to flash /tmp/tinygo446225229/main.uf2: rename /media/ayke/CPLAYBTBOOT/flash.uf2.tmp /media/ayke/CPLAYBTBOOT/flash.uf2: no such file or directory

It still works, but this (somewhat cosmetic) error should probably be fixed eventually.

[sago35]

USB-CDC now works without adding sendZlp() in handleUSBIRQ().
But it's not yet known which changes are affecting them.
I've only fixed the code in SAMD51 (usb.go + machine_atsamd51.go).
Tested with pyportal (samd51) + windows10 only.

The changes are outlined below.

• made to resemble the arduino source (github.com/adafruit/ArduinoCore-samd/cores/arduino/USB/USBCore.cpp)
  • Added usbPackMessage (probably unnecessary)
  • Fix handleStandardSetup().
  • Fix sendDescriptor()
• ClearBits() and then SetBits() from SetBits().
• Return Ack for Start of frame.
• Removed sendZlp() in handleUSBIRQ().

I'm going to organize it tomorrow so that it's only a necessary change.
If possible, I would like you to check the operation on Linux, etc.
@deadprogram

sago35@22b4a22
sago35@7b03ada

[niaow]

Works for me!

[sago35]

Works fine.

[deadprogram]

This branch now appears good to merge.