there return of a read operation that times out with no data received
is inconsistent:
```
Adafruit CircuitPython 7.3.0-beta.1-31-g73f6b4867-dirty on 2022-04-30; Adafruit Feather RP2040 with rp2040
>>>
>>> import board, busio
>>> print(board.UART().read(5))
None
Adafruit CircuitPython 6.3.0 on 2021-06-01; FeatherS2 with ESP32S2
>>> import board,busio
>>> print(board.UART().read(5))
None
Adafruit CircuitPython 7.3.0-beta.1 on 2022-04-07; Adafruit Feather STM32F405 Express with STM32F405RG
>>> import board, busio
>>> print(board.UART().read(5))
None
Adafruit CircuitPython 7.3.0-beta.1-31-g73f6b4867-dirty on 2022-04-28; Teensy 4.1 with IMXRT1062DVJ6A
>>> import board, busio
>>> print(board.UART().read(5))
b''
```
Since I have a PR on this file anyway, I thought I would put in the change to make it consistent
with the other 3 board types I tried. Can not say about any of the others.
The existing code was setup that allowed you to specify an RTS
pin to be used as an RS485 direction pin, however there are no
RTS pins that are exposed on any of the Teensy 4.x boards.
Instead Arduino code base allowed you to specify any GPIO pin to
work instead. So I added the code in to facilitate this.
In addition the alternative code to wrap your own GPIO pin set high and low
around call(s) to uart.write() will not currently work, unless maybe you
fudge it and add your own delays as the write will return after the last
byte was pushed onto the UART’s hardware FIFO queue and as such if you
then immediately set the IO pin low, it will corrupt your output stream.
The code I added detects that you are setup to use the RS485 pin and
before it returns will wait for the UART’s Transfer complete status flag
to be set.
Now a 'once' and a 'loop' buffer can be specified.
'once' is useful for things like writing a neopixel strip in the background,
if you can guarantee the buffer contents are stable until the write is complete.
'loop' is useful for periodic things, like pwm & servos.
both together are useful for some special cases of pwm/servo, where a
transitional waveform needs to be played for one repetition and then
a new waveform needs to be played after that.
The API is renamed to reflect that it's a more generic 'background'
operation.
The old formulation
* wouldn't work if there were ID3 tags at the end
* would choose whether to background-refill the inbuf
based on a check before skipping to the next sync word, which
could be incorrect.
I think it was aspect "B" that ended up triggering the erroneous EOF
problem fixed in the prior commit. This would depend on specific data
sizes and offsets occuring in the file such that a read would be
scheduled but then the buffer would be filled and left 100% full by
find_sync_word(). It's just lucky(?) that a particular person produced
such a file, and/or many files produced by Audacity have those
characteristics.
