So that the user can explicitly deactivate UART(0) if needed. See
issue #4314.
This introduces some risk to "brick" the device, if the user disables the
REPL without providing an alternative REPL (eg WebREPL), or any way to
reenable it. In such a case the device needs to be erased and
reprogrammed. This seems unavoidable, given the desire to have the option
to use the UART for something other than the REPL.
This patch makes it so that UART(0) can by dynamically attached to and
detached from the REPL by using the uos.dupterm function. Since WebREPL
uses dupterm slot 0 the UART uses dupterm slot 1 (a slot which is newly
introduced by this patch). UART(0) must now be attached manually in
boot.py (or otherwise) and inisetup.py is changed to provide code to do
this. For example, to attach use:
import uos, machine
uart = machine.UART(0, 115200)
uos.dupterm(uart, 1)
and to detach use:
uos.dupterm(None, 1)
When attached, all incoming chars on UART(0) go straight to stdin so
uart.read() will always return None. Use sys.stdin.read() if it's needed
to read characters from the UART(0) while it's also used for the REPL (or
detach, read, then reattach). When detached the UART(0) can be used for
other purposes.
If there are no objects in any of the dupterm slots when the REPL is
started (on hard or soft reset) then UART(0) is automatically attached.
Without this, the only way to recover a board without a REPL would be to
completely erase and reflash (which would install the default boot.py which
attaches the REPL).
This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.
