Previously, the machine.UART0 object had two meanings:
- it was the first UART on the chip
- it was the default output for println
These two meanings conflict, and resulted in workarounds like:
- Defining UART0 to refer to the USB-CDC interface (atsamd21,
atsamd51, nrf52840), even though that clearly isn't an UART.
- Defining NRF_UART0 to avoid a conflict with UART0 (which was
redefined as a USB-CDC interface).
- Defining aliases like UART0 = UART1, which refer to the same
hardware peripheral (stm32).
This commit changes this to use a new machine.Serial object for the
default serial port. It might refer to the first or second UART
depending on the board, or even to the USB-CDC interface. Also, UART0
now really refers to the first UART on the chip, no longer to a USB-CDC
interface.
The changes in the runtime package are all just search+replace. The
changes in the machine package are a mixture of search+replace and
manual modifications.
This commit does not affect binary size, in fact it doesn't affect the
resulting binary at all.
Instead of trying to modify periperhals directly, external functions are
called. For example, __tinygo_gpio_set sets a GPIO pin to a specified
value (high or low). It is expected that binaries made this way will be
linked with some extra libraries that implement support for these
functions.
One particularly interesting case is this experimental board simulator:
https://github.com/aykevl/tinygo-play
Compiling code to WebAssembly with the correct build tag for a board
will enable this board to be simulated in the browser.
Atmel/Microchip based SAMD boards are not currently supported, because
their I2C/SPI support is somewhat uncommon and harder to support in the
machine API. They may require a modification to the machine API for
proper support.
