On i.MX the SysTick IRQ cannot wake the CPU from a WFI so the CPU was
blocked on WFI waiting for USB data in mp_hal_stdin_rx_chr() even though it
had already arrived (because it may arrive just after calling the check
tud_cdc_available()). This commit fixes this problem by using SEV/WFE to
indicate that there has been a USB event.
The mp_hal_stdout_tx_strn() function is also fixed so that it doesn't
overflow the USB buffers.
Signed-off-by: Damien George <damien@micropython.org>
This is an extremely minimal port to the NXP i.MX RT, in the style of the
SAMD port It's largely based on the TinyUSB mimxrt implementation, using
the NXP SDK. It currently supports the Teensy 4.0 board with a REPL over
the USB-VCP interface.
This commit also adds the NXP SDK submodule (also from TinyUSB) to
lib/nxp_driver.
Note: if you already have the tinyusb submodule initialized recursively you
will need to run the following as the tinyusb sub-submodules have been
rearranged (upstream):
git submodule deinit lib/tinyusb
rm -rf .git/modules/lib/tinyusb
git submodule update --init lib/tinyusb
