This is a start to make a more consistent machine.RTC class across ports.
The stm32 pyb.RTC class at least has the datetime() method which behaves
the same as esp8266 and esp32, and with this patch the ntptime.py script
now works with stm32.
If both FS and HS USB peripherals are enabled for a board then the active
one used for the REPL will now be auto-detected, by checking to see if both
the DP and DM lines are actively pulled low. By default the code falls
back to use MICROPY_HW_USB_MAIN_DEV if nothing can be detected.
When going out of memory-mapped mode to do a control transfer to the QSPI
flash, the MPU settings must be changed to forbid access to the memory
mapped region. And any ongoing transfer (eg memory mapped continuous read)
must be aborted.
The Cortex-M7 CPU will do speculative loads from any memory location that
is not explicitly forbidden. This includes the QSPI memory-mapped region
starting at 0x90000000 and with size 256MiB. Speculative loads to this
QSPI region may 1) interfere with the QSPI peripheral registers (eg the
address register) if the QSPI is not in memory-mapped mode; 2) attempt to
access data outside the configured size of the QSPI flash when it is in
memory-mapped mode. Both of these scenarios will lead to issues with the
QSPI peripheral (eg Cortex bus lock up in scenario 2).
To prevent such speculative loads from interfering with the peripheral the
MPU is configured in this commit to restrict access to the QSPI mapped
region: when not memory mapped the entire region is forbidden; when memory
mapped only accesses to the valid flash size are permitted.
Damien George