This is a hack to free up TIM3 so that it can be used by the user.
Instead we use the PVD irq to call the USB VCP polling function, and
trigger it from SysTick (so SysTick itself does not do any processing).
The feature is enabled for pyboard lite only, since it lacks timers.
The STMCube examples define both USE_USB_HS and USE_USB_HS_IN_FS when they
use the HS in FS mode.
The STM32F401 doesn't have a USB_HS at all, so the USB_OTG_HS instance
doesn't even exist.
uwTick can only change in the SysTick IRQ so this IRQ function does not
need to take special care with this variable. It's important to make
this IRQ function as efficient as possible.
Using SysTick to do the counting and dispatch of the flash storage idle
handler is more efficient than requiring a dedicated hardware timer.
No new counter is needed, just the existing uwTick variable. The
processing is not actually done in the SysTick IRQ, it is deferred to
the flash IRQ (which runs at lower priority).
Turning on each DMA block increases the current consumption
by about 8 mA. This code adds an idle timer for each DMA
block and turns off the clocks when no streams are in use
for 128 msec. Having a small timeout allows for improved
performance when back-to-back transfers are being performed.
The 128 msec is basically a guess.
py/mphal.h contains declarations for generic mp_hal_XXX functions, such
as stdio and delay/ticks, which ports should provide definitions for. A
port will also provide mphalport.h with further HAL declarations.
