Polling mode will cause failures with the mass-erase command due to USB
timeouts, because the USB IRQs are not being serviced. Swiching from
polling to IRQ mode fixes this because the USB IRQs can be serviced between
page erases.
Note that when the flash is being programmed or erased the MCU is halted
and cannot respond to USB IRQs, because mboot runs from flash, as opposed
to the built-in bootloader which is in system ROM. But the maximum delay
in responding to an IRQ is the time taken to erase a single page, about
100ms for large pages, and that is short enough that the USB does not
timeout on the host side.
Recent tests have shown that in the current mboot code IRQ mode is pretty
much the same speed as polling mode (within timing error), code size is
slightly reduced in IRQ mode, and IRQ mode idles at about half of the power
consumption as polling mode.
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