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Supported for all SAMD51 devices and SAMD21 with external flash.  For
interrupt events, IRQ_RX and IRQ_TXIDLE are provided.

IRQ_RX is called for every received byte.  This may not be useful for high
data rates, but can be used to build a wrapper class providing an
IRQ_RXIDLE event or to signal just the first byte of a message.

IRQ_TXIDLE is called only when messages are longer than 5 bytes and
triggers when still 5 bytes are due to be sent.

The SAMD hardware does not support implementing IRQ_RXIDLE.
Signed-off-by: robert-hh <robert@hammelrath.com>