The first step of a DMA transmission is storing bytes in the transmit buffer and configuring EasyDMA. This is achieved by writing the initial address pointer to TXD.PTR, and the number of bytes to transmit from the RAM buffer to TXD.MAXCNT. The UARTE transmission is started by triggering the STARTTX task.
After each byte has been sent over the TXD line, a TXDRDY event will be generated.
When all bytes have been transmitted, the transmission will automatically end and the ENDTX event will be generated.
A UARTE transmission sequence is stopped by triggering the STOPTX task. A TXSTOPPED event will be generated when the UARTE transmitter has stopped.
If the ENDTX event has not already been generated when the UARTE transmitter has come to a stop, UARTE will generate the ENDTX event explicitly even though all bytes specified in the TXD.MAXCNT register have not been transmitted.
If flow control is enabled through the HWFC field in the CONFIG register, a transmission will be automatically suspended when CTS is deactivated and resumed when CTS is activated again, as shown in the following figure. A byte that is in transmission when CTS is deactivated will be fully transmitted before the transmission is suspended.
The UARTE transmitter is in its lowest activity level consuming the least amount of energy when it is stopped. That is, before it is started via STARTTX or after it has been stopped via STOPTX and the TXSTOPPED event has been generated. See POWER — Power control for more information about power modes.