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tells whether TX is expecting data for the left or right channel. It will be set with TXBL if right data is
expected. The receiver will set RXDATAVRIGHT if there is at least one right element in the buffer, and
RXFULLRIGHT if the buffer is full of right elements.
When using I2S with DMA, separate DMA requests can be used for left and right data by setting
DMASPLIT in USARTn_I2SCTRL.
In both master and slave mode the USART always starts transmitting on the LEFT channel after being
enabled. In master mode, the transmission will stop if TX becomes empty. In that case, TXC is set.
Continuing the transmission in this case will make the data-stream continue where it left off. To make
the USART start on the LEFT channel after going empty, disable and re-enable TX.
17.3.4 PRS-triggered Transmissions
If a transmission must be started on an event with very little delay, the PRS system can be used
to trigger the transmission. The PRS channel to use as a trigger can be selected using TSEL in
USARTn_TRIGCTRL. When a positive edge is detected on this signal, the receiver is enabled if RXTEN
in USARTn_TRIGCTRL is set, and the transmitter is enabled if TXTEN in USARTn_TRIGCTRL is set.
Only one signal input is supported by the USART.
The AUTOTX feature can also be enabled via PRS. If an external SPI device sets a pin high when there is
data to be read from the device, this signal can be routed to the USART through the PRS system and be
used to make the USART clock data out of the external device. If AUTOTXTEN in USARTn_TRIGCTRL
is set, the USART will transmit data whenever the PRS signal selected by TSEL is high given that there
is enough room in the RX buffer for the chosen frame size. Note that if there is no data in the TX buffer
when using AUTOTX, the TX underflow interrupt will be set.
AUTOTXTEN can also be combined with TXTEN to make the USART transmit a command to the
external device prior to clocking out data. To do this, disable TX using the TXDIS command, load the
TX buffer with the command and enable AUTOTXTEN and TXTEN. When the selected PRS input goes
high, the USART will now transmit the loaded command, and then continue clocking out while both the
PRS input is high and there is room in the RX buffer
17.3.5 PRS RX Input
The USART can be configured to receive data directly from a PRS channel by setting RXPRS in
USARTn_INPUT. The PRS channel used is selected using RXPRSSEL in USARTn_INPUT. This way,
for example, a differential RX signal can be input to the ACMP and the output routed via PRS to the
USART.
17.3.6 DMA Support
The USART has full DMA support. The DMA controller can write to the transmit buffer using the
registers USARTn_TXDATA, USARTn_TXDATAX, USARTn_TXDOUBLE and USARTn_TXDOUBLEX,
and it can read from the receive buffer using the registers USARTn_RXDATA, USARTn_RXDATAX,
USARTn_RXDOUBLE and USARTn_RXDOUBLEX. This enables single byte transfers, 9 bit data +
control/status bits, double byte and double byte + control/status transfers both to and from the USART.
A request for the DMA controller to read from the USART receive buffer can come from the following
source:
• Data available in the receive buffer.
• Data available in the receive buffer and data is for the RIGHT I2S channel. Only used in I2S mode.
A write request can come from one of the following sources:
• Transmit buffer and shift register empty. No data to send.
• Transmit buffer has room for more data.
2013-05-08 - Wonder Gecko Family - d0233_Rev0.50
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