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19.3.9 PRS RX Input
In addition to receiving data on an external pin the LEUART can be configured to receive data directly from a PRS channel by setting
RX_PRS in LEUARTn_INPUT. The PRS channel used can be selected using RX_PRS_SEL in LEUARTn_INPUT. See the PRS chapter
for more details on the PRS block.
For example the output of a comparator could be routed to the LEUART through the PRS to allow for receiving a signal with low peak-
to-peak voltage or a significant DC offset.
19.3.10 DMA Support
The LEUART has full DMA support in energy modes EM0 Active – EM2 Deep Sleep. The DMA controller can write to the transmit buf-
fer using the registers LEUARTn_TXDATA and LEUARTn_TXDATAX, and it can read from receive buffer using the registers
LEUARTn_RXDATA and LEUARTn_RXDATAX. This enables single byte transfers and 9 bit data + control/status bits transfers both to
and from the LEUART. The DMA will start up the HFRCO and run from this when it is waken by the LEUART in EM2. The HFRCO is
disabled once the transaction is done.
A request for the DMA controller to read from the receive buffer can come from one of the following sources:
• Receive buffer full
A write request can come from one of the following sources:
• Transmit buffer and shift register empty. No data to send.
• Transmit buffer empty
In some cases, it may be sensible to temporarily stop DMA access to the LEUART when a parity or framing error has occurred. This is
enabled by setting ERRSDMA in LEUARTn_CTRL. When this bit is set, the DMA controller will not get requests from the receive buffer
if a framing error or parity error is detected in the received byte. The ERRSDMA bit applies only to the RX DMA.
When operating in EM2 Deep Sleep, the DMA controller must be powered up in order to perform the transfer. This is automatically
performed for read operations if RXDMAWU in LEUARTn_CTRL is set and for write operations if TXDMAWU in LEUARTn_CTRL is set.
To make sure the DMA controller still transfers bits to and from the LEUART in low energy modes, these bits must thus be configured
accordingly.
Note:
When RXDMAWU or TXDMAWU is set, the system will not be able to go to EM2 Deep Sleep/EM3 Stop before all related LEUART
DMA requests have been processed. This means that if RXDMAWU is set and the LEUART receives a frame, the system will not be
able to go to EM2 Deep Sleep/EM3 Stop before the frame has been read from the LEUART. In order for the system to go to EM2 during
the last byte transmission, LEUART_CTRL_TXDMAWU must be cleared in the DMA interrupt service routine. This is because TXBL will
be high during that last byte transfer.
Reference Manual
LEUART - Low Energy Universal Asynchronous Receiver/Transmitter
silabs.com
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