Universal Asynchronous Receiver/Transmitter (UART)
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3’b000 4th
Modem Status
CTS, DSR, RI, or DCD
Read the MSR
3’b100 5th
Monitor Baud Status
Rx path monitor done interrupt
Read the REG_MON_BAUD_STS
14.3.6
DMA Flow Control
GDMA is always DMA flow controller for UART Tx, while either GDMA or UART can be DMA flow controller for UART Rx. Only Rx DMA mode is
illustrated here.
When GDMA is DMA flow controller, GDMA sets expected block length before blocks transfer, and terminates block transfer when Rx length is
equal to block length. Rx_dma_req is asserted when Rx size in Rx FIFO isn’t less than burst size, and rx_dma_single is asserted when Rx FIFO
isn’t empty.
When UART is DMA flow controller, UART asserts rx_dma_last to terminate block transfer when UART has received whole packet. The finish
flag is defined that no new Rx character comes in for rx_timeout_thres time after the last Rx character.
Fig 14-7 gives the DMA interface timing diagram on condition that block size isn’t multiple of burst transaction size. T1 and T2 are burst
transfer, T3 and T4 are single transfer. T2 is the last burst transfer of block transfer. After timeout, there are two entries in Rx FIFO which is less
than burst transaction transfer size. Then UART starts two single transfers to finish block transfer.
rx_dma_req
rx_dma_single
rx_dma_last
rx_dma_ack
timeout
T1
T2
T3
T4
Fig 14-7 DMA interface timing diagram
Fig 14-8 gives the DMA interface timing diagram on condition that block size is multiple of burst transaction size. Block transfer is actually done
after last burst transfer of T2. While UART can’t judge the end of block transfer until timeout happens. In T3, UART starts a fake single last
transfer to inform GDMA, on the same time, UART sets DUMMY_FLAG in the MISCR register for software judgement.
rx_dma_req
rx_dma_single
rx_dma_last
rx_dma_ack
timeout
T1
T2
T3
Fig 14-8 DMA interface timing diagram
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2019-05-15 10:08:03