Universal synchronous asynchronous receiver transmitter (USART) RM0351
1214/1693 DocID024597 Rev 3
36.5.15 USART continuous communication in DMA mode
The USART is capable of performing continuous communication using the DMA. The DMA
requests for Rx buffer and Tx buffer are generated independently.
Note: Please refer to Section 36.4: USART implementation on page 1180 to determine if the DMA
mode is supported. If DMA is not supported, use the USART as explained in Section 36.5.2:
USART transmitter or Section 36.5.3: USART receiver. To perform continuous
communication, the user can clear the TXE/ RXNE flags In the USARTx_ISR register.
Transmission using DMA
DMA mode can be enabled for transmission by setting DMAT bit in the USARTx_CR3
register. Data is loaded from a SRAM area configured using the DMA peripheral (refer to
Section 10: Direct memory access controller (DMA) on page 295) to the USARTx_TDR
register whenever the TXE bit is set. To map a DMA channel for USART transmission, use
the following procedure (x denotes the channel number):
1. Write the USARTx_TDR register address in the DMA control register to configure it as
the destination of the transfer. The data is moved to this address from memory after
each TXE event.
2. Write the memory address in the DMA control register to configure it as the source of
the transfer. The data is loaded into the USARTx_TDR register from this memory area
after each TXE event.
3. Configure the total number of bytes to be transferred to the DMA control register.
4. Configure the channel priority in the DMA register
5. Configure DMA interrupt generation after half/ full transfer as required by the
application.
6. Clear the TC flag in the USARTx_ISR register by setting the TCCF bit in the
USARTx_ICR register.
7. Activate the channel in the DMA register.
When the number of data transfers programmed in the DMA Controller is reached, the DMA
controller generates an interrupt on the DMA channel interrupt vector.
In transmission mode, once the DMA has written all the data to be transmitted (the TCIF flag
is set in the DMA_ISR register), the TC flag can be monitored to make sure that the USART
communication is complete. This is required to avoid corrupting the last transmission before
disabling the USART or entering Stop mode. Software must wait until TC=1. The TC flag
remains cleared during all data transfers and it is set by hardware at the end of transmission
of the last frame.
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