Continuous Communication Using Dma - ST STM32F100 Series Reference Manual

Universal synchronous asynchronous receiver transmitter (USART)
23.3.13

Continuous communication using DMA

The USART is capable of continuous communication using the DMA. The DMA requests for
Rx buffer and Tx buffer are generated independently.
Transmission using DMA
DMA mode can be enabled for transmission by setting DMAT bit in the USART_CR3
register. Data is loaded from a SRAM area configured using the DMA peripheral (refer to the
DMA specification) to the USART_DR 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 USART_DR register address in the DMA control register to configure it as the
destination of the transfer. The data are 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 are loaded into the USART_DR 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 bit in the SR register by writing 0 to it.
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 the Stop mode. The software must wait until TC=1. The TC
flag remains cleared during all data transfers and it is set by hardware at the last frame's
end of transmission.
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