14.5.4.1 Peripheral to Memory Transfer

XDMAC reads data from the source peripheral and writes to the destination memory location.

Figure 14-4. Peripheral to Memory Transfer Hierarchy

It is a peripheral synchronized transfer, which means the memory transaction is synchronized with the hardware trigger that comes from the corresponding peripheral. It is also possible to use software trigger to initiate data transfer. Peripheral to memory transfer has totally five levels of data transactions. They are Host, Block, Microblock, Burst, and Chunk level transactions. Host, Block, Microblock, and Burst level transactions work exactly the same way as explained earlier in the memory to memory data transfer section. In peripheral to memory data transfer, the burst level transaction is further split into chunk level data transaction to have higher granularity.

XDMAC Chunk and Incomplete Chunk: When a peripheral to memory transfer is activated, the burst level transaction is further split into a number of data chunks. The chunk size is configured in CSIZE field of XDMAC Channel Configuration Register (XDMAC_CCx). The chunk size denotes the number of ‘data’ to be transferred from the corresponding peripheral receive register to memory. In general, the chunk size is set as ‘1 data’ in most of the peripherals (example: - UART, SPI, TWI, etc.), as the maximum size of their receive register is ‘1 data’. In specific scenarios, the chunk size is chosen more than 1 data. For example, the data receive/input registers of AES and HSMCI modules can hold more than ‘1 data’. So, the chunk size can be chosen as '2/4/8/16 data' accordingly. In this case, the larger the chunk size is, the better the performance is. When the amount of data chunks read becomes equal to the memory burst size, the actual data transaction starts (as a memory burst). During ‘peripheral to memory’ transfer, the data chunks are first read and stored into XDMAC's internal FIFO buffer. If their size becomes equal to the memory burst size, the FIFO buffer gets flushed out automatically, which makes ‘memory burst transfer’. When the microblock size is not a multiple of the chunk size, the last chunk being transferred contains the last trailing data.

Note: If the chunk size is chosen as more than '1 data' for peripherals like UART, SPI, TWI, etc., XDMAC reads the same data register (receive/input register) multiple times. As a result, multiple copies of the same data are stored in the memory.