SWRU626 December 2025 CC3501E , CC3551E
In a DMA block read operation (single or multiple), the request signal SDMARREQN is asserted to its active level when a complete block is written in the buffer. The block size transfer is specified in the SD_BLK[10:0] BLEN field.
The SDMARREQN signal is deasserted to its inactive level when the DMA has read one single word from the buffer. Only one request is sent per block; the DMA controller can make a 1-shot read access or several DMA bursts, in which case the DMA controller must manage the number of burst accesses, according to block size BLEN field.
New DMA requests are internally masked if the DMA has not read exactly BLEN bytes and a new complete block is not ready. As DMA accesses are in 32-bit, then the number of DMA read is Integer(BLEN/4)+1.
The receive buffer never overflows. In multiple block transfers for block size above 512 bytes, when the buffer gets full, the SDMMC_CLK clock signal (provided to the card) is momentarily stopped until the DMA or the MCU performs a read access, which reads a complete block in the buffer.
Figure 20-10 provides a summary:
Figure 20-10 DMA
Receive Mode