SPRUI30H November 2015 – May 2024 DRA745 , DRA746 , DRA750 , DRA756
When the Write FIFO is disabled, transmit DMA requests pass through directly from the McASP to the host/DMA controller. Whether the WFIFO is enabled or disabled, the McASP generates transmit DMA requests as needed; the AFIFO is “invisible” to the McASP. When the Write FIFO is enabled, transmit DMA requests from the McASP are sent to the AFIFO, which in turn generates transmit DMA requests to the host/DMA controller. If the Write FIFO is enabled, upon a transmit DMA request from the McASP, the WFIFO writes WNUMDMA 32-bit words to the McASP if and when there are at least WNUMDMA words in the Write FIFO. If there are not, the WFIFO waits until this condition has been satisfied. At that point, it writes WNUMDMA words to the McASP. (See description for WFIFOCTL[7:0] WNUMDMA.) If the host CPU writes to the Write FIFO, independent of a transmit DMA request, the WFIFO will accept host writes until full. After this point, excess data will be discarded. Note that when the WFIFO is first enabled, it will immediately issue a transmit DMA request to the host. This is because it begins in an empty state, and is therefore ready to accept data.