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Read FIFO Control Register
Rx DMA Req.
Tx DMA Req.
Write FIFO
Read FIFO
Master
Wrtie FIFO Control Register
Rx DMA Req.
Tx DMA Req.
McBSP
Slave
32-Bit
Slave
32
32
Slave
32
32
Config Bus
32-Bit
32-Bit
Master
Master
Host
or DMA
Controller
Config Bus
Config Bus
Architecture
1222
SPRUH82C – April 2013 – Revised September 2016
Copyright © 2013–2016, Texas Instruments Incorporated
Multichannel Buffered Serial Port (McBSP)
25.2.7.6 McBSP Buffer FIFO (BFIFO)
The BFIFO contains two FIFOs: one Read FIFO (RFIFO) and one Write FIFO (WFIFO). To ensure
backward compatibility with existing software, both the Read and Write FIFOs are disabled by default. See
for a high-level block diagram of the BFIFO.
The BFIFO may be enabled/disabled and configured via the Write FIFO control register (WFIFOCTL) and
the Read FIFO control register (RFIFOCTL). Note that if the Read or Write FIFO is to be enabled, it must
be enabled prior to initializing the receive/transmit section of the McBSP (see
for details).
Figure 25-32. McBSP Buffer FIFO (BFIFO) Block Diagram
Note that the McBSP Buffer FIFO (BFIFO) has a different memory-map (see your device-specific data manual) than
the McBSP memory-mapped registers (MMRs); hence, the BFIFO is accessible by way of a different Configuration
Bus.
25.2.7.6.1 BFIFO Data Transmission
When the Write FIFO is disabled, transmit DMA requests pass through directly from the McBSP to the
host/DMA controller. Whether the WFIFO is enabled or disabled, the McBSP generates transmit DMA
requests as needed; the BFIFO is "invisible" to the McBSP.
When the Write FIFO is enabled, transmit DMA requests from the McBSP are sent to the BFIFO, 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 McBSP, the WFIFO writes
WNUMDMA
32-bit words to the McBSP, 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 this point, it writes
WNUMDMA
words to the McBSP.
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.