UART/IrDA Functional Description
12-93
UART Devices
12.9.6 FIFO DMA Mode Operation
12.9.6.1
DMA Signaling
There are four modes of DMA operation: DMA mode 0, DMA mode 1, DMA
mode 2, and DMA mode 3. They can be selected as follows.
-
When SCR[0] = 0:
J
Setting FCR[3] to 0 enables DMA mode 0.
J
Setting FCR[3] to 1 enables DMA mode 1.
-
When SCR[0] = 1:SCR[2:1] determine DMA mode 0 to 3 according to
supplementary control register (SCR) description.
So for instance:
-
If no DMA operation is desired, set SCR[0] to 1 and SCR[2:1] to 00 (FCR[3]
is disregarded).
-
If DMA mode 1 is desired, either set SCR[0] to 0 and FCR[3] to 1 or set
SCR[0] to 1 SCR[2:1] to 01 (FCR[3] is disregarded).
If the FIFOs are disabled (FCR[0] = 0), DMA occurs in single character trans-
fers. When DMA mode 0 has been programmed, the signals associated with
DMA operation are not active.
12.9.6.2
DMA Transfers (DMA Mode 1, 2, or 3)
Figure 12–20 shows DMA operations at receive; Figure 12–21 shows DMA
operations at transmit.
Figure 12–20. Receive FIFO DMA Request Generation
Threshold reads
From system DMA
Programmable threshold
Receive FIFO level
Zero byte
time
DMA request
time
DMA request
Active low