Cirrus Logic EP93xx - Manual (Page 426)

10-32 DS785UM1
Copyright 2007 Cirrus Logic
DMA Controller
EP93xx User’s Guide
1
0
1
0
10
of the source and destination addresses to avoid any
problems in the case where software erroneously
programs a byte-aligned address. The SCT bit is used
only when in M2M software-triggered transfer mode.
DoneIntEn: Setting this bit to “1” enables the generation of the DONE
Interrupt which indicates if the transfer completed
successfully.
ENABLE: Setting this bit to 1 enables the channel, clearing this bit
disables the channel. The channel must always be
enabled after writing the Source/Destination Base address
registers and the BCR register. When a channel is
disabled, the external peripheral signals will be placed in
their inactive state.
START: Start Transfer. When this bit is set, the DMA begins M2M
transfer in accordance with the values in the control
registers. START is cleared automatically after one clock
cycle and is always read as a logic 0. This bit, in effect,
provides a “Software-triggered DMA capability”. A channel
must be configured and enabled before setting the START
bit. This bit is not used for external DMA transfers, or for
IDE and SSP transfers. For a double-buffer software
triggered DMA transfer, the START bit need only be set
once, that is, at the very beginning of transfer. It is
sufficient for software to program the ‘other’ buffer
descriptor only, in order to guarantee rollover to the
second buffer when the byte count of the first buffer has
been reached.
BWC: Bandwidth Control. These 4 bits are used to indicate the
number of bytes in a block transfer. When the BCR
register value is within 15 bytes of a multiple of the BWC
value, the DMA releases the bus by negating the AHB bus
request strobe allowing lower priority masters to be
granted control of the bus. BWC = 0000 specifies the
maximum transfer rate: other values specify a transfer rate
limit.
The BWC bits should only be set for software triggered
M2M transfers, where HREQ stays asserted throughout
the transfer. For transfer to/from external devices, HREQ
is released after every transfer, and so bandwidth control
is not needed.
The BWC bits are ignored when in external DMA transfer
mode.