Freescale Semiconductor ColdFire MCF52210 - Manual (Page 288)

DMA Controller Module
MCF52211 ColdFire® Integrated Microcontroller Reference Manual, Rev. 2
17-14 Freescale Semiconductor
As soon as the channel has been initialized, it is started by writing a one to DCRn[START] or a peripheral
DMA request, depending on the status of DCRn[EEXT]. Programming the channel for internal requests
causes the channel to request the bus and start transferring data immediately. If the channel is programmed
for external request, a peripheral DMA request must be asserted before the channel requests the bus.
Changes to DCRn are effective immediately while the channel is active. To avoid problems with changing
a DMA channel setup, write a one to DSRn[DONE] to stop the DMA channel.
17.4.4 Data Transfer
This section describes auto-alignment and bandwidth control for DMA transfers.
17.4.4.1 Auto-Alignment
Auto-alignment allows block transfers to occur at the optimal size based on the address, byte count, and
programmed size. To use this feature, DCRn[AA] must be set. The source is auto-aligned if DCRn[SSIZE]
indicates a transfer size larger than DCRn[DSIZE]. Source alignment takes precedence over the
destination when the source and destination sizes are equal. Otherwise, the destination is auto-aligned. The
address register chosen for alignment increments regardless of the increment value. Configuration error
checking is performed on registers not chosen for alignment.
If BCRn is greater than 16, the address determines transfer size. Bytes, words, or longwords are transferred
until the address is aligned to the programmed size boundary, at which time accesses begin using the
programmed size.
If BCRn is less than 16 at the start of a transfer, the number of bytes remaining dictates transfer size. For
example, AA equals 1, SARn equals 0x0001, BCRn equals 0x00F0, SSIZE equals 00 (longword), and
DSIZE equals 01 (byte). Because SSIZE > DSIZE, the source is auto-aligned. Error checking is performed
on destination registers. The access sequence is as follows:
1. Read byte from 0x0001—write 1 byte, increment SARn.
2. Read word from 0x0002—write 2 bytes, increment SARn.
3. Read longword from 0x0004—write 4 bytes, increment SARn.
4. Repeat longwords until SARn = 0x00F0.
5. Read byte from 0x00F0—write byte, increment SARn.
If DSIZE is another size, data writes are optimized to write the largest size allowed based on the address,
but not exceeding the configured size.
17.4.4.2 Bandwidth Control
Bandwidth control makes it possible to force the DMA off the bus to allow access to another device.
DCRn[BWC] provides seven levels of block transfer sizes. If the BCRn decrements to a multiple of the
decode of the BWC, the DMA bus request negates until the bus cycle terminates. If a request is pending,
the arbiter may then pass bus mastership to another device. If auto-alignment is enabled,
DCRn[AA] equals 1, the BCRn may skip over the programmed boundary, in which case, the DMA bus
request is not negated.