Intel PXA255 - Manual (Page 162)

5-12 Intel® PXA255 Processor Developer’s Manual
DMA Controller
5.2.1.1 Using Flow-Through DMA Read Cycles to Service Internal
Peripherals
A flow-through DMA read for an internal peripheral begins when the internal peripheral sends a
request, via the PREQ bus, to a DMAC channel that is running and configured for a flow-through
read. The number of bytes to be transferred is specified with DCMDx[SIZE]. When the request is
the highest priority request, the following process begins:
1. The DMAC sends the memory controller a request to read the number of bytes addressed by
DSADRx[31:0] into a 32-byte staging buffer in the DMAC.
2. The DMAC transfers the data to the I/O device addressed in DTADRx[31:0].
DCMD[WIDTH] specifies the width of the internal peripheral to which the data is transferred.
3. At the end of the transfer, DSADRx is increased by the smaller value of DCMDx[LENGTH]
and DCMD[SIZE]. DCMDx[LENGTH] is decreased by the same value.
For a flow-through DMA read to an internal peripheral, use the following settings for the DMAC
register bits:
• DSADR[SRCADDR] = external memory address
• DTADR[TRGADDR] = internal peripheral’s address
• DCMD[INCSRCADDR] = 1
• DCMD[FLOWSRC] = 0
• DCMD[FLOWTRG] = 1
5.2.1.2 Using Flow-Through DMA Write Cycles to Service Internal
Peripherals
A flow-through DMA write for an internal peripheral begins when the internal peripheral sends a
request, via the PREQ bus, to a DMAC channel that is running and configured for a flow-through
write. The number of bytes to be transferred are specified with DCMDx[SIZE]. When the request
is the highest priority request, the following process begins:
1. The DMAC transfers the required number of bytes from the I/O device addressed by
DSADRx[31:0] to the DMAC write buffer.
2. The DMAC transfers the data to the memory controller via the internal bus. DCMD[WIDTH]
specifies the width of the internal peripheral to which the transfer is being made.
3. At the end of the transfer, DTADRx is increased by the smaller value of DCMDx[LENGTH]
and DCMD[SIZE]. DCMDx[LENGTH] is decreased by the same number.
For a flow-through DMA write to an internal peripheral, use the following settings for the DMAC
register bits:
• DSADR[SRCADDR] = internal peripheral address
• DTADR[TRGADDR] = external memory address
• DCMD[INCTRGADDR] = 1
• DCMD[FLOWSRC] = 1
• DCMD[FLOWTRG] = 0