DMA Interface
MOTOROLA
MC68322 USER’S MANUAL
8-7
Support for byte-sized transfers is handled automatically by the DMA interface through data
packing and it requires no external logic. Byte-sized data received from the core or PPI are
packed into words prior to requesting a DRAM access. Data sent to the core as bytes are
read in as words from DRAM and unpacked to bytes for transfer. All data transfers to and
from memory are in big endian format, thus assuring compatibility with the processor’s data
organization in memory.
8.6.1 PDMA Transfers
Handshaking for the PDMA channel is transparent and handled internally between the PPI
and the PDMA channel. A typical transfer cycle for the PPI is described in Section 9 Parallel
Port Interface.
8.6.2 GDMA MC68322 Bus Read and Write Cycles
GDMA handshaking uses the DREQ and DACK signals. DREQ is an input signal that has a
programmable level or edge sensitivity with polarity control. The default configuration is
active low level. DACK is an output signal that is used by the external DMA device to
acknowledge that a GDMA cycle is in progress. The polarity for the DACK output is fixed to
provide an active low output.
A GDMA cycle is requested when DREQ is asserted by the external DMA device, such as
PROM or I/O. After the DMA interface internally synchronizes DREQ, it arbitrates for the
MC68322 bus and, when granted, asserts DACK. After DACK is asserted, the internal bus
interface unit (BIU) performs the GDMA cycle defined by the GDMA configuration register.
After the cycle completes, DACK is negated and the DMA cycle terminates.
During a GDMA bus cycle, CS
× can be disabled. This option supports external DMA devices
that require CS
× to be inactive during a DMA operation. The DS bit in the GDMA
configuration register controls the operation of CS
× during a DMA-generated MC68322 bus
cycle.
A GDMA MC68322 bus write cycle occurs when the GDMA channel is configured to transfer
from DRAM to an external DMA device. A word-sized GDMA write cycle asserts the WRU
and WRL signals and a byte-sized GDMA write cycle asserts WRL and negates WRU.
Figure 8-5 illustrates the typical timing for a fast DMA read or write cycle. The bus cycle
timing shown is obtained using minimum values for all timing parameters.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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