6- 6
MC68341 USER'S MANUAL
MOTOROLA
request, DREQ≈ should be held asserted (bursting) into the following DMA bus cycle to
allow another transfer to occur. The recognized request will immediately be serviced. If
DREQ≈ is negated before DACK≈ is asserted, a new request is not recognized, and the
DMA channel releases ownership of the bus.
6.3.2.2 EXTERNAL CYCLE STEAL MODE. For external devices that generate a pulsed
signal for each operand to be transferred, the cycle steal request mode uses the DREQ≈
signal as a falling-edge-sensitive input. The DREQ≈ pulse generated by the device must be
asserted during two consecutive falling edges of the clock to be recognized as valid.
Therefore, if a peripheral generates it asynchronously, it must be at least two clock periods
long.
The DMA channel responds to cycle steal requests the same as all other requests.
However, if subsequent DREQ≈ pulses are generated before DACK≈ is asserted in
response to each request, they are ignored. If DREQ≈ is asserted after the DMA channel
asserts DACK≈ for the previous request but before DACK≈ is negated, then the new
request is serviced before bus ownership is released. If a new request is not generated by
the time DACK≈ is negated, the bus is released.
6.3.2.3 EXTERNAL REQUEST WITH OTHER MODULES. The DMA controller can be
externally connected to the serial module and used in conjunction with the serial module to
send or receive data. The DMA takes the place of a separate service routine for accessing
or storing data that is sent or received by the serial module. Using the DMA also lowers the
CPU32 overhead required to handle the data transferred by the serial module. Figure 6-4
shows the external connections required for using the DMA with the serial module.
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DMA MODULE
DREQ1
DREQ2
TxRDYA
RxRDYA
SERIAL MODULE
Figure 6-4. DMA External Connections to Serial Module
For serial receive, the DMA reads data from the serial receive buffer register (when the
serial module has filled the buffer on input) and writes data to memory. For serial transmit,
the DMA reads data from memory and writes data to the serial transmit buffer register. Only
dual-address mode can be used with the serial module. The MC68341 on-chip peripherals
do not support single-address transfers.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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