Typewriter User Manual

6- 4 MC68340 USER’S MANUAL MOTOROLA
6.2 DMA MODULE SIGNAL DEFINITIONS
This section contains a brief description of the DMA module signals used to provide
handshake control for either a source or destination external device.
NOTE
The terms
assertion and negation are used throughout this
section to avoid confusion when dealing with a mixture of
active-low and active-high signals. The term
assert or assertion
indicates that a signal is active or true, independent of the level
represented by a high or low voltage. The term
negate or
negation indicates that a signal is inactive or false.
6.2.1 DMA Request (DREQ≈)
This active-low input is asserted by a peripheral device to request an operand transfer
between that peripheral and memory. The assertion of
DREQ≈ starts the DMA process.
The assertion level in external burst mode is level sensitive; in external cycle steal mode,
it is falling-edge sensitive.
6.2.2 DMA Acknowledge (DACK≈)
This active-low output is asserted by the DMA to signal to a peripheral that an operand is
being transferred in response to a previous transfer request.
6.2.3 DMA Done (DONE≈)
This active-low bidirectional signal is asserted by the DMA or a peripheral device during
any DMA bus cycle to indicate that the last data transfer is being performed.
DONE≈ is an
active input in any mode. As an output,
DONE≈ is only active in external request mode. An
external pullup resistor is required even if operating only in the internal request mode.
6.3 TRANSFER REQUEST GENERATION
The DMA channel supports two types of request generation methods: internal and
external. Internally generated requests can be programmed to limit the amount of bus
utilization. Externally generated requests can be either burst mode or cycle steal mode.
The request generation method used for the channel is programmed by the channel
control register (CCR) in the REQ field.
6.3.1 Internal Request Generation
Internal requests are accessed in two clocks by the intermodule bus (IMB). The channel is
started as soon as the STR bit in the CCR is set. The channel immediately requests the
bus and begins transferring data. Only internal requests can limit the amount of bus
utilization. The percentage of the bandwidth that the DMA channel can use during a
transfer can be selected by the CCR BB field.
Frees
cale Semiconductor,
I
Freescale Semiconductor, Inc.
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