Block Transfer Termination - Motorola MC68302 User Manual

System Integration Block (SIB)
DREQ input to the IDMA is level-sensitive and is sampled at certain points to determine
when a valid request is asserted by the device. The device requests service by asserting
DREQ and leaving it asserted. In response, the IDMA arbitrates for the system bus and
begins to perform an operand transfer. During each access to the device, the IDMA will
assert DACK to indicate to the device that a request is being serviced. If DREQ remains
asserted when the IDMA completes the peripheral cycle (the cycle during which DACK is
asserted by the IDMA) one setup time (see specification. 80) before the S5 falling edge
(i.e., before or with DTACK), then a valid request for another operand transfer is recog-
nized, and the IDMA will service the next request immediately. If DREQ is negated one
setup time (see specification 80) before the S5 falling edge, a new request will not be rec-
ognized, and the IDMA will relinquish the bus.
If 8 to 16 bit packing occurs, then the DREQ is sampled during
the last 8-bit cycle.
External Cycle Steal
For external devices that generate a pulsed signal for each operand to be transferred, the
external cycle steal mode uses DREQ as a falling edge-sensitive input. The IDMA will re-
spond to cycle-steal requests in the same manner as for all other requests. However, if
subsequent DREQ pulses are generated before DACK is asserted in response to each
request, they will be ignored. If DREQ is asserted after the IDMA asserts DACK for the
previous request but one setup time (see specification 80) before the S5 falling edge, then
the new request will be serviced before the bus is relinquished. If a new request has not
been generated by one setup time (see specification 80) before the S5 falling edge, the
bus will be released to the next bus master.

3.1.4.5 Block Transfer Termination

The user may stop the channel by clearing STR. Additionally, the channel operation can be
terminated for any of the following reasons: transfer count exhausted, external device termi-
nation, or error termination. This is independent of how requests are generated to the IDMA.
Transfer Count Exhausted
When the channel begins an operand transfer, if the current value of the BCR is one or
two (according to the operand size in the CMR), DONE is asserted during the last bus cy-
cle to the device to indicate that the channel operation will be terminated when the current
operand transfer has successfully completed. In the memory to memory case, DONE is
asserted during the last access to memory (source or destination) as defined by the ECO
bit. When the operand transfer has completed and the BCR has been decremented to ze-
ro, the channel operation is terminated, STR is cleared, and an interrupt is generated if
INTN is set. The SAPR and/or DAPR are also incremented in the normal fashion.
If the channel is started with BCR value set to zero, the channel
will transfer 64K bytes.
3-12
NOTE:
NOTE
MC68302 USER'S MANUAL MOTOROLA