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Figure D-10 illustrates the activation of external burst mode by using the DREQ signal as a
level-sensitive input to the IDMA. If DREQ is asserted when the IDMA is accessing the pe-
ripheral (indicated by DACK being asserted) as shown in Figure D-10, then the IDMA will
continue servicing the peripheral by performing another sequence of operand transfer cy-
cles. The external burst mode stops when the DREQ signal is deasserted prior to the trailing
edge of S3 in the cycle where DACK is asserted.
READ CYCLE
S0 S1
S2
CLKO
AS
DTACK
DREQ
DACK
NOTE:
DREQ is sampled on the falling edge of clock.
LEGEND:
1
DREQ asserted prior to DTACK = continue burst mode transfer
2
DREQ negated prior to DTACK = relinquish the bus
D.5.7 Internal Interrupt Sequence
An interrupt acknowledge cycle (IACK) occurs when an allowed internal or external interrupt
request is pending and the priority of the interrupt is higher than the current microprocessor
run level. The interrupt acknowledge cycle begins at the conclusion of instruction execution
in state S0. All internal resources, including the IDMA, generate INRQ requests at level 4.
The four registers used in interrupt processing are as follows:
1. The interrupt mask register (IMR) contains the flags that, when set, allow the INRQ
source to initiate service.
2. The interrupt pending register (IPR) contains bits that correspond to the INRQ source
requesting service.
3. The interrupt in-service register (ISR) indicates which internal interrupts are currently
being processed (usually only one at a time).
4. The global interrupt mode register (GIMR) has bits that specify interrupt modes such
as the edge or level of an input that triggers an interrupt.
A level 4 interrupt may be generated by the IDMA upon completion of a data block transfer.
Interrupt processing of IDMA transfers is possible by 1) setting the IDMA interrupt enable
(bit 11) in the IMR and 2) setting one or both interrupt enable (INTN and INTE) bits in the
CMR (see Table D-1). Once in the interrupt handler, four bits in the CSR indicate the reason
for termination of an IDMA data block (see Table D-2).
MOTOROLA
WRITE CYCLE
S3 S4 S5 S6 S7
1
Figure D-10. Burst Mode Cycles
MC68302 USER'S MANUAL
MC68302 Applications
READ CYCLE
WRITE CYCLE
2
D-29
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