
Signal Description
MOTOROLA
MC68302 USER’S MANUAL
5-9
ters. If DTACK is generated internally, then it is an output. It is an input when the IMP ac-
cesses an external device not within the range of the chip-select logic or when
programmed to be generated externally.
RMC/IOUT1—Read-Modify-Write Cycle Indication/Interrupt Output 1
This signal functions as RMC in normal operation. RMC is an output signal that is asserted
when a read-modify-write cycle is executed. It indicates that the cycle is indivisible.
When the M68000 core is disabled, this pin operates as IOUT1. IOUT2–IOUT0 provide
the interrupt request output signals from the IMP interrupt controller to an external CPU
when the M68000 core is disabled.
IAC—Internal Access
This output indicates that the current bus cycle accesses an on-chip location. This in-
cludes the on-chip 4K byte block of internal RAM and registers (both real and reserved
locations), and the system configuration registers ($0F0–$0FF). The above-mentioned
bus cycle may originate from the M68000 core, the IDMA, or an external bus master. Note
that, if the SDMA accesses the internal dual-port RAM, it does so without arbitration on
the M68000 bus; therefore, the IAC pin is not asserted in this case. The timing of IAC is
identical to that of the CS3–CS0 pins.
IAC can be used to disable an external address/data buffer when the on-chip dual-port
RAM and registers are accessed, thus preventing bus contention. Such a buffer is option-
al and is only required in larger systems. An external address/data buffer with its output
enable (E) and direction control (dir) may be placed between the two bus segments as
shown in Figure 5-7. The IAC signal saves the propagation delay and logic required to OR
all the various system chip-select lines together to determine when to enable the external
buffers.
Figure 5-7. External Address/Data Buffer
IMP
OTHER MASTER/SLAVE
OTHER MASTER/SLAVE
OTHER SLAVE
ROM
RAM
IAC
E
DIR
R/W
BUFFERS