MC68336/376
SYSTEM INTEGRATION MODULE
MOTOROLA
USER’S MANUAL
Rev. 15 Oct 2000
5-29
Fast termination cycles use internal handshaking signals generated by the chip-select
logic. To initiate a transfer, the MCU asserts an address and the SIZ[1:0] signals.
When AS, DS, and R/W are valid, a peripheral device either places data on the bus
(read cycle) or latches data from the bus (write cycle). At the appropriate time, chip-
select logic asserts data and size acknowledge signals.
The DSACK option fields in the chip-select option registers determine whether inter-
nally generated DSACK or externally generated DSACK is used. The external DSACK
lines are always active, regardless of the setting of the DSACK field in the chip-select
option registers. Thus, an external DSACK can always terminate a bus cycle. Holding
a DSACK line low will cause all external bus cycles to be three-cycle (zero wait states)
accesses unless the chip-select option register specifies fast accesses.
For fast termination cycles, the fast termination encoding (%1110) must be used.
To use fast termination, an external device must be fast enough to have data ready
within the specified setup time (for example, by the falling edge of S4). Refer to Table When fast termination is in use, DS is asserted during read cycles but not during write
cycles. The STRB field in the chip-select option register used must be programmed
with the address strobe encoding to assert the chip-select signal for a fast termination
write.
5.6.4 CPU Space Cycles
Function code signals FC[2:0] designate which of eight external address spaces is
accessed during a bus cycle. Address space 7 is designated CPU space. CPU space
is used for control information not normally associated with read or write bus cycles.
for more information on codes and encoding.
During a CPU space access, ADDR[19:16] are encoded to reflect the type of access
being made. Figure 5-12 shows the three encodings used by 68300 family microcon-
trollers. These encodings represent breakpoint acknowledge (Type $0) cycles, low
power stop broadcast (Type $3) cycles, and interrupt acknowledge (Type $F) cycles.
Refer to 5.8 Interrupts for information about interrupt acknowledge bus cycles.