are driven from the system clock, the bus cycle termination is inherently synchronized
with the system clock.
If multiple chip selects are to be used to select the same device that can support fast
termination, and match conditions can occur simultaneously, program the DSACK
field in each associated chip-select option register for fast termination. Alternately, pro-
gram one DSACK field for fast termination and the remaining DSACK fields for exter-
nal termination.
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 are used. For fast termination
cycles, the F-term encoding (%1110) must be used. Refer to 4.8.1 Chip-Select Reg-
isters for information about fast-termination setup.
4
To use fast-termination, an external device must be fast enough to have data ready,
within the specified setup time, by the falling edge of S4. Refer to APPENDIX A ELEC-
TRICAL CHARACTERISTICS for tabular information about fast termination timing.
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.
4.5.4 CPU Space Cycles
Function code signals FC[2:0] designate which of eight external address spaces is ac-
cessed 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.
Function codes are valid only while AS is asserted. Refer to 4.4.1.7 Function Codes
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 4-11 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 4.7 Interrupts for information about interrupt acknowledge bus cycles.
MOTOROLA
4-26
SYSTEM INTEGRATION MODULE
MC68331
USER’S MANUAL
MOTOROLA [ MOTOROLA ]
