MOTOROLA
SYSTEM INTEGRATION MODULE
M68HC16 Z SERIES
5-40
USER’S MANUAL
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 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 essentially all external bus cycles to be three-cycle (zero
wait states) accesses unless the chip-select option register specifies fast accesses.
NOTE
There are certain exceptions to the three-cycle rule when one or both
DSACK lines are asserted. Check the current device and mask set
errata for details.
For fast termination cycles, the fast termination encoding (%1110) must be used. Re-
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 AP- 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.
5.6.4 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.
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. Three encodings are used by the MCU, as shown in Figure 5-14. These
encodings represent breakpoint acknowledge (type $0) cycles, low power stop broad-
cast (type $3) cycles, and interrupt acknowledge (type $F) cycles. Type $0 and type
$3 cycles are discussed in the following paragraphs. Refer to 5.8 Interrupts for infor-
mation about interrupt acknowledge bus cycles.