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-
fer to
for information about fast termination setup.
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
PENDIX A ELECTRICAL CHARACTERISTICS
for 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.
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.
Function codes are valid only while AS is asserted. Refer to
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
. 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
for infor-
mation about interrupt acknowledge bus cycles.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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