MC68F375
SINGLE-CHIP INTEGRATION MODULE 2 (SCIM2E)
MOTOROLA
REFERENCE MANUAL
Rev. 25 June 03
4-42
Fast termination cycles use internal handshaking signals generated by the chip-select
logic. To initiate a transfer, the MCU drives the address bus 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 the DSACK[1:0] signals.
The DSACK field in the chip-select option registers determine whether internally gen-
erated DSACK or externally generated DSACK is used. For fast termination cycles,
Cycles for information about fast termination setup.
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 termi-
nate 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 reg-
ister specifies fast termination accesses.
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
tion timing.
When a fast termination cycle is issued, DS is asserted for reads but not for writes. 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.
4.6.4 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 as 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 4.5.1.7 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. Three encodings are used by the MCU, as shown in Figure 4-13. These
encodings represent breakpoint acknowledge (type 0x0) cycles, low power stop
broadcast (type 0x3) cycles, and interrupt acknowledge (type 0xF) cycles. Type 0x0
and type 0x3 cycles are discussed in the following paragraphs. Refer to 4.8 Interrupts for information about interrupt acknowledge bus cycles.
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Freescale Semiconductor, Inc.
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