MC68336/376
SYSTEM INTEGRATION MODULE
MOTOROLA
USER’S MANUAL
Rev. 15 Oct 2000
5-26
fied. When these specifications are met, the MCU is guaranteed to recognize the
appropriate signal on a specific edge of the CLKOUT signal.
For a read cycle, when assertion of DSACK is recognized on a particular falling edge
of the clock, valid data is latched into the MCU on the next falling clock edge, provided
that the data meets the data setup time. In this case, the parameter for asynchronous
operation can be ignored.
When a system asserts DSACK for the required window around the falling edge of S2
and obeys the bus protocol by maintaining DSACK and BERR or HALT until and
throughout the clock edge that negates AS (with the appropriate asynchronous input
hold time), no wait states are inserted. The bus cycle runs at the maximum speed of
three clocks per cycle.
To ensure proper operation in a system synchronized to CLKOUT, when either BERR
or BERR and HALT is asserted after DSACK, BERR (or BERR and HALT) assertion
must satisfy the appropriate data-in setup and hold times before the falling edge of the
clock cycle after DSACK is recognized.
5.6.2 Regular Bus Cycles
The following paragraphs contain a discussion of cycles that use external bus control
cycles.
To initiate a transfer, the MCU asserts an address and the SIZ[1:0] signals. The SIZ
signals and ADDR0 are externally decoded to select the active portion of the data bus.
device either places data on the bus (read cycle) or latches data from the bus (write
cycle), then asserts a DSACK[1:0] combination that indicates port size.
The DSACK[1:0] signals can be asserted before the data from a peripheral device is
valid on a read cycle. To ensure valid data is latched into the MCU, a maximum period
between DSACK assertion and DS assertion is specified.
There is no specified maximum for the period between the assertion of AS and
DSACK. Although the MCU can transfer data in a minimum of three clock cycles when
the cycle is terminated with DSACK, the MCU inserts wait cycles in clock period incre-
ments until either DSACK signal goes low.
If bus termination signals remain unasserted, the MCU will continue to insert wait
states, and the bus cycle will never end. If no peripheral responds to an access, or if
an access is invalid, external logic should assert the BERR or HALT signals to abort
the bus cycle (when BERR and HALT are asserted simultaneously, the CPU32 acts
as though only BERR is asserted). When enabled, the SIM bus monitor asserts BERR
when DSACK response time exceeds a predetermined limit. The bus monitor timeout
period is determined by the BMT[1:0] field in SYPCR. The maximum bus monitor tim-
eout period is 64 system clock cycles.