MC68F375
SINGLE-CHIP INTEGRATION MODULE 2 (SCIM2E)
MOTOROLA
REFERENCE MANUAL
Rev. 25 June 03
4-39
4.6.1 Synchronization to CLKOUT
External devices connected to the MCU bus can operate at a clock frequency different
from the frequencies of the MCU as long as the external devices satisfy the interface
signal timing constraints. Although bus cycles are classified as asynchronous, they are
interpreted relative to the MCU system clock output (CLKOUT).
Descriptions are made in terms of individual system clock states, labelled {S0, S1,
S2,..., SN}. The designation “state” refers to the logic level of the clock signal, and
does not correspond to any implemented machine state. A clock cycle consists of two
more information.
Bus cycles terminated by DSACK assertion normally require a minimum of three
CLKOUT cycles. To support systems that use CLKOUT to generate DSACK and other
inputs, asynchronous input setup time and asynchronous input hold times are speci-
fied. When these specifications are met, the MCU is guaranteed to recognize the
appropriate signal on a specific edge of the CLKOUT signal.
4.6.2 Regular Bus Cycle
The following paragraphs contain a discussion of cycles that use external bus control
To initiate a transfer, the MCU drives the address bus and the SIZ[1:0] signals. The
SIZ signals and ADDR0 are externally decoded to select the active portion of the data
W are valid, a peripheral device either places data on the bus (read cycle) or latches
data from the bus (write cycle), then asserts a DSACK[1:0] combination to indicate the
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 by the MCU, a maximum period
between MCU assertion of DS and supplied assertion of DSACK[1:0] is specified.
There is no specified maximum for the period between MCU assertion of AS and sup-
plied assertion of DSACK[1:0]. 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 increments until either DSACK1 or DSACK0 goes low.
If the DSACK 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 sig-
nals to abort the bus cycle (when BERR and HALT are asserted simultaneously, the
CPU32 acts as though only BERR is asserted). When enabled, the SCIM2E bus mon-
itor 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 maxi-
mum bus monitor timeout period is 64 system clock cycles.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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