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MOTOROLA
MC68349 USER'S MANUAL
3- 21
internally and respond for the slave. If no slave responds or the access is invalid, external
control logic may assert BERR to abort the bus cycle or BERR with HALT to retry the bus
cycle.
DSACK≈ can be asserted before the data from a slave device is valid on a read cycle. The
length of time that DSACK≈ may precede data must not exceed a specified value in any
asynchronous system to ensure that valid data is latched into the MC68349. (See Section
11 Electrical Characteristics for timing parameters.) Note that no maximum time is
specified from the assertion of AS to the assertion of DSACK≈ . Although the MC68349 can
transfer data in a minimum of three clock cycles when the cycle is terminated with
DSACK≈ , the MC68349 inserts wait cycles in clock-period increments until DSACK≈ is
recognized. BERR and/or HALT can be asserted after DSACK≈ is asserted. BERR and or
HALT must be asserted within the time specified after DSACK≈ is asserted in any
asynchronous system. If this maximum delay time is violated, the MC68349 may exhibit
erratic behavior.
3.2.5 Synchronous Operation with
DSACK
Although cycles terminated with DSACK≈ are classified as asynchronous, cycles
terminated with DSACK≈ can also operate synchronously in that signals are interpreted
relative to clock edges. The devices that use these cycles must synchronize the response
to the MC68349 clock (CLKOUT) to be synchronous. Since the devices terminate bus
cycles with DSACK≈ , the dynamic bus sizing capabilities of the MC68349 are available.
The minimum cycle time for these cycles is also three clocks. To support systems that use
the system clock to generate DSACK≈ and other asynchronous inputs, the asynchronous
input setup time and the asynchronous input hold time are given. If the setup and hold
times are met for the assertion or negation of a signal such as DSACK≈ , the MC68349 is
guaranteed to recognize that signal level on that specific falling edge of the system clock.
If the assertion of DSACK≈ is recognized on a particular falling edge of the clock, valid
data is latched into the MC68349 (for a read cycle) on the next falling clock edge if the
data meets the data setup time. In this case, the parameter for asynchronous operation
can be ignored. The timing parameters are described in Section 11 Electrical
Characteristics.
If a system asserts DSACK≈ for the required window around the falling edge of S2 and
obeys the proper bus protocol by maintaining DSACK≈ (and/or BERR /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 its maximum speed for bus cycles
terminated with DSACK≈ (three clocks per cycle). When BERR (or BERR and HALT) is
asserted after DSACK≈ , BERR (and HALT) must meet the appropriate setup time prior to
the falling clock edge one clock cycle after DSACK≈ is recognized. This setup time is
critical, and the MC68349 may exhibit erratic behavior if it is violated. When operating
synchronously, the data-in setup and hold times for synchronous cycles may be used
instead of the timing requirements for data relative to DS .