Bus Exception Control Cycles - Motorola MC68340 User Manual

3.5 BUS EXCEPTION CONTROL CYCLES

The bus architecture requires assertion of DSACK from an external device to signal that
a bus cycle is complete. Neither DSACK nor AVEC is asserted in the following cases:
• DSACK / AVEC is programmed to respond internally.
• The external device does not respond.
• Various other application-dependent errors occur.
The MC68340 provides BERR when no device responds by asserting DSACK / AVEC
within an appropriate period of time after the MC68340 asserts AS . This mechanism
allows the cycle to terminate and the MC68340 to enter exception processing for the error
condition. HALT is also used for bus exception control. This signal can be asserted by an
external device for debugging purposes to cause single bus cycle operation, or, in
combination with BERR , a retry of a bus cycle in error. To properly control termination of a
bus cycle for a retry or a bus error condition, DSACK , BERR , and HALT can be asserted
and negated with the rising edge of the MC68340 clock. This assures that when two
signals are asserted simultaneously, the required setup and hold time for both is met for
the same falling edge of the MC68340 clock. This or an equivalent precaution should be
designed into the external circuitry to provide these signals. Alternatively, the internal bus
monitor could be used. The acceptable bus cycle terminations for asynchronous cycles
are summarized in relation to DSACK assertion as follows (case numbers refer to Table
3-4):
• Normal Termination: DSACK is asserted; BERR and HALT remain negated (case 1).
• Halt Termination: HALT is asserted at the same time as or before DSACKx , and
BERR remains negated (case 2).
• Bus Error Termination: BERR is asserted in lieu of, at the same time as, or before
DSACK (case 3) or after DSACK (case 4), and HALT remains negated; BERR is
negated at the same time as or after DSACK .
• Retry Termination: HALT and BERR are asserted in lieu of, at the same time as, or
before DSACK (case 5) or after DSACK (case 6); BERR is negated at the same
time as or after DSACK , and HALT may be negated at the same time as or after
BERR .
Table 3-4 lists various combinations of control signal sequences and the resulting bus
cycle terminations. To ensure predictable operation, BERR and HALT should be negated
according to the specifications given in Section 11 Electrical Characteristics. DSACK
BERR , and HALT may be negated after AS . If DSACK or BERR remain asserted into S2
of the next bus cycle, that cycle may be terminated prematurely.
EXAMPLE A: A system uses a bus monitor timer to terminate accesses to an unpopulated
address space. The timer asserts BERR after timeout (case 3).
3-32
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