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3- 26
MC68306 USER'S MANUAL
MOTOROLA
S0
S2
S4
S6
CLK
FC2-FC0
A23–A1
S8
S0
S2
S4
S6
AS
LDS/UDS
R/W
DTACK
D15–D0
BERR
HALT
1 CLOCK PERIOD
≥
READ
HALT
RETRY
Figure 3-25. Retry Bus Cycle Timing Diagram
The processor terminates the bus cycle, and remains in this state until
HALT
is negated.
Then the processor retries the preceding cycle using the same function codes, address,
and data (for a write operation).
BERR
should be negated at least one clock cycle before
HALT
is negated.
NOTE
To guarantee that the entire read-modify-write cycle runs
correctly and that the write portion of the operation is
performed without negating the address strobe, the processor
does not retry a read-modify-write cycle. When
BERR
occurs
during a read-modify-write operation, a bus error operation is
performed whether or not
HALT
is asserted.
3.4.3 Halt Operation
HALT
performs a halt/run/single-step operation. When
HALT
is asserted by an external
device, the processor halts and remains halted as long as the signal remains asserted, as
shown in Figure 3-26.
While the processor is halted, bus arbitration is performed as usual. Should a bus error
occur while
HALT
is asserted, the processor performs the retry operation previously
described.
NOTE
If a RESET instruction is executed while
HALT
is asserted, the
CPU will be reset.