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MC68306 USER'S MANUAL
MOTOROLA
that uses the read-modify-write cycle) only operates on bytes. Thus, all read-modify-write
cycles are byte operations. The read-modify-write flowchart is shown in Figure 3-8 and the
timing diagram is shown in Figure 3-9.
BUS MASTER
ADDRESS THE DEVICE
1) SET R/W TO READ
2) PLACE FUNCTION CODE ON FC2–FC0
3) PLACE ADDRESS ON ADDRESS BUS
4) ASSERT ADDRESS STROBE (AS)
5) ASSERT UPPER DATA STROBE (UDS)
OR LOWER DATA STROBE (LDS)
TERMINATE THE CYCLE
OUTPUT THE DATA
1) DECODE ADDRESS
2) PLACE DATA ON D7–D0 OR D15–D0
3) ASSERT DATA TRANSFER
ACKNOWLEDGE (DTACK)
SLAVE
START NEXT CYCLE
1) REMOVE DATA FROM D7–D0
OR D15–D8
2) NEGATE DTACK
1) LATCH DATA
1) NEGATE UDS AND LDS
2) START DATA MODIFICATION
ACQUIRE THE DATA
START OUTPUT TRANSFER
1) SET R/W TO WRITE
2) PLACE DATA ON D7–D0 OR D15–D8
3) ASSERT UPPER DATA STROBE (UDS)
OR LOWER DATA STROBE (LDS)
TERMINATE OUTPUT TRANSFER
1) NEGATE UDS OR LDS
2) NEGATE AS
3) REMOVE DATA FROM D7–D0 OR
D15–D8
4) SET R/W TO READ
INPUT THE DATA
1) STORE DATA ON D7–D0 OR D15–D8
2) ASSERT DATA TRANSFER
ACKNOWLEDGE (DTACK)
TERMINATE THE CYCLE
1) NEGATE DTACK
Figure 3-8. Read-Modify-Write Cycle Flowchart