Motorola MC68020 User Manual page 91

State 5
MC68020—The processor negates AS, DS, and DBEN during S5. If more than one read
cycle is required to read in the operand(s), S0–S5 are repeated for each read cycle.
When the read cycle(s) are complete, the processor holds the address, R/W , and
FC2–FC0 valid in preparation for the write portion of the cycle.
The external device keeps its data and DSACK1/DSACK0 signals asserted until it
detects the negation of AS or DS (whichever it detects first). The device must remove
the data and negate DSACK1/DSACK0 within approximately one clock period after
sensing the negation of AS or DS. DSACK1/DSACK0 signals that remain asserted
beyond this limit may be prematurely detected for the next portion of the operation.
MC68EC020—The processor negates AS, DS, and DBEN during S5. If more than one
read cycle is required to read in the operand(s), S0–S5 are repeated for each read
cycle. When the read cycle(s) is complete, the processor holds the address, R/W, and
FC2–FC0 valid in preparation for the write portion of the cycle.
The external device keeps its data and DSACK1/DSACK0 signals asserted until it
detects the negation of AS or DS (whichever it detects first). The device must remove
the data and negate DSACK1/DSACK0 within approximately one clock period after
sensing the negation of AS or DS. DSACK1/DSACK0 signals that remain asserted
beyond this limit may be prematurely detected for the next portion of the operation.
Idle States
MC68020/EC020—The processor does not assert any new control signals during the
idle states, but it may internally begin the modify portion of the cycle at this time. S6–
S11 are omitted if no write cycle is required. If a write cycle is required, the R/W signal
remains in the read mode until S6 to prevent bus conflicts with the preceding read
portion of the cycle; the data bus is not driven until S8.
State 6
MC68020—The processor asserts ECS and OCS in S6 to indicate that another external
cycle is beginning. The processor drives R/W low for a write cycle. Depending on the
write operation to be performed, the address lines may change during S6.
MC68EC020—During S6, the processor drives R/W low for a write cycle. Depending on
the write operation to be performed, the address lines may change during S6.
State 7
MC68020—During S7, the processor asserts AS, indicating that the address on the
address bus is valid. The processor also asserts DBEN, which can be used to enable
data buffers. In addition, ECS (and OCS, if asserted) is negated during S7.
MC68EC020—During S7, the processor asserts AS, indicating that the address on the
address bus is valid.
State 8
MC68020/EC020—During S8, the processor places the data to be written onto the data
bus.
MOTOROLA M68020 USER'S MANUAL 5- 43