Typewriter User Manual
3- 18 MC68340 USER’S MANUAL MOTOROLA
3.3.2 Write Cycle
During a write cycle, the MC68340 transfers data to memory or a peripheral device. Figure
3-8 is a flowchart of a word write cycle.
1. NEGATE DS AND AS
2. REMOVE DATA FROM D15–D0
BUS MASTER SLAVE
ADDRESS DEVICE
1. SET R/W TO WRITE
2. DRIVE ADDRESS ON A31–A0
3. DRIVE FUNCTION CODE ON FC3–FC0
4. DRIVE SIZE PINS FOR OPERAND SIZE
5. ASSERT AS
6. PLACE DATA ON D15–D0
7. ASSERT DS
TERMINATE OUTPUT TRANSFER
START NEXT CYCLE
1. DECODE ADDRESS
2. LATCH DATA FROM D15–D0
ACCEPT DATA
3. ASSERT DSACKx SIGNALS
TERMINATE CYCLE
1. NEGATE DSACKx
Figure 3-8. Word Write Cycle Flowchart
State 0—The write cycle starts in S0. During S0, the MC68340 places a valid address on
A31–A0 and valid function codes on FC3–FC0. The function codes select the address
space for the cycle. The MC68340 drives R/
W low for a write cycle. SIZ1/SIZ0 become
valid, indicating the number of bytes to be transferred.
State 1—One-half clock later during S1, the MC68340 asserts
AS, indicating a valid
address on the address bus.
State 2—During S2, the MC68340 places the data to be written onto D15–D0, and
samples
DSACK≈ at the end of S2.
State 3—The MC68340 asserts
DS during S3, indicating that data is stable on the data
bus. As long as at least one of the
DSACK≈ signals is recognized by the end of S2
(meeting the asynchronous input setup time requirement), the cycle terminates one clock
later. If
DSACK≈ is not recognized by the start of S3, the MC68340 inserts wait states
instead of proceeding to S4 and S5. To ensure that wait states are inserted, both
DSACK1 and DSACK0 must remain negated throughout the asynchronous input setup
and hold times around the end of S2. If wait states are added, the MC68340 continues to
sample
DSACK≈ on the falling edges of the clock until one is recognized. The selected
device uses R/
W, SIZ1/SIZ0, and A0 to latch data from the appropriate byte(s) of D15–D8
and D7–D0. SIZ1/SIZ0 and A0 select the bytes of the data bus. If it has not already done
so, the device asserts
DSACK≈ to signal that it has successfully stored the data.
Frees
cale Semiconductor,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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