
MC68331
USER’S MANUAL
SYSTEM INTEGRATION MODULE
MOTOROLA
4-25
4
4.5.2.2 Write Cycle
During a write cycle, the MCU transfers data to an external memory or peripheral de-
vice. If the instruction specifies a long-word or word operation, the MCU attempts to
write two bytes at once. For a byte operation, the MCU writes one byte. The portion of
the data bus upon which each byte is written depends on operand size, peripheral ad-
dress, and peripheral port size.
Refer to
4.4.2 Dynamic Bus Sizing
and
4.4.4 Misaligned Operands
for more infor-
mation.
Figure 4-10
is a flowchart of a write-cycle operation for a word transfer. Refer
to the SIM Reference Manual (SIMRM/AD) for more information.
Figure 4-10 Write Cycle Flowchart
4.5.3 Fast Termination Cycles
When an external device has a fast access time, the chip-select circuit fast-termination
option can provide a two-cycle external bus transfer. Because the chip-select circuits
WR CYC FLOW
MCU
PERIPHERAL
ADDRESS DEVICE (S0)
1) DECODE ADDRESS
2) LATCH DATA FROM DATA BUS
3) ASSERT DSACK SIGNALS
ACCEPT DATA (S2 + S3)
TERMINATE CYCLE
1) SET R/W TO WRITE
2) DRIVE ADDRESS ON ADDR[23:0]
3) DRIVE FUNCTION CODE ON FC[2:0]
4) DRIVE SIZ[1:0] FOR OPERAND SIZE
1) NEGATE DS AND AS
2) REMOVE DATA FROM DATA BUS
TERMINATE OUTPUT TRANSFER (S5)
START NEXT CYCLE
ASSERT AS (S1)
PLACE DATA ON DATA[15:0] (S2)
ASSERT DS AND WAIT FOR DSACK (S3)
OPTIONAL STATE (S4)
NO CHANGE
1) NEGATE DSACK