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.
MCU
PERIPHERAL
ADDRESS DEVICE (S0)
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
4
ASSERT AS (S1)
PLACE DATA ON DATA[15:0] (S2)
ASSERT DS AND WAIT FOR DSACK (S3)
ACCEPT DATA (S2 + S3)
1) DECODE ADDRESS
2) LATCH DATA FROM DATA BUS
3) ASSERT DSACK SIGNALS
OPTIONAL STATE (S4)
NO CHANGE
TERMINATE OUTPUT TRANSFER (S5)
1) NEGATE DS AND AS
2) REMOVE DATA FROM DATA BUS
TERMINATE CYCLE
1) NEGATE DSACK
START NEXT CYCLE
WR CYC FLOW
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
MC68331
SYSTEM INTEGRATION MODULE
MOTOROLA
4-25
USER’S MANUAL
MOTOROLA [ MOTOROLA ]
