MC68340 datasheet(58/441 Pages) FREESCALE

58 / 441 page

MOTOROLA

MC68340 USER’S MANUAL

3- 9

3.2.3.3 BYTE OPERAND TO 16-BIT PORT, ODD (A0 = 1). The MC68340 drives the

address bus with the desired address and the SIZx pins to indicate a single-byte operand.

DSACK0

DSACK1

SIZ0

SIZ1

0110

D15

(OP0)

OP0

BYTE OPERAND

DATA BUS

CYCLE 1

For a read operation, the slave responds by placing data on bits 7–0 of the data bus and

asserting

DSACK1 to indicate a 16-bit port. The MC68340 then reads the operand byte

from bits 7–0 and ignores bits 15–8.

For a write operation, the MC68340 drives the single-byte operand on both bytes of the

data bus because it does not know the port size until the

DSACK

≈ signals are read. The

slave device reads the operand from bits 7–0 of the data bus and uses the address to

place the operand in the specified location. The slave then asserts

DSACK1 to terminate

the bus cycle.

3.2.3.4 WORD OPERAND TO 8-BIT PORT, ALIGNED. The MC68340 drives the address

bus with the desired address and the SIZx pins to indicate a word operand.

D15

OP0

OP1

OP0

OP1

DSACK0

DSACK1

SIZ0

SIZ1

10010

01110

WORD OPERAND

DATA BUS

CYCLE 1

CYCLE 2

(OP1)

For a read operation, the slave responds by placing the most significant byte of the

operand on bits 15–8 of the data bus and asserting

DSACK0 to indicate an 8-bit port. The

MC68340 reads the most significant byte of the operand from bits 15–8 and ignores bits

7–0. The MC68340 then decrements the transfer size counter, increments the address,

and reads the least significant byte of the operand from bits 15–8 of the data bus.

For a write operation, the MC68340 drives the word operand on bits 15–0 of the data bus.

The slave device then reads the most significant byte of the operand from bits 15–8 of the

data bus and asserts

DSACK0 to indicate that it received the data but is an 8-bit port.

The MC68340 then decrements the transfer size counter, increments the address, and

writes the least significant byte of the operand to bits 15–8 of the data bus.

Freescale Semiconductor, Inc.

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