SPMCK16Z4MPV20 datasheet(138/500 Pages) FREESCALE

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SYSTEM INTEGRATION MODULE

M68HC16 Z SERIES

5-32

USER’S MANUAL

5.5.1.5 Read/Write Signal

The read/write signal (R/W) determines the direction of the transfer during a bus cycle.

This signal changes state, when required, at the beginning of a bus cycle, and is valid

while AS is asserted. R/W only transitions when a write cycle is preceded by a read

cycle or vice versa. The signal may remain low for two consecutive write cycles.

5.5.1.6 Size Signals

Size signals (SIZ[1:0]) indicate the number of bytes remaining to be transferred during

an operand cycle. They are valid while AS is asserted. Table 5-13 shows SIZ0 and

SIZ1 encoding.

5.5.1.7 Function Codes

The CPU generates function code signals (FC[2:0]) to indicate the type of activity oc-

curring on the data or address bus. These signals can be considered address exten-

sions that can be externally decoded to determine which of eight external address

spaces is accessed during a bus cycle.

Because the CPU16 always operates in supervisor mode (FC2 = 1), address spaces

0 to 3 are not used. Address space 7 is designated CPU space. CPU space is used

for control information not normally associated with read or write bus cycles. Function

codes are valid while AS is asserted. Table 5-14 shows address space encoding.

5.5.1.8 Data Size Acknowledge Signals

During normal bus transfers, external devices assert the data size acknowledge sig-

nals (DSACK[1:0]) to indicate port width to the MCU. During a read cycle, these sig-

nals tell the MCU to terminate the bus cycle and to latch data. During a write cycle, the

signals indicate that an external device has successfully stored data and that the cycle

can terminate. DSACK[1:0] can also be supplied internally by chip-select logic. Refer

to 5.9 Chip-Selects for more information.

Table 5-13 Size Signal Encoding

SIZ1

SIZ0

Transfer Size

Byte

Word

3 Byte

Long Word

Table 5-14 Address Space Encoding

FC2

FC1

FC0

Address Space

Reserved

Data space

Program space

CPU space

Freescale Semiconductor, Inc.

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