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.
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.
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
Table 5-13 Size Signal Encoding
SIZ1
SIZ0
Transfer Size
0
1
Byte
1
0
Word
1
1
3 Byte
0
0
Long Word
Table 5-14 Address Space Encoding
FC2
FC1
FC0
Address Space
1
0
0
Reserved
1
0
1
Data space
1
1
0
Program space
1
1
1
CPU space
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Freescale Semiconductor, Inc.
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