MC68HC16Z1CPV16 Datasheet, PDF (138/500 Pages) Freescale Semiconductor, Inc – Symbols and Operators, CPU16 Register Mnemonics

Freescale Semiconductor, Inc.
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.
Table 5-13 Size Signal Encoding
SIZ1
0
1
1
0
SIZ0
1
0
1
0
Transfer Size
Byte
Word
3 Byte
Long Word
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.
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
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.
5-32
SYSTEM INTEGRATION MODULE
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