
Central Processing Unit (CPU)
MC9S12T64Revision 1.1.1
56
Central Processing Unit (CPU)
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MOTOROLA
n
Write PPAGE register. An
n
cycle is used only in CALL and RTC instructions to write the destination
value of the PPAGE register and is not visible on the external bus. Since the PPAGE register is an
internal 8-bit register, an
n
cycle is never stretched.
O
Optional cycle. An
O
cycle adjusts instruction alignment in the instruction queue. An
O
cycle can be a
free cycle (
f
) or a program word access cycle (
P
). When the first byte of an instruction with an odd
number of bytes is misaligned, the
O
cycle becomes a
P
cycle to maintain queue order. If the first
byte is aligned, the
O
cycle is an
f
cycle.
The $18 prebyte for a page-two opcode is treated as a special one-byte instruction. If the prebyte is
misaligned, the
O
cycle at the beginning of the instruction becomes a
P
cycle to maintain queue
order. If the prebyte is aligned, the
O
cycle is an
f
cycle. If the instruction has an odd number of
bytes, it has a second
O
cycle at the end. If the first
O
cycle is a
P
cycle (prebyte misaligned), the
second
O
cycle is an
f
cycle. If the first
O
cycle is an
f
cycle (prebyte aligned), the second
O
cycle is
a
P
cycle.
An
O
cycle that becomes a
P
cycle can be extended to two bus cycles if the MCU is operating with an
8-bit external data bus and the program is stored in external memory. There can be additional
stretching when the address space is assigned to a chip-select circuit programmed for slow memory.
An
O
cycle that becomes an
f
cycle is never stretched.
P
Program word access. Program information is fetched as aligned 16-bit words. A
P
cycle is extended
to two bus cycles if the MCU is operating with an 8-bit external data bus and the program is stored
externally. There can be additional stretching when the address space is assigned to a chip-select
circuit programmed for slow memory.
r
8-bit data read. An
r
cycle is stretched only when controlled by a chip-select circuit programmed for
slow memory.
R
16-bit data read. An
R
cycle is extended to two bus cycles if the MCU is operating with an 8-bit
external data bus and the corresponding data is stored in external memory. There can be additional
stretching when the address space is assigned to a chip-select circuit programmed for slow memory.
An
R
cycle is also stretched if it corresponds to a misaligned access to a memory that is not
designed for single-cycle misaligned access.
s
Stack 8-bit data. An
s
cycle is stretched only when controlled by a chip-select circuit programmed for
slow memory.
S
Stack 16-bit data. An
S
cycle is extended to two bus cycles if the MCU is operating with an 8-bit
external data bus and the SP is pointing to external memory. There can be additional stretching if the
address space is assigned to a chip-select circuit programmed for slow memory. An
S
cycle is also
stretched if it corresponds to a misaligned access to a memory that is not designed for single-cycle
misaligned access. The internal RAM is designed to allow single cycle misaligned word access.
w
8-bit data write. A
w
cycle is stretched only when controlled by a chip-select circuit programmed for
slow memory.
W
16-bit data write. A
W
cycle is extended to two bus cycles if the MCU is operating with an 8-bit
external data bus and the corresponding data is stored in external memory. There can be additional
stretching when the address space is assigned to a chip-select circuit programmed for slow memory.
A
W
cycle is also stretched if it corresponds to a misaligned access to a memory that is not designed
for single-cycle misaligned access.
u
Unstack 8-bit data. A
W
cycle is stretched only when controlled by a chip-select circuit programmed
for slow memory.
Table 10 Access Detail Notation (Continued)
F
Freescale Semiconductor, Inc.
n
.