CHAPTER 4 CLOCK GENERATION FUNCTION
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User’s Manual U15109EJ3V0UD
4.3.1 Control registers
(1) Processor clock control register (PCC)
This is a specific register. It can be written to only when a specified combination of sequences is used (see
3.4.9
Specific registers
). This register can be read/written in 8- or 1-bit units.
After reset:
03H
R/W
Address: FFFFF074H
<7>
<6>
5
4
3
<2>
1
0
PCC
FRC
MCK
0
0
0
CK2
CK1
CK0
FRC
Selection of internal feedback resistor for subclock
0
Use
1
Do not use
MCK
Operation of main clock
0
Operate
1
Stop
CK2
Notes 1, 2
CK1
CK0
Selection of CPU clock
0
0
0
f
XX
0
0
1
f
XX
/2
0
1
0
f
XX
/4
0
1
1
f
XX
/8
1
X
X
f
XT
(subclock)
Notes 1.
It is recommended to manipulate CK2 in 1-bit units. However, when manipulating the PCC register
in 8-bit units, be sure not to change the values of CK1 and CK0.
2.
Do not set the STOP mode when the CPU is operating on the subclock (CK2 = 1).
Cautions 1. Do not change the CPU clock (the values of CK2 to CK0 in the PCC register) while
CLKOUT is being output.
2. Even if the MCK bit is set to 1 during main clock operation, the main clock is not stopped.
The CPU clock stops after the subclock is selected.
3. Always set bits 5 to 3 to 0.
Remark
X: don’t care
(a) Example of main clock operation
→
→
→
→
subclock operation setting
<1> CK2
←
1:
Bit manipulation instructions are recommended. Do not change CK1 and
CK0.
<2> Subclock operation:
It takes up to the following number of instructions for the subclock to start
operating after the CK2 bit is set.
(CPU clock frequency before setting / subclock frequency)
×
2
It is therefore necessary to insert waits equivalent to this number using a
program.