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7.6 Changing System Clock and CPU Clock Settings
7.6.1 Time required for switchover between system clock and CPU clock
The system clock and CPU clock can be switched over by means of bits 0 to 2 (PCC0 to PCC2) and bit 4 (CSS)
of the processor clock control register (PCC).
The actual switchover operation is not performed directly after writing to the PCC, but operation continues on the
pre-switchover clock for several instructions (see Table 7-3).
Determination as to whether the system is operating on the main system clock or the subsystem clock is performed
by bit 5 (CLS) of the PCC register.
Set Values before
Switchover
CSS PCC2 PCC1 PCC0 CSS PCC2 PCC1 PCC0 CSS PCC2 PCC1 PCC0 CSS PCC2 PCC1 PCC0 CSS PCC2 PCC1 PCC0 CSS PCC2 PCC1 PCC0 CSS PCC2 PCC1 PCC0
0
0
0
0
0
0
1
0
1
0
0
1
1
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1
0
0
×
×
×
1
Caution
Selection of the CPU clock cycle dividing factor (PCC0 to PCC2) and switchover from the main
system clock to the subsystem clock (changing CSS from 0 to 1) should not be performed
simultaneously.
Simultaneous setting is possible, however, for selection of the CPU clock cycle dividing factor
(PCC0 to PCC2) and switchover from the subsystem clock to the main system clock (changing
CSS from 1 to 0).
Remarks 1. One instruction is the minimum instruction execution time with the pre-switchover CPU clock.
2. Values in parentheses apply to operation with f
168
CHAPTER 7 CLOCK GENERATOR
Table 7-3. Maximum Time Required for CPU Clock Switchover
0
0
0
0
0
0
0
16 instructions
8 instructions
4 instructions
4 instructions
2 instructions
2 instructions
1 instruction
1 instruction
1 instruction
1 instruction
Set Values after Switchover
1
0
0
1
0
0
0
16 instructions
16 instructions
8 instructions
8 instructions
4 instructions
2 instructions
1 instruction
1 instruction
1 instruction
1 instruction
= 10.0 MHz or f
X
1
1
0
1
0
0
1
16 instructions
f
/4f
X
XT
(77 instructions)
8 instructions
f
/8f
X
XT
(39 instructions)
4 instructions
f
/16f
X
(20 instructions)
2 instructions
f
/32f
X
(10 instructions)
f
/64f
X
(5 instructions)
1 instruction
= 32.768 kHz.
XT
×
×
×
instructions
instructions
instructions
XT
instructions
XT
instructions
XT
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