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CHAPTER 5 CLOCK GENERATOR
User’s Manual U15798EJ2V0UD
5.6 Changing System Clock and CPU Clock Settings
5.6.1 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 PCC; operation continues on the pre-
switchover clock for several instructions (see
Table 5-3
).
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.
Table 5-3. Maximum Time Required for CPU Clock Switchover
Remarks 1.
One instruction is the minimum instruction execution time with the pre-switchover CPU clock.
2.
Figures in parentheses are for operation with f
X
= 10 MHz and f
XT
= 32.768 kHz.
Cautions 1. Selection of the CPU clock cycle division ratio (PCC0 to PCC2) and switchover from the main
system clock to the subsystem clock (changing CSS from 0 to 1) should not be set
simultaneously.
Simultaneous setting is possible, however, for selection of the CPU clock cycle division ratio
(PCC0 to PCC2) and switchover from the subsystem clock to the main system clock
(changing CSS from 1 to 0).
2. If the clock must be switched over between the subsystem clock being used in
×
4 mode and
the main system clock, be sure to set the CPU clock frequency when the main system clock
is used to 280 kHz or more.
Set Value Before
Set Value After Switchover
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
0
1
0
0
1
0
0
0
1
1
0
1
0
0
1
×
0
0
0
0
16 instructions
16 instructions
16 instructions
16 instructions
f
X
/2f
XT
instruction
(153 instructions)
0
0
1
8 instructions
8 instructions
8 instructions
8 instructions
f
X
/4f
XT
instruction
(77 instructions)
0
1
0
4 instructions
4 instructions
4 instructions
4 instructions
f
X
/8f
XT
instruction
(39 instructions)
0
1
1
2 instructions
2 instructions
2 instructions
2 instructions
f
X
/16f
XT
instruction
(20 instructions)
1
0
0
1 instruction
1 instruction
1 instruction
1 instruction
f
X
/32f
XT
instruction
(10 instructions)
1
×
1 instruction
1 instruction
1 instruction
1 instruction
1 instruction
×
×
×
×
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