
125
CHAPTER 7 CLOCK GENERATOR
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 bit 0 to bit 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 7-2
).
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 7-2. Maximum Time Required for CPU Clock Switchover
Set Values before Switchover
Set Values After Switchover
MCS 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
X
X
X
X
0
0
0
0
8 instructions 4 instructions 2 instructions
1 instruction
1 instruction
0
0
1
16 instructions
4 instructions 2 instructions
1 instruction
1 instruction
0
1
0
16 instructions
8 instructions
2 instructions
1 instruction
1 instruction
0
1
1
16 instructions
8 instructions 4 instructions
1 instruction
1 instruction
1
0
0
16 instructions
8 instructions 4 instructions 2 instructions
1 instruction
1
1
X
X
X
f
X
/2f
XT
instruction
f
X
/4f
XT
instruction
f
X
/8f
XT
instruction
f
X
/16f
XT
instruction
f
X
/32f
XT
instruction
(77 instructions)
(39 instructions)
(20 instructions)
(10 instructions)
(5 instructions)
0
f
X
/4f
XT
instruction
f
X
/8f
XT
instruction
f
X
/16f
XT
instruction f
X
/32f
XT
instruction
f
X
/64f
XT
instruction
(39 instructions)
(20 instructions)
(10 instructions)
(5 instructions)
(3 instructions)
Caution
Selection of the CPU clock cycle scaling 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 scaling 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.
Figures in parentheses apply to operation with f
X
= 5.0 MHz and f
XT
= 32.768 kHz.
2.