CHAPTER 5 CLOCK GENERATOR
Preliminary User’s Manual U16315EJ1V0UD
123
5.7 Changing System Clock and CPU Clock Settings
5.7.1 Time required for switching between system clock and CPU clock
The system clock and CPU clock can be switched using 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 immediately after rewriting to the PCC; operation continues on
the pre-switchover clock for several instructions (see
Table 5-6
).
Whether the system is operating on the X1 input clock (or Ring-OSC clock) or the subsystem clock can be
ascertained using bit 5 (CLS) of the PCC register.
Table 5-6. Maximum Time Required for CPU Clock Switchover
Set Value Before
Switchover
Set Value After 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
16 clocks
16 clocks
16 clocks
16 clocks
f
XP
/f
XT
clocks
(306 clocks)
0
0
1
8 clocks
8 clocks
8 clocks
8 clocks
f
XP
/2f
XT
clocks
(153 clocks)
0
1
0
4 clocks
4 clocks
4 clocks
4 clocks
f
XP
/4f
XT
clocks
(77 clocks)
0
1
1
2 clocks
2 clocks
2 clocks
2 clocks
f
XP
/8f
XT
clocks
(39 clocks)
0
1
0
0
1 clock
1 clock
1 clock
1 clock
f
XP
/16f
XT
clocks
(20 clocks)
1
×
×
×
1 clock
1 clock
1 clock
1 clock
1 clock
Remarks 1.
The maximum time is the number of clocks of the CPU clock before switching.
2.
Figures in parentheses apply to operation with f
XP
= 10 MHz and f
XT
= 32.768 kHz.
Caution
Selection of the CPU clock cycle division factor (PCC0 to PCC2) and switchover from the X1
input 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 factor
(PCC0 to PCC2) and switchover from the subsystem clock to the X1 input clock (changing CSS
from 1 to 0).