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CHAPTER 5 CLOCK GENERATOR
User’s Manual U19780EJ2V0UD
166
5.6.8 Time required for switchover of CPU clock and main system clock
By setting bits 0 to 2 (PCC0 to PCC2) and bit 4 (CSS) of the processor clock control register (PCC), the CPU clock
can be switched (between the main system clock and the subsystem clock) and the division ratio of the main system
clock can be changed.
The actual switchover operation is not performed immediately after rewriting to PCC; operation continues on the
pre-switchover clock for several clocks (see
Tables 5-7
and
5-8
).
Whether the CPU is operating on the main system clock or the subsystem clock
Note
can be ascertained using bit 5
(CLS) of the PCC register.
Note
The 78K0/KB2-A is not provided with a subsystem clock.
Table 5-7. Time Required for Switchover of CPU Clock and Main System Clock Cycle Division Factor
(78K0/KB2-A)
Set Value Before
Switchover
Set Value After Switchover
PCC2 PCC1 PCC0 PCC2 PCC1 PCC0 PCC2 PCC1 PCC0 PCC2 PCC1 PCC0 PCC2 PCC1 PCC0
PCC2 PCC1 PCC0
0 0 0 0 0 1 0 1 0 0 1 1 1 0 0
0
0
0
16 clocks
16 clocks
16 clocks
16 clocks
0
0
1
8 clocks
8 clocks
8 clocks
8 clocks
0
1
0
4 clocks
4 clocks
4 clocks
4 clocks
0
1
1
2 clocks
2 clocks
2 clocks
2 clocks
1
0
0
1 clock
1 clock
1 clock
1 clock
Remark
The number of clocks listed in Table 5-7 is the number of CPU clocks before switchover.
Table 5-8. Time Required for Switchover of CPU Clock and Main System Clock Cycle Division Factor
(78K0/KC2-A)
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
2f
XP
/f
SUB
clocks
0
0
1
8 clocks
8 clocks
8 clocks
8 clocks
f
XP
/f
SUB
clocks
0
1
0
4 clocks
4 clocks
4 clocks
4 clocks
f
XP
/2f
SUB
clocks
0
1
1
2 clocks
2 clocks
2 clocks
2 clocks
f
XP
/4f
SUB
clocks
0
1
0
0
1 clock
1 clock
1 clock
1 clock
f
XP
/8f
SUB
clocks
1
×
×
×
2 clocks
2 clocks
2 clocks 2
clocks 2
clocks
Caution Selection of the main system clock cycle division factor (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 main system clock cycle division
factor (PCC0 to PCC2) and switchover from the subsystem clock to the main system clock
(changing CSS from 1 to 0).
Remark 1.
The number of clocks listed in Table 5-8 is the number of CPU clocks before switchover.