6 changing system clock and cpu clock settings – NEC uPD78P078 User Manual
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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 7-3).
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 7-3. Maximum Time Required for CPU Clock Switchover
Set Values after Switchover
Set Values before 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.
2. Figures in parentheses apply to operation with f
X
= 5.0 MHz and f
XT
= 32.768 kHz.