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CHAPTER 3 APPLICATION OF SYSTEM CLOCK SELECTION
CHAPTER 3 APPLICATION OF SYSTEM CLOCK SELECTION
The 78K/0 series allows you to select a CPU clock and controls the operation of the oscillator by rewriting the
contents of the processor clock control register (PCC).
When the CPU clock is changed, the time shown in Table 3-1 and 3-2 is required since the contents of the PCC
have been rewritten until the CPU clock is actually changed. It is therefore not apparent for a while after the contents
of the PCC have been rewritten, whether the processor operates on the new or old clock. To stop the main system
clock or execute the STOP instruction, therefore, the wait time shown in Table 3-1 and 3-2 is necessary.
Table 3-1. Maximum Time Required for Changing CPU Clock
(
μ
PD78002, 78002Y, 78014, 78014Y, 78018F, 78018FY, 780024, 780024Y, 780034, 780034Y, 78014H subseries)
Set Value before Change
Set Value after Change
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
/4f
XT
instructions
(77 instructions)
0
0
1
8 instructions
8 instructions
8 instructions
8 instructions
f
X
/8f
XT
instructions
(39 instructions)
0
1
0
4 instructions
4 instructions
4 instructions
4 instructions
f
X
/16f
XT
instructions
(20 instructions)
0
1
1
2 instructions
2 instructions
2 instructions
2 instructions
f
X
/32f
XT
instructions
(10 instructions)
1
0
0
1 instruction
1 instruction
1 instruction
1 instruction
f
X
/64f
XT
instructions
(5 instructions)
1
×
×
×
1 instruction
1 instruction
1 instruction
1 instruction
1 instruction
Caution Do not select dividing the CPU clock (PCC0-PCC2) and changing from the main system clock to
subsystem clock (by setting CSS to 0
→
1) at the same time.
However, dividing the CPU clock (PCC0-PCC2) can be selected at the same time as changing from
the subsystem clock to the main system clock.
Remarks 1.
One instruction is the minimum instruction execution time of the CPU clock before change.
2.
( ): f
X
= 10.0 MHz, f
XT
= 32.768 kHz