37
CHAPTER 3 SYSTEM CLOCK SWITCHING APPLICATION
CHAPTER 3 SYSTEM CLOCK SWITCHING APPLICATION
The 78K/0 Series can control the selection of the CPU clock and oscillator operation by rewriting the
processor clock control register (PCC).
The display mode registers 0 and 1 (DSPM0, DSPM1) can be used to set mode of the noise eliminator
for the subsystem clock and enable or disable display operation (except for the
μ
PD780228 subseries).
When the CPU clock is changed, it takes the time shown in Tables 3-1 and 3-2 from when a rewrite
instruction is used to the PCC until the CPU clock is actually changed. For a while after an instruction to
rewrite the PCC is issued, therefore, it cannot be determined which clock, old or new, is used by the CPU.
When a main system clock is to be stopped or a STOP instruction is to be executed, a wait enough to assure
instructions listed in Tables 3-1 and 3-2 have been executed is needed.
Table 3-1. Maximum Time Required to Change the CPU Clock
(
μ
PD78044F,
μ
PD78044H, and
μ
PD780208 Subseries)
Caution Selecting of the frequency division of the CPU clock (PCC0-PCC2) and switching from
main system clock to subsystem clock (CSS: 0 -> 1) must not be performed simultane-
ously.
However, selecting of the frequency division of the CPU clock (PCC0-PCC2) and switching
from subsystem clock to main system clock (CSS: 1 -> 0) can be performed simultane-
ously.
Remarks 1.
The execution time of one instruction is the minimum instruction execution time of the CPU
clock before switching.
2.
Time enclosed in parentheses is required when f
X
= 5.0 MHz and f
XT
= 32.768 kHz.
CSS
PCC2 PCC1 PCC0
CSS
0
PCC2
0
PCC1
0
PCC0
0
CSS
0
PCC2
0
PCC1
0
PCC0
1
CSS
0
PCC2
0
PCC1
1
PCC0
0
CSS
0
PCC2
0
PCC1
1
PCC0
1
CSS
0
PCC2
1
PCC1
0
PCC0
0
CSS
1
PCC2
x
PCC1
x
PCC0
x
Setting before switching
Setting after switching
0
0
0
0
16 instructions
16 instructions
16 instructions
16 instructions
f
X
/2f
XT
instructions
(64)
0
0
1
8 instructions
8 instructions
8 instructions
8 instructions
f
X
/4f
XT
instructions
(32)
0
1
0
4 instructions
4 instructions
4 instructions
4 instructions
f
X
/8f
XT
instructions
(16)
0
1
1
2 instructions
2 instructions
2 instructions
2 instructions
f
X
/16f
XT
instructions
(8)
1
0
0
1 instruction
1 instruction
1 instruction
1 instruction
f
X
/32f
XT
instructions
(4)
1
x
x
x
1 instruction
1 instruction
1 instruction
1 instruction
1 instruction