
56
μ
PD75237
(6)
Time required for system clock and CPU clock switching
The system clock and the CPU clock can be switched to each other with the least significant bit of the SCC
and the lower 2 bits of the PCC. This switching is not executed just after register rewrite and operation continues
with the previous clock during the specified machine cycle. Thus, to stop main system clock oscillation, it is
necessary to execute the STOP instruction or to set SCC.3 after the specified switching time.
Table 4-2 Maximum Time Required for System Clock and CPU Clock Switching
SCC
0
PCC
1
PCC
0
SCC0
0
PCC1
0
PCC0
0
SCC0
0
PCC1
0
PCC0
1
SCC0
0
PCC1
1
PCC0
0
SCC0
0
PCC1
1
PCC0
1
SCC0
1
f
X
PCC1
×
Set Value before
Switching
Set Value after Switching
0
0
1
1
×
0
1
0
1
0
1
×
4 machine cycle
8 machine cycle
16 machine cycle
1 machine cycle
1 machine cycle
8 machine cycle
16 machine cycle
Setting prohibited
1 machine cycle
4 machine cycle
16 machine cycle
1 machine cycle
1 machine cycle
4 machine cycle
8 machine cycle
1 machine cycle
f
X
4f
(64 machine cycle)
machine cycle
f
X
8f
XT
(23 machine cycle)
machine cycle
Setting prohibited
64f
XT
(3 machine cycle)
machine cycle
PCC0
×
Remarks
1.
CPU clock
Φ
is a clock to be supplied to the internal CPU of
μ
PD75237 and its inverse number is the
minimum instruction time (defined as “one machine cycle” in this manual).
Values in parentheses are when f
X
= 6.0 MHz and f
XT
= 32.768 kHz.
2.
Note
When PCC = 0001B (
Φ
= f
X
/16 selected), do not set SCC.0 to “1”. When switching from the main system
clock to the subsystem clock, do so after setting PCC to another value (PCC
≠
0001B).
Do not set PCC = 0001B while in operation with the subsystem clock.