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58
μ
PD75518(A)
(6) Time required to change the system clock and CPU clock
The system clock and CPU clock can be changed by using the least significant bit of the SCC and the low-
order two bits of the PCC. This switching is not performed immediately after the contents of the registers
are rewritten, but the system operates with the previous clock for some machine cycles. Accordingly, after
this time period, the STOP instruction must be executed or SCC.3 must be set to 1 to terminate main system
clock generation.
Table 4-5 Maximum Time Required to Change the System Clock and CPU Clock
Remarks 1.
Time enclosed in parentheses is required when f
X
= 6.0 MHz and f
XT
= 32.768 kHz.
2.
×
: Don’t care
3.
CPU clock
Φ
is supplied to the CPU. The reciprocal of this frequency is a minimum instruction
time (defined as one machine cycle in this manual).
Caution When the four bits of PCC are set to 0001B (
Φ
= f
X
/16), do not set SCC.0 to 1. Before switching
the main system clock to the subsystem clock, be sure to manipulate the PCC bits so other than
0001B is set. When the system operates on the subsystem clock, the PCC bits must also be
other than 0001B.
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
/64f
XT
machine
cycles
(3 machine cycles)
PCC1
×
PCC0
×
Setting before
switching
PCC
1
0
Setting after switching
0
1
1 machine cycle
8 machine cycles
16 machine cycles
Not to be set
1 machine cycle
4 machine cycles
16 machine cycles
1 machine cycle
1 machine cycle
4 machine cycles
8 machine cycles
1 machine cycle
4 machine cycles
8 machine cycles
16 machine cycles
1 machine cycle
SCC0
0
Not to be set
f
X
/8f
XT
machine
cycles
(23 machine cycles)
f
X
/4
f
XT
machine
cycles
(46 machine cycles)
PCC
0
SCC
0
1
0
1
×
0
0
1
1
×