SIC63616-(Rev. 1.0) NO. P41
3240-0412
4.4.5 Switching the CPU clock
Either the OSC1 clock or the OSC3 clock can be selected as the CPU system clock using the CLKCHG
register.
The OSC3 oscillation circuit can be turned off (OSCC = "0") to save power while the CPU is operating with
the OSC1 clock (CLKCHG = "0").
If the system needs high speed operation, turn the OSC3 oscillation circuit on (OSCC = "1") and switch over
the system clock to OSC3 (CLKCHG = "0"
→
"1").
In this case, since 1 msec to several tens of msec are necessary for the oscillation to stabilize after turning the
OSC3 oscillation circuit on, you should switch over the clock after the stabilization time has elapsed. For the
oscillation start time, refer to Chapter 8, "Electrical Characteristics".
After the clock is switched from OSC3 to OSC1, the OSC3 oscillation circuit can be turned off immediately.
When switching the clock from OSC3 to OSC1 (CLKCHG = "1"
→
"0"), be sure to switch OSC3 oscillation
off with separate instructions. Using a single instruction to process simultaneously can cause a malfunction
of the CPU.
Figure 4.4.5.1 indicates the status transition diagram for the clock changeover.
RESET
OSCC=1
OSCC=0
CLKCHG=1
CLKCHG=0
ON
ON or OFF
STOP
HALT status
OSC1
OSC3
CPU clock
OFF
OFF
STOP
SLEEP status
OSC1
OSC3
CPU clock
Program Execution Status
HALT instruction
SLP instruction
Interrupt
Interrupt
(Key input interrupt)
Standby Status
*
*
ON
ON
OSC3
High speed operation
OSC1
OSC3
CPU clock
ON
ON
OSC1
Low speed operation
OSC1
OSC3
CPU clock
Low speed and
low power operation
ON
OFF
OSC1
OSC1
OSC3
CPU clock
* The return destination from the standby status becomes the program execution status prior to shifting to the standby status.
Fig. 4.4.5.1 Status transition diagram for the clock changeover