Switching The Cpu Clocks - Epson 0C88832 Technical Manual

5 PERIPHERAL CIRCUITS AND THEIR OPERATION (Oscillation Circuits and Operating Mode)

5.3.6 Switching the CPU clocks

You can use either OSC1 or OSC3 as the system
clock for the CPU and you can switch over by
means of software.
You can save power by turning the OSC3 oscilla-
tion circuit off while the CPU is operating in OSC1.
When you must operate on OSC3, you can change
to high speed operation by turning the OSC3
oscillation circuit ON and switching over the
system clock. In this case, since several 100 µsec to
several 10 msec are necessary for the oscillation to
stabilize after turning the OSC3 oscillation circuit
ON, you should switch over the clock after
stabilization time has elapsed. (The oscillation start
time will vary somewhat depending on the oscilla-
tor and on the externally attached parts. Refer to
the oscillation start time example indicated in
Chapter 7, "ELECTRICAL CHARACTERISTICS".)
High speed mode
OSC1
OSC3
CPU clock
×
VDC0=
VDC1=1
Normal mode
OSC1
OSC3
CPU clock
VDC0=0
VDC1=0
Low power mode
OSC1
OSC3
CPU clock
Interrupt
OSC1
OSC3
CPU clock
* The return destination from the standby status becomes the program execution status prior to shifting to the standby
status
Fig. 5.3.6.1 Status transition diagram for the operation mode and clock changeover
30
Program Execution Status
High speed mode
OSCC=1
ON OSC1
OFF OSC3
OSC1 CPU clock
OSCC=0
VDC0=0
VDC1=0
RESET
OSCC=1
ON OSC1
OFF OSC3
OSC1 CPU clock
OSCC=0
VDC0=1
VDC1=0
ON
OFF
OSC1
*
HALT instruction
HALT status
ON
ON or OFF
STOP
Standby Status
When switching over from the OSC3 to the OSC1,
turn the OSC3 oscillation circuit OFF immediately
following the clock changeover.
The basic clock switching procedure is as described
above, however, you must also combine it with the
changeover of the operating mode to permit low
current consumption and high speed operation.
Figure 5.3.6.1 indicates the status transition dia-
gram for the operation mode and clock changeover.
Note: When turning ON the OSC3 oscillation circuit
after switching the operating mode, you should
allow a minimum waiting time of 5 msec.
CLKCHG=1
ON
ON
OSC1
CLKCHG=0
Normal mode
CLKCHG=1
ON
ON
OSC1
CLKCHG=0
*
Interrupt
(Input interrupt)
SLEEP status
OSC1
OSC3
CPU clock
EPSON
High speed mode
OSC1 ON
OSC3 ON
CPU clock OSC3
Normal mode
OSC1 ON
OSC3 ON
CPU clock OSC3
SLP instruction
OFF
OFF
STOP
E0C88832/88862 TECHNICAL MANUAL