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II CORE BLOCK: CLG (Clock Generator)
B-II-6-4 EPSON S1C33L03 FUNCTION PART
Setting and Switching Over the CPU Operating Clock
Setting the CPU operating clock frequency
When operating the CPU with the high-speed (OSC3) clock, the operating frequency can be switched over in
four steps. Use CLKDT[1:0] (D[7:6]) / Power control register (0x40180) for this switchover.
Table 6.3 Setting of CPU Operating Clock
CLKDT1 CLKDT0 Division ratio
11 fout/8
10 fout/4
01 fout/2
00 fout/1
fout: PLL output
The clock thus set becomes the system clock, which is used as the CPU operating clock and the bus clock.
At initial reset, the division ratio is set to fout/1, so the CPU is operated directly by the PLL output clock.
Since the device's current consumption can be decreased by reducing the CPU operating speed, switch over
the operating frequency as necessary.
This setting is effective only for the high-speed (OSC3) clock, and has no effect when the low-speed (OSC1)
clock is used as the system clock.
Note: Writing to CLKDT[1:0] is effective only when the power-control register protection flag is set to
"0b10010110".
Switching over the CPU operating clock
Note: The CPU operating clock can be switched from OSC3 to OSC1 only when the low-speed (OSC1)
oscillation circuit in the Peripheral Block is used.
After an initial reset, the CPU starts operating using the OSC3 clock. All internal peripheral circuits also
operate.
In cases in which some peripheral circuits (e.g., programmable timer, serial interface, A/D converter, and
ports) do not need to be operate or processing in low-speed operation is possible, and the CPU can process its
jobs at a low clock speed, the CPU operating clock can be switched to the OSC1 clock, thereby reducing
current consumption. Use CLKCHG (D2) / Power control register (0x40180) to switch over the operating
clock.
Procedure for switching over from the OSC3 clock to the OSC1 clock
1. Turn on the low-speed (OSC1) oscillation circuit (by writing "1" to SOSC1).
2. Wait until the OSC1 oscillation stabilizes (three seconds or more).
3. Change the CPU operating clock (by writing "0" to CLKCHG).
4. Turn off the high-speed (OSC3) oscillation circuit (by writing "0" to SOSC3).
∗ Steps 1 and 2 are required only when the low-speed (OSC1) oscillation circuit is inactive.
Notes:•Use separate instructions to switch from OSC3 to OSC1 and turn the OSC3 oscillation off. If
these operations are processed simultaneously using one instruction, the CPU may operate
erratically.
•Make sure the operation of the peripheral circuits, such as the programmable timer and serial
interface is terminated before the OSC3 oscillation is turned off in order to prevent them from
operating erratically or the prescaler clock is set as OSC1. In addition, in order to prevent
incorrect operation, a setup of prescaler must be performed before changing the CPU clock.
Procedure for switching over from the OSC1 clock to the OSC3 clock
1. Turn on the high-speed (OSC3) oscillation circuit (by writing "1" to SOSC3).
2. Wait until the OSC3 oscillation stabilizes (10 ms or more for a 3.3-V crystal resonator).
3. Switch over the CPU operating clock (by writing "1" to CLKCHG).
Note: The operating clock switchover by CLKCHG is effective only when both oscillation circuits are on
and the power-control register protection flag is set to "0b10010110".