
CHAPTER 6 CLOCK GENERATOR
User’s Manual U17260EJ6V0UD
160
6.6 Controlling Clock
6.6.1 Controlling high-speed system clock
The following two types of high-speed system clocks are available.
X1 clock:
Crystal/ceramic resonator is connected across the X1 and X2 pins.
External main system clock: External clock is input to the EXCLK pin.
When the high-speed system clock is not used, the X1/P121 and X2/EXCLK/P122 pins can be used as I/O port
pins.
Caution
The X1/P121 and X2/EXCLK/P122 pins are in the I/O port mode after a reset release.
The following describes examples of setting procedures for the following cases.
(1) When oscillating X1 clock
(2) When using external main system clock
(3) When using high-speed system clock as CPU clock and peripheral hardware clock
(4) When stopping high-speed system clock
(1) Example of setting procedure when oscillating the X1 clock
<1> Setting frequency (OSCCTL register)
Using AMPH, set the gain of the on-chip oscillator according to the frequency to be used.
AMPH
Note
Operating Frequency Control
0
1 MHz
≤ fXH ≤ 10 MHz
1
10 MHz < fXH
≤ 20 MHz
Note
Set AMPH before setting the peripheral functions after a reset release. The value of AMPH can
be changed only once after a reset release. When AMPH is set to 1, the clock supply to the CPU
is stopped for 4.06 to 16.12
s.
Remark
fXH: High-speed system clock oscillation frequency
<2> Setting P121/X1 and P122/X2/EXCLK pins and selecting X1 clock or external clock (OSCCTL register)
When EXCLK is cleared to 0 and OSCSEL is set to 1, the mode is switched from port mode to X1
oscillation mode.
EXCLK
OSCSEL
Operation Mode of High-
Speed System Clock Pin
P121/X1 Pin
P122/X2/EXCLK Pin
0
1
X1 oscillation mode
Crystal/ceramic resonator connection
<3> Controlling oscillation of X1 clock (MOC register)
If MSTOP is cleared to 0, the X1 oscillator starts oscillating.
<4> Waiting for the stabilization of the oscillation of X1 clock
Check the OSTC register and wait for the necessary time.
During the wait time, other software processing can be executed with the internal high-speed oscillation
clock.
Caution
1. Do not change the value of EXCLK and OSCSEL while the X1 clock is operating.