7 CLOCK GENERATOR (CLG)
7-4
Seiko Epson Corporation
S1C17651 TECHNICAL MANUAL
OSC3A Oscillator
7.3.2
The OSC3A oscillator is a high-precision, high-speed oscillator circuit that uses either a crystal resonator or a ceramic
resonator. It can be switched for use with the OSC3B oscillator. Figure 7.3.2.1 shows the OSC3A oscillator configura-
tion.
OSC3AWT[1:0]
fOSC3A
OSC3AEN
Oscillation stabilization
wait circuit
VSS
OSC4
OSC3
Rf
RD
CD3
CG3
X'tal3
or
Ceramic
SLEEP/NORMAL
3.2.1 OSC3A Oscillator Circuit
Figure 7.
A crystal resonator (X’tal3) or ceramic resonator (Ceramic) should be connected between the OSC3 and OSC4
pins. Additionally, two capacitors (CG3 and CD3) should be connected between the OSC3/OSC4 pins and VSS.
For the effective frequency range and oscillation characteristics, see the “Electrical Characteristics” chapter.
OSC3A oscillation on/off
The OSC3A oscillator circuit starts oscillating when OSC3AEN/CLG_CTL register is set to 1 and stops oscil-
lating when set to 0. The OSC3A oscillator circuit stops oscillating in SLEEP mode.
After an initial reset, OSC3AEN is set to 0, and the OSC3A oscillator circuit is halted.
Stabilization wait time at start of OSC3A oscillation
The OSC3A oscillator circuit includes an oscillation stabilization wait circuit to prevent malfunctions due to
unstable clock operations at the start of OSC3A oscillation—e.g., when the OSC3A oscillator is turned on with
software. The OSC3A clock is not supplied to the system until the time set for this circuit has elapsed. Use OS-
C3AWT[1:0]/CLG_WAIT register to select one of four oscillation stabilization wait times. For the oscillation
start time, see the “Electrical Characteristics” chapter.
3.2.1 OSC3A Oscillation Stabilization Wait Time Settings
Table 7.
OSC3AWT[1:0]
Oscillation stabilization wait time
0x3
128 cycles
0x2
256 cycles
0x1
512 cycles
0x0
1024 cycles
(Default: 0x0)
This is set to 1,024 cycles (OSC3A clock) after an initial reset.
When the system clock is switched to OSC3A immediately after the OSC3A oscillator circuit is turned on, the
OSC3A clock is supplied to the system after the OSC3A clock system supply wait time indicated below (at a
maximum) has elapsed. For the oscillation start time, see the “Electrical Characteristics” chapter.
OSC3A clock system supply wait time
≤ OSC3A oscillation start time (max.) + OSC3A oscillation
stabilization wait time
Note: Oscillation stability will vary, depending on the resonator and other external components. Carefully
consider the OSC3A oscillation stabilization wait time before reducing the time.
OSC1 Oscillator
7.3.3
The S1C17651 has two low-speed oscillator circuits (OSC1A and OSC1B) and either one can be selected as the
OSC1 oscillator.
The OSC1 clock is generally used as the timer operation clock (for the real-time clock, clock timer, watchdog
timer, and 16-bit PWM timer) and an operation clock for the UART, sound generator, and LCD driver. It can be
used as the system clock instead of the OSC3B or OSC3A clock to reduce power consumption when no high-speed
processing is required.