
iiiPeriPheraLModuLes1(systeM):reaL-tiMeCLoCk(rtC)
s1C33L17teChniCaLManuaL
ePson
iii-3-11
III
RTC
iii.3.5osC1oscillatorCircuit
The S1C33L17 contains an oscillator circuit (OSC1) used to generate a 32.768 kHz (typ.) clock as the clock source
for timekeeping operation of the RTC.
The OSC1 clock can also be used as a power-saving operating clock for the core system or peripheral circuits. For
details, see Section III.1, “Clock Management Unit (CMU).”
iii.3.5.1input/outputPinsoftheosC1oscillatorCircuit
Table III.3.5.1.1 lists the input/output pins of the OSC1 oscillator circuit.
TableIII.3.5.1.1Input/OutputPinsoftheLow-speed(OSC1)OscillatorCircuit
Pinname
RTC_CLKI
RTC_CLKO
i/o
I
O
Function
OSC1inputpin:Crystaloscillatororexternalclockinput
OSC1outputpin:Crystaloscillatoroutput(leftopenwhenexternalclockisinput)
iii.3.5.2structureoftheosC1oscillatorCircuit
The OSC1 oscillator circuit accommodates a crystal oscillator and external clock input. As for the RTC, VDD is
used to supply power to this circuit.
Figure III.3.5.2.1 shows the structure of the OSC1 oscillator circuit.
Lowlevel
Oscillationcircuit
controlsignal
Oscillationcircuit
controlsignal
OSC1
RTC_CLKO
RTC_CLKI
External
clock
N.C.
VSS
VDD
OSC1
(1)Crystaloscillationcircuit
VSS
RTC_CLKO
RTC_CLKI
Oscillationcircuit
controlsignal
(3)Whennotused
(2)Externalclockinput
VSS
RTC_CLKO
RTC_CLKI
CD1
CG1
X'tal1
Rf
Rd
FigureIII.3.5.2.1OSC1OscillatorCircuit
For use as a crystal oscillator circuit, connect a crystal resonator X’tal1 (32.768 kHz, typ.), feedback resistor (Rf),
two capacitors (CG1, CD1), and, if necessary, a drain resistor (Rd) to the RTC_CLKI and RTC_CLKO pins and VSS,
as shown in Figure III.3.5.2.1 (1).
To use an external clock, leave the RTC_CLKO pin open and input a VDD level clock (whose duty cycle is 50%) to
the RTC_CLKI pin. Do not input VDDH or other I/O level clocks.
The oscillator frequency/input clock frequency is 32.768 kHz (typ.). Make sure the crystal resonator or external
clock used in the RTC has this clock frequency. With any other clock frequencies, the RTC cannot be used for
timekeeping purposes.
For details of oscillation characteristics and the input characteristics of external clock, see “Electrical
Characteristics.”
When not using the OSC1 oscillator circuit, connect the RTC_CLKI pin to VSS and leave the RTC_CLKO pin open.