
54
EPSON
S1C88409 TECHNICAL MANUAL
CHAPTER 5: PERIPHERAL CIRCUITS AND OPERATION (Oscillation Circuit)
5.4 Oscillation Circuit
5.4.1 Configuration of oscillation circuit
The S1C88409 has been designed as a twin clock
system with two internal oscillation circuits (OSC1
and OSC3). The OSC1 oscillation circuit generates
the 32.768 kHz (Typ.) operating clock, and the
OSC3 oscillation circuit generates the high-speed
clock for the CPU and the peripheral circuits
(serial interface, programmable timer, A/D
converter, etc.).
Figure 5.4.1.1 shows the configuration of the
oscillation circuit.
To CPU (CLK)
OSC1
oscillation circuit
Clock
switch
OSC3
oscillation circuit
To peripheral
circuit (fOSC1)
OSCC
Oscillation circuit
control circuit
CLKCHG
CPU clock
selection circuit
To peripheral
circuit (fOSC3)
SLEEP
status
Fig. 5.4.1.1 Configuration of oscillation circuit
At initial reset, the OSC1 oscillation clock is
selected for the CPU operating clock.
Turning the OSC3 oscillation circuit on/off and
switching the system clock (OSC1
OSC3) can be
controlled by software. The OSC3 oscillation
circuit is used when the CPU and the peripheral
circuits require high-speed operation. When the
CPU has to handle low-speed operation (e.g. clock
control), stop the OSC3 oscillation and use OSC1
as the operating clock to reduce current consump-
tion.
5.4.2 Mask option
OSC1 oscillation circuit
s
s Crystal oscillation circuit
s
s CR oscillation circuit
s
s External clock input
s
s Crystal oscillation circuit (gate capacitor built-in)
OSC3 oscillation circuit
s
s Crystal oscillation circuit
s
s Ceramic oscillation circuit
s
s CR oscillation circuit
s
s External clock input
Either "Crystal oscillation", "CR oscillation",
"Crystal oscillation (gate capacitor built-in)" or
"External clock input" can be selected by mask
option as a kind of OSC1 oscillation circuit.
Either "Crystal oscillation", "Ceramic oscillation",
"CR oscillation" or "External clock input" can be
selected as a kind of OSC3 oscillation circuit same
as the OSC1.
5.4.3 OSC1 oscillation circuit
The OSC1 oscillation circuit generates the 32.768
kHz (Typ.) system clock. The OSC1 oscillation
clock is used for the low-speed (low power)
operation clock of the CPU. Furthermore, it is used
as the source clock for the clock timer and watch-
dog timer even when OSC3 is used as the system
clock. The OSC1 oscillation circuit stops when the
SLP instruction is executed.
Either "Crystal oscillation", "CR oscillation",
"Crystal oscillation (gate capacitor built-in)" or
"External clock input" can be selected by mask
option as a kind of OSC1 oscillation circuit.
Figure 5.4.3.1 shows the structure of the OSC1
oscillation circuit.
VSS
OSC2
OSC1
X'tal1
CG1
fOSC1
SLEEP status
(1) Crystal oscillation circuit
OSC2
OSC1
fOSC1
SLEEP status
(2) CR oscillation circuit
OSC2
OSC1
External
clock
N.C.
VSS
VDD
VSS
fOSC1
SLEEP status
(3) External clock input
VSS
OSC2
OSC1
fOSC1
SLEEP status
VSS
X'tal1
(4) Crystal oscillation circuit (gate capacitor built-in)
Fig. 5.4.3.1 OSC1 oscillation circuit