Epson S1C88650 - Manual (Page 49)

S1C88650 TECHNICAL MANUAL EPSON 41
5 PERIPHERAL CIRCUITS AND THEIR OPERATION (Oscillation Circuits)
5.4 Oscillation Circuits
5.4.1 Configuration of oscillation circuits
The S1C88650 is twin clock system with two
internal oscillation circuits (OSC1 and OSC3).
The OSC3 oscillation circuit generates the main-
clock (Max. 8.2 MHz) to run the CPU and some
peripheral circuits in high speed, and the OSC1
oscillation circuit generates the sub-clock (Typ.
32.768 kHz) for low-power operation.
Figure 5.4.1.1 shows the configuration of the
oscillation circuit.
5.4.3 OSC1 oscillation circuit
The OSC1 oscillation circuit generates the 32.768
kHz (Typ.) system clock which is utilized during
low speed operation (low power mode) of the CPU
and peripheral circuits. Furthermore, even when
OSC3 is utilized as the system clock, OSC1
continues to generate the source clock for the clock
timer and stopwatch timer.
This oscillation circuit stops when the SLP instruc-
tion is executed.
In terms of the oscillation circuit types, either
crystal oscillation or CR oscillation can be selected
with the mask option.
Figure 5.4.3.1 shows the configuration of the OSC1
oscillation circuit.
Fig. 5.4.1.1 Configuration of oscillation circuits
At initial reset, OSC3 oscillation circuit is selected
for the CPU operating clock. ON/OFF switching of
the OSC3 oscillation circuit and switching of the
system clock between OSC3 and OSC1 are control-
led in software. OSC3 circuit is utilized when high
speed operation of the CPU and some peripheral
circuits become necessary. Otherwise, OSC1 should
be used to generate the operating clock and OSC3
circuit placed in a stopped state in order to reduce
current consumption.
5.4.2 Mask option
OSC1 oscillation circuit
■ ■ Crystal oscillation circuit
■ ■ CR oscillation circuit
OSC3 oscillation circuit
■ ■ Crystal oscillation circuit
■ ■ Ceramic oscillation circuit
■ ■ CR oscillation circuit
In terms of the oscillation circuit types for OSC1,
either crystal oscillation or CR oscillation can be
selected with the mask option.
In terms of the oscillation circuit types for OSC3,
either crystal oscillation, ceramic oscillation or CR
oscillation can be selected with the mask option, in
the same way as OSC1.
Note: Do not select CR oscillation for the OSC1
oscillation circuit when crystal oscillation is
selected for the OSC3 oscillation circuit.
When such a selection is made, the OSC3
clock may be supplied to the internal circuits
even though the OSC3 oscillation has not
stabilized.
(2) CR oscillation circuit
Fig. 5.4.3.1 OSC1 oscillation circuit
When crystal oscillation is selected, a crystal
oscillation circuit can be easily formed by connect-
ing a crystal oscillator X'tal1 (Typ. 32.768 kHz)
between the OSC1 and OSC2 terminals along with
a trimmer capacitor C
G1 (5–25 pF) between the
OSC1 terminal and V SS .
When CR oscillation is selected, the CR oscillation
circuit (Max. 200 kHz) is formed merely by
connecting a resistor (R
CR1 ) between OSC1 and
OSC2 terminals.
To CPU (CLK)
OSC1
oscillation circuit
Clock
switch
OSC3
oscillation circuit
To peripheral
circuit
SOSC3
Oscillation circuit
control signal
CLKCHG
CPU clock
selection signal
To some peripheral
circuit
SLEEP
status
Prescaler
(f OSC1 )
(f OSC3 )
OSC2
OSC1
R CR1
SLEEP status
f OSC1
f OSC1
V SS
V SS
OSC2
OSC1
X'tal1
C G1
SLEEP status
(1) Crystal oscillation circuit