
56
EPSON
S1C88409 TECHNICAL MANUAL
CHAPTER 5: PERIPHERAL CIRCUITS AND OPERATION (Oscillation Circuit)
5.4.5 Switching of CPU clock
and operating voltage VD1
The OSC3 oscillation circuit can be turned on and
off. It should be turned on when operating periph-
eral circuit (serial interface, programmable timer,
A/D converter, etc.) that needs a high-speed clock.
Further, the CPU operating clock can be switched
from OSC1 to OSC3 to execute the program in
high-speed.
In the S1C88409, the internal operating voltage VD1
can also be switched by software for stable opera-
tion in large supply voltage and operating fre-
quency ranges and for saving power.
Switching operating voltage VD1
When operating the OSC3 oscillation circuit, it is
necessary to switch the internal operating voltage
VD1 according to the oscillation frequency. Table
5.4.5.1 shows the relation between the supply
voltage to be used, oscillation frequency and the
internal operating voltage VD1 that should be set.
Table 5.4.5.1 Oscillation frequency and internal
operating voltage VD1
OSC3
oscillator
OFF
ON
OSC3
frequency
0.03–1.1 MHz
0.03–4.4 MHz
0.03–6.6 MHz
0.03–8.8 MHz
Supply
voltage
1.8–5.5 V
2.6–5.5 V
3.5–5.5 V
4.5–5.5 V
1.6 V
q
×
2.4 V
v
q
×
4.2 V
v
q
VD1
3.2 V
v
q
×
q
v
×
Operation with OSC1 clock
Can be set
Can be set (However, it increases current consumption)
Cannot be set
VD1 is switched with the VD1 level setting register
VD1C.
Table 5.4.5.2 VD1 settings
VD1C1
1
0
VD1C0
1
0
1
0
Operating voltage VD1
4.2 V
3.2 V
1.6 V
2.4 V
At initial reset, VD1 is set to 2.4 V (Typ.). VD1
should be switched to 1.6 V (Typ.) when using an
OSC3 clock lower than 1.1 MHz or the OSC3
oscillation is stopped.
Note: The VD1 level must be switched while the
OSC3 oscillation circuit is off (before
turning on and after turning off). Switch-
ing during operation may cause malfunc-
tion.
The VD1 voltage required at least 5 msec
of voltage stabilizing time after switching.
Do not turn the OSC3 oscillation circuit on
during this period.
VD1 cannot be switched directly to a level
that is two or three levels different from
the current level. The middle level must
be set between switching.
To switch from 1,6 (3.2) V to 3.2 (1.6) V:
1.6 V
→ 2.4 V → 3.2 V
1.6 V
← 2.4 V ← 3.2 V
To switch from 1.6 (4.2) V to 4.2 (1.6) V:
1.6 V
→ 2.4 V → 3.2 V → 4.2 V
1.6 V
← 2.4 V ← 3.2 V ← 4.2 V
To switch from 2.4 (4.2) V to 4.2 (2.4) V:
2.4 V
→ 3.2 V → 4.2 V
2.4 V
← 3.2 V ← 4.2 V
A 5 msec interval is required for each
switching step.
Turning the OSC3 oscillation circuit on and off
The OSC3 oscillation circuit can be turned on and
off using the OSCC register. It is necessary to
switch the VD1 voltage.
The switching procedure is as follows. The follow-
ing procedures are described assuming the VD1 has
been set to 1.6 V before turning the OSC3 oscilla-
tion on.
[OSC3 on sequence]
OSC3 = 0.03 MHz–1.1 MHz
1. Write "1" to the OSCC register.
(turning the OSC3 oscillation circuit on)
2. Wait at least 20 msec.
OSC3 = 0.03 MHz–4.4 MHz
1. Write "00B" to the VD1C register. (VD1 = 2.4 V)
2. Wait at least 5 msec.
3. Write "1" to the OSCC register.
(turning the OSC3 oscillation circuit on)
4. Wait at least 20 msec.
OSC3 = 0.03 MHz–6.6 MHz
1. Write "00B" to the VD1C register. (VD1 = 2.4 V)
2. Wait at least 5 msec.
3. Write "10B" to the VD1C register. (VD1 = 3.2 V)
4. Wait at least 5 msec.
5. Write "1" to the OSCC register.
(turning the OSC3 oscillation circuit on)
6. Wait at least 20 msec.