24
EPSON
S1C63454 TECHNICAL MANUAL
CHAPTER 4: PERIPHERAL CIRCUITS AND OPERATION (Oscillation Circuit)
4.3.4 Switching of operating voltage
(1) When OSC1 crystal oscillation circuit is used
The CPU system clock is switched to OSC1 or OSC3 by the software (CLKCHG register). In this case,
to obtain stable operation, the operating voltage V
D1
for the internal circuits must be switched by the
software (VDC register).
OSC1 (crystal oscillation) operation:
V
D1
= 1.3 V
(VDC = "0")
OSC3 operation:
V
D1
= 2.2 V
(VDC = "1")
When OSC3 is to be used as the CPU system clock, it should be done as the following procedure using
the software: first switch the operating voltage V
D1
, turn the OSC3 oscillation ON after waiting 2.5
msec or more for the above operation to stabilize, switch the clock after waiting 5 msec or more for
oscillation stabilization.
When switching from OSC3 to OSC1, turn the OSC3 oscillation circuit OFF after switching the clock
then set the operating voltage V
D1
to 1.3 V.
OSC1
→
OSC3
OSC3
→
OSC1
1. Set VDC to "1" (1.3 V
→
2.2 V).
1. Set CLKCHG to "0" (OSC3
→
OSC1).
2. Maintain 2.5 msec or more.
2. Set OSCC to "0" (OSC3 oscillation OFF).
3. Set OSCC to "1" (OSC3 oscillation ON).
3. Set VDC to "0" (2.2 V
→
1.3 V).
4. Maintain 5 msec or more.
5. Set CLKCHG to "1" (OSC1
→
OSC3).
(2) When OSC1 CR oscillation circuit is used
When the CR oscillation circuit has been selected as the OSC1 oscillation circuit by mask option, 2.2 V
of V
D1
necessary to operate with OSC1 and OSC3.
OSC1 (CR oscillation) operation:
V
D1
= 2.2 V
OSC3 operation:
V
D1
= 2.2 V
Since the S1C63454 fixes the V
D1
voltage value at 2.2 V when the CR oscillation circuit is selected as
the OSC1 oscillation circuit by mask option, it is not necessary to switch the operating voltage V
D1
by
software (VDC register). However, software control to switch the CPU system clock using the
CLKCHG register is necessary.
When OSC3 is to be used as the CPU system clock, it should be done as the following procedure using
the software: turn the OSC3 oscillation ON, switch the clock after waiting 5 msec or more for oscilla-
tion stabilization.
When switching from OSC3 to OSC1, turn the OSC3 oscillation circuit OFF after switching the clock.
OSC1
→
OSC3
OSC3
→
OSC1
1. Set OSCC to "1" (OSC3 oscillation ON).
1. Set CLKCHG to "0" (OSC3
→
OSC1).
2. Maintain 5 msec or more.
2. Set OSCC to "0" (OSC3 oscillation OFF).
3. Set CLKCHG to "1" (OSC1
→
OSC3).
4.3.5 Clock frequency and instruction execution time
Table 4.3.5.1 shows the instruction execution time according to each frequency of the system clock.
Table 4.3.5.1 Clock frequency and instruction execution time
Clock frequency
OSC1: 32.768 kHz
OSC1: 60 kHz
OSC3: 4 MHz
Instruction execution time (
µ
sec)
1-cycle instruction
2-cycle instruction
3-cycle instruction
61
122
183
33
67
100
0.5
1
1.5
