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EPSON
S1C63358 TECHNICAL MANUAL
CHAPTER 4: PERIPHERAL CIRCUITS AND OPERATION (Oscillation Circuit)
CLKCHG: CPU system clock switching register (FF00H•D3)
The CPU's operation clock is selected with this register.
When "1" is written: OSC3 clock is selected
When "0" is written: OSC1 clock is selected
Reading: Valid
When the CPU clock is to be OSC3, set CLKCHG to "1"; for OSC1, set CLKCHG to "0".
After turning the OSC3 oscillation ON (OSCC = "1"), switching of the clock should be done after waiting
5 msec or more.
When V
D1
is 1.35 V (VDC = "0") and when OSC3 oscillation is OFF (OSCC = "0"), setting of CLKCHG =
"1" becomes invalid and switching to OSC3 is not performed. When the CR oscillation circuit has been
selected as the OSC1 oscillation circuit by mask option, setting VDC to "0" makes no difference.
At initial reset, this register is set to "0".
4.4.7 Programming notes
(1) When switching the CPU system clock from OSC1 to OSC3, first set V
D1
. After that maintain 2.5 msec
or more, and then turn the OSC3 oscillation ON.
When switching from OSC3 to OSC1, set V
D1
after switching to OSC1 and turning the OSC3 oscilla-
tion OFF. However, when the CR oscillation circuit has been selected as the OSC1 oscillation circuit, it
is not necessary to set V
D1
.
(2) It takes at least 5 msec from the time the OSC3 oscillation circuit goes ON until the oscillation stabi-
lizes. Consequently, when switching the CPU operation clock from OSC1 to OSC3, do this after a
minimum of 5 msec have elapsed since the OSC3 oscillation went ON.
Further, the oscillation stabilization time varies depending on the external oscillator characteristics
and conditions of use, so allow ample margin when setting the wait time.
(3) When switching the clock form OSC3 to OSC1, use a separate instruction for switching the OSC3
oscillation OFF. An error in the CPU operation can result if this processing is performed at the same
time by the one instruction.
(4) When the CR oscillation circuit has been selected as the OSC1 oscillation circuit by mask option, it is
not necessary to switch the operating voltage V
D1
using the VDC register and the V
D1
voltage is fixed
at 2.25 V. The V
D1
level does not change even if any data is written to the VDC register.
