HT47C20 Holtek Semiconductor Inc, HT47C20 Datasheet - Page 16

• Manufacturer Part Number: HT47C20
• Description: 8-Bit Microcontroller
• Manufacturer: Holtek Semiconductor Inc

Oscillator configuration
There are two oscillator circuits in the HT47C20.
Both are designed for system clocks; the RC oscil-
lator and the crystal oscillator, which are deter-
mined by mask option. No matter what oscillator
type is selected, the signal provides the system
clock. The halt mode stops the system oscillator
and ignores an external signal to conserve power.
The OSC1 and OSC2 are at the same level
when the system enters the power down mode.
If an RC oscillator is used, an external resistor
between OSC1 and OSC2 is need and the resis-
tance must range from 51kW to 1MW. The RC
oscillator provides the most cost effective solu-
tion. However, the frequency of the oscillation
may vary with VDD, temperature and the chip
itself due to process variations. It is, therefore,
not suitable for timing sensitive operations
where accurate oscillator frequency is desired.
If a crystal oscillator is used, a crystal across
OSC1 and OSC2 is needed to provide the feed-
back and phase shift needed for oscillator, no
other external components are needed. Instead
of a crystal, a resonator can also be connected
between OSC1 and OSC2 to get a frequency ref-
erence, but two external capacitors in OSC1
and OSC2 are required.
There is another oscillator circuit designed for
the real time clock. In this case, only the
32768kHz crystal oscillator can be applied. The
crystal should be connected between OSC3 and
OSC4, and two external capacitors along with
one external resistor are required for the oscil-
lator circuit in order to get a stable frequency.
The RTC oscillator circuit can be controlled to
oscillate quickly by setting ²SAVE² bit (bit 4 of
RTCC) to ²0². After power on reset, the ²SAVE²
bi t ini ti al v a lu e i s ² 0 ² t ha t i s on the
System oscillator
16
quick-oscillate mode. It¢s recommended to turn
it off by setting the ²SAVE² bit ²1² after a period
ofabout2secondstoavoiddrainingextrapower.
The WDT oscillator is a free running on-chip
RC oscillator, and no external components are
required. Even if the system enters the power
down mode, the system clock is stopped, but the
WDT oscillator still works with a period of ap-
proximately 90ms. The WDT oscillator can be
disabled by mask option to conserve power.
Watchdog timer - WDT
The clock source of the WDT(f
by a dedicated RC oscillator (WDT oscillator) or
a instruction clock (system clock divided by 4)
or a real time clock oscillator (RTC oscillator),
decided by mask options. The timer is designed
to prevent a software malfunction or sequence
jumping to an unknown location with unpre-
dictable results. The watchdog timer can be dis-
abled by a mask option. If the watchdog timer is
disabled, all the executions related to the WDT
result in no operation.
If the clock source of WDT chooses the internal
WDT oscillator, the time-out period may vary
with temperature, VDD, and process varia-
tions. On the other hand, if the clock source se-
lects the instruction clock and the ²HALT²
instruction is executed, WDT may stop count-
ing and lose its protecting purpose, and the
logic can only be restarted by external logic.
When the device operates in a noisy environ-
ment, using the on-chip RC oscillator (WDT
OSC) is strongly recommended, since the HALT
can cease the system clock.
The WDT overflow under normal operation will
initialize ²chip reset² and set the status bit TO.
Whereas in the halt mode, the overflow will ini-
tialize a ²warm reset² only the PC and SP are
reset to zero. To clear the contents of WDT, three
RTC oscillator
s
) is implemented
January 18, 2000
HT47C20