HT45R35V Holtek Semiconductor Inc., HT45R35V Datasheet - Page 14

• Manufacturer Part Number: HT45R35V
• Description: C/r To F Type 8-bit Otp Mcu
• Manufacturer: Holtek Semiconductor Inc.

External RC Oscillator
Using the external system RC oscillator requires that a
resistor, with a value between 24k and 1.5M , is con-
nected between OSC1 and VDD, and a capacitor is con-
nected to ground. The generated system clock divided
by 4 will be provided on OSC2 as an output which can
be used for external synchronization purposes. Note
that as the OSC2 output is an NMOS open-drain type, a
pull high resistor should be connected if it to be used to
monitor the internal frequency. Although this is a cost ef-
fective oscillator configuration, the oscillation frequency
can vary with VDD, temperature and process variations
and is therefore not suitable for applications where tim-
ing is critical or where accurate oscillator frequencies
are
refer to the Holtek website for typical RC Oscillator vs.
Temperature and VDD characteristics graphics.
that it is the only microcontroller internal circuitry to-
gether with the external resistor, that determine the fre-
quency of the oscillator. The external capacitor shown
on the diagram does not influence the frequency of os-
cillation.
Watchdog Timer Oscillator
The WDT oscillator is a fully self-contained free running
on-chip RC oscillator with a typical period of 65 s at 5V
requiring no external components. When the device en-
ters the Power Down Mode, the system clock will stop
running but the WDT oscillator continues to free-run and
to keep the watchdog active. However, to preserve
power in certain applications the WDT oscillator can be
disabled via a configuration option.
Rev. 1.00
required.For the value of the external resistor R
External RC Oscillator
Watchdog Timer
Note
OSC
14
Watchdog Timer - WDT
The WDT clock can be sourced from its own dedicated
internal oscillator (WDT oscillator), or from the or in-
struction clock, which is the system clock divided by 4.
The choice is determined via a configuration option. The
WDT timer is designed to prevent a software malfunc-
tion or sequence from jumping to an unknown location
with unpredictable results. The Watchdog Timer can be
disabled by a configuration option. If the Watchdog
Timer is disabled, any executions related to the WDT re-
sult in no operation.
The WDT clock source is first divided by 256. If the inter-
nal WDT oscillator is used ,this gives a nominal time-out
period of approximately 17ms at 5V. This time-out pe-
riod may vary with temperatures, VDD and process vari-
ations. By using the WDT prescaler, longer time-out
periods can be realised. Writing data to the WS2, WS1,
WS0 bits in the WDTS register, can give different
time-out periods. If WS2, WS1 and WS0 are all equal to
1, the division ratio will be 1:128, and the maximum
time-out period will be 2.1s at 5V. If the internal WDT os-
cillator is disabled, the WDT clock may still come from
the instruction clock and operate in the same manner
except that in the Power Down state the WDT will stop
counting and lose its protecting purpose. The high nib-
ble and bit 3 of the WDTS can be used for user defined
flags.
If the device operates in a noisy environment, using the
internal WDT oscillator is the recommended choice,
since the HALT instruction will stop the system clock.
WS2
0
0
0
0
1
1
1
1
WS1
0
0
1
1
0
0
1
1
WDTS (09H) Register
WS0
0
1
0
1
0
1
0
1
Division Ratio
January 15, 2009
HT45R35V
1:128
1:16
1:32
1:64
1:1
1:2
1:4
1:8