HT45R35V datasheet(14/42 Pages) HOLTEK

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HT45R35V

Rev. 1.00

January 15, 2009

External RC Oscillator

Using the external system RC oscillator requires that a

resistor, with a value between 24k

W and 1.5MW,iscon-

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 required.For the value of the external resistor ROSC

refer to the Holtek website for typical RC Oscillator vs.

Temperature and VDD characteristics graphics. Note

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

msat5V

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.

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

WS1

WS0

Division Ratio

000

1:1

001

1:2

010

1:4

011

1:8

1:16

1:32

1:64

1:128

WDTS (09H) Register

O S C 1

4 7 0 p F

V D D

R O S C

O S C 2

f S Y S / 4 N M O S O p e n D r a i n

External RC Oscillator

S y s t e m C l o c k / 4

8 - b i t C o u n t e r

W D T P r e s c a l e r

7 - b i t C o u n t e r

8 - t o - 1 M U X

W D T T i m e - o u t

W S 0 ~ W S 2

O p t i o n

S e l e c t

W D T

O S C

Watchdog Timer

HT45R35V Datasheet(PDF) 14 Page - Holtek Semiconductor Inc

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