HT36F2 datasheet(10/36 Pages) HOLTEK

10 / 36 page

HT36F2

Rev. 1.00

August 15, 2005

On the other hand, if the crystal oscillator is selected, a

crystal across OSC1 and OSC2 is needed to provide the

feedback and phase shift required for the oscillator, and

no other external components are required. A resonator

may be connected between OSC1 and OSC2 to replace

the crystal and to get a frequency reference, but two ex-

ternal capacitors in OSC1 and OSC2 are required.

The WDT oscillator is a free running on-chip RC oscilla-

tor, 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 approximately 78

ms. The WDT oscillator can

be disabled by mask option to conserve power.

Watchdog Timer

- WDT

The WDT clock source is implemented by a dedicated

RC oscillator (WDT oscillator) or instruction clock (sys-

tem clock of the MCU divided by 4), determined by mask

options. This timer is designed to prevent a software

malfunction or sequence jumping to an unknown loca-

tion with unpredictable results. The Watchdog Timer can

be disabled by mask option. If the Watchdog Timer is

disabled, all the executions related to the WDT result in

no operation.

Once the internal WDT oscillator (RC oscillator with a

period of 78

ms normally) is selected, it is first divided by

256 (8-stages) to get the nominal time-out period of ap-

proximately 20ms. This time-out period may vary with

temperature, VDD and process variations. By invoking

the WDT prescaler, longer time-out periods can be real-

ized. Writing data to WS2, WS1, WS0 (bit 2,1,0 of the

WDTS) can give different time-out periods. If WS2,

WS1, WS0 all equal to 1, the division ratio is up to 1:128,

and the maximum time-out period is 2.6 seconds.

If the WDT oscillator is disabled, the WDT clock may still

come from the instruction clock and operate in the same

manner except that in the HALT state the WDT may stop

counting and lose its protecting purpose. In this situation

the logic can only be restarted by external logic. The

high nibble and bit 3 of the WDTS are reserved for user

defined flags, and the programmer may use these flags

to indicate some specified status.

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

If the device operates in a noisy environment, using the

on-chip RC oscillator (WDT OSC) is strongly recom-

mended, since the HALT will stop 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 initialize a

²warm reset²

only the Program Counter and SP are reset to zero. To

clear the WDT contents (including the WDT prescaler ),

3 methods are implemented; external reset (a low level

to RES), software instructions, or a

²HALT² instruction.

The software instructions include

²CLR WDT² and the

other set

-²CLR WDT1² and ²CLR WDT2². Of these

two types of instructions, only one can be active de-

pending on the mask option

-²CLR WDT times selec-

tion option

².Ifthe ²CLR WDT² is selected (i.e. CLRWDT

times equal one), any execution of the

²CLR WDT² in-

struction will clear the WDT. In case

²CLR WDT1² and

²CLR WDT2² are chosen (i.e. CLRWDT times equal

two), these two instructions must be executed to clear

the WDT; otherwise, the WDT may reset the chip be-

cause of time-out.

S y s t e m C l o c k / 8

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

M a s k

O p t i o n

S e l e c t

W D T

O S C

Watchdog Timer

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