HT46R652 datasheet(14/47 Pages) HOLTEK

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HT46R652

Rev. 1.00

December 19, 2006

Watchdog Timer

- WDT

The WDT clock source is implemented by a dedicated

RC oscillator (WDT oscillator) or an instruction clock

(system clock/4) or a real time clock oscillator (RTC os-

cillator). The timer is designed to prevent a software

malfunction or sequence from jumping to an unknown

location with unpredictable results. The WDT can be

disabled by options. But if the WDT is disabled, all exe-

cutions related to the WDT lead to no operation.

If the internal WDT oscillator, which is an RC oscillator

with a nominal period of 65

ms at 5V, is selected, it is di-

vided by 2

12~215 , the actual ratio chosen by configura-

tion option, to get the WDT time-out period. The

minimum period for the WDT time-out period is about

300ms~600ms. This time-out period may vary with tem-

perature, VDD and process variations. By selection the

WDT configuration option, longer time-out periods can

be realised. If the WDT time-out is selected as 2

15, the

maximum time-out period is divided by 2

15~216 about

2.1s~4.3s. If the WDT oscillator is disabled, the WDT

clock may still come from the instruction clock and oper-

ate in the same manner except that in the Power Down

mode the WDT will stop counting and lose its protecting

function. If the device operates in a noisy environment,

using the WDT internal RC oscillator is strongly recom-

mended, since the HALT instruction will stop the system

clock.

The WDT overflow under normal operation initiates a de-

vice reset which sets the status bit

²TO². In the Power

Down mode, the overflow initiates a warm reset, in which

only the Program Counter and Stack Pointer are reset to

zero. To clear the contents of the WDT, there are three

methods that can be adopted. These are, an external re-

set, which is a low level on the RES pin, a software in-

struction and a

²HALT² instruction. There are two types of

software instructions; a single

²CLR WDT² instruction or

the pair of instructions,

²CLR WDT1² and ²CLR WDT2².

Of these two types of instruction, only one type of instruc-

tion can be active at a time depending on a configuration

option

-²CLR WDT² times selection option. If the ²CLR

WDT

² is selected (i.e., CLR WDT times equal one), any

execution of the

²CLR WDT² instruction clears the WDT.

In the case where the two

²CLR WDT1² and ²CLR

WDT2

² instruction are chosen (i.e., CLR WDT times

equal two), these two instructions have to be executed to

clear the WDT; otherwise, the WDT may reset the chip

due to a time-out.

Multi-function Timer

The device provides a multi-function timer for the WDT,

time base and RTC but with different time-out periods.

The multi-function timer consists of an 8-stage divider

and a 7-bit prescaler, with the clock source coming from

the WDT OSC, the RTC OSC or the instruction clock,

which is the system clock divided by 4. The multi-func-

tion timer also provides a selectable frequency signal,

whose division ratio ranges from fS/2

2 to f

S/2

8, for LCD

driver circuits, and a selectable frequency signal, rang-

ing from fS/2

2 to f

S/2

9, for the buzzer output selectable via

configuration options.

It is recommended to select a frequency as close as

possible to 4kHz for the LCD driver circuits to obtain the

best display clarity.

Time Base

The time base offers a periodic time-out period to gener-

ate a regular internal interrupt. Its time-out period

ranges from 2

12/f

S to 2

15/f

S selected by a configuration

option. If a time base time-out occurs, the related inter-

rupt request flag, TBF; bit 5 of INTC1, will be set. If the

interrupt is enabled, and the stack is not full, a subrou-

tine call to location 14H occurs.

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

D i v i d e r

R O M

C o d e

O p t i o n

W D T

O S C

1 2 k H z

R T C

O S C 3 2 7 6 8 H z

f S

f S / 2 8

T i m e - o u t R e s e t

2 1 5 / f S ~ 2 1 6 / f S

2 1 4 / f S ~ 2 1 5 / f S

2 1 3 / f S ~ 2 1 4 / f S

2 1 2 / f S ~ 2 1 3 / f S

W D T

P r e s c a l e r

M a s k O p t i o n

W D T C l e a r

C K

Watchdog Timer

f s

D i v i d e r

P r e s c a l e r

R O M C o d e O p t i o n

L C D D r i v e r ( f S / 2 2 ~ f S / 2 8 )

B u z z e r ( f S / 2 2 ~ f S / 2 9 )

T i m e B a s e I n t e r r u p t

2 1 2 / f S ~ 2 1 5 / f S

R O M

C o d e

O p t i o n

Time Base

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