HT9480 datasheet(16/57 Pages) HOLTEK

16 / 57 page

Watchdog timer

mode initializes a “warm reset” only when the

PC and SP are reset to zero. To clear the con-

tents of the WDT (including the WDT pres-

caler), there are three methods to be adopted

namely, external reset (a low level to RES),

software instruction(s), and a “HALT” instruc-

tion. There are two types of software instruc-

tions,

“CLR WDT” and “CLR WDT1”/“CLR

WDT2”. But only one of these two types of in-

structions can be active at a time depending on

the mask option

− “CLR WDT times selection

option”. If the “CLR WDT” is selected (i.e.,

CLRWDT times equal one), any execution of the

“CLR WDT” instruction clears the WDT. In the

case that “CLR WDT1” and “CLR WDT2” are

chosen (i.e., CLRWDR times equal two), these

two instructions should be executed to clear the

WDT; otherwise, the WDT may reset the chip

due to a time-out.

Powerdown operation – HALT

The HALT mode is initialized by the “HALT”

instruction and results in the following.

The system turns off. The low power oscillator,

tone generator, LCD driver, pager decoder, and

WDT oscillator all keep running (if the WDT

oscillator is selected).

The contents of the on–chip RAM and of the

registers remain unchanged.

The WDT and the WDT prescaler are cleared

and counted again (if the WDT clock is from the

WDT oscillator).

All the I/O ports remain in their original status.

The PD flag is set but the TO flag is cleared.

The system can quit the HALT mode by an

external reset, an interrupt, an external falling

edge signal on port A, or a WDT overflow. An

external reset leads to device initialization and

the WDT overflow performs a “warm reset”.

After the TO and PD flags are examined, the

reason for the chip reset is determined. The PD

flag that is cleared on power-up is set after the

“HALT” instruction is executed. The TO flag is

set when the WDT time-out occurs, which

causes a wake-up that resets only the PC and

SP, and leaves the others in their original status.

The port A wake-up and interrupt methods can

be considered as a continuation of normal exe-

cution. Every bit in port A can be independently

selected to wake up the device by mask option.

Awakening from an I/O port stimulation, the

program resumes execution of the next instruc-

tion. However, if the program awakens from an

interrupt, two sequences may occur. The pro-

gram will resume execution at the next instruc-

tion if the related interrupt(s) is (are) disabled

or the interrupt(s) is (are) enabled but the stack

is full. A regular interrupt response, on the

other hand, may take place if the interrupt is

enabled and the stack is not full.

If the wake-up event(s) occurs and the wake-up

results from an interrupt acknowledge, the actual

interrupt subroutine execution is delayed by one

or more cycles. On the other hand, if the wake-

up brings about the following instruction execu-

tion, the actual interrupt subroutine is executed

immediately after the dummy period is completed.

To minimize power consumption, the I/O pins

should all be carefully managed before entering

the HALT status.

HT9480

23th Feb ’98

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