HT83F02 datasheet(43/64 Pages) HOLTEK

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HT83F02

Rev. 1.20

November 19, 2010

Low Voltage Reset

- LVR

The microcontroller contains a low voltage reset circuit

in order to monitor the supply voltage of the device,

which is selected via a configuration option. If the supply

voltage of the device drops to within a range of

0.9V~VLVR such as might occur when changing the

battery, the LVR will automatically reset the device inter-

nally.

Low Voltage Detector

- LVD

This Low Voltage Detect internal function provides a

means for the user to monitor when the power supply

voltage falls below a certain fixed level as specified in

the DC characteristics. Bit 2 of PWMC register and Low

Voltage Detector option are used to control the overall

function of the LVD. Low Voltage Detector option is the

enable/disable control bit, when select disable the over-

all function of the LVD will be disabled.

Bit 2 is the LVD detector output bit and is known as

LVDF. Under normal operation, and when the power

supply voltage is above the specified VLVD value in the

DC characteristic section, the LVDF bit will remain at a

zero value. If the power supply voltage should fall below

this VLVD value then the LVDF bit will change to a high

value indicating a low voltage condition. Note that the

LVDF bit is a read-only bit. By polling the LVDF bit in the

PWMC register, the application program can therefore

determine the presence of a low voltage condition.

Operation

The Low Voltage Detector must first be enabled using a

configuration option.

The LVD control bit is bit 2 of the PWMC regsiter and is

known as LVDF. Under normal operation, and when the

power supply voltage is above the specified VLVD value

in the DC characteristic section, the LVDF bit will remain

at a zero value. If the power supply voltage should fall be-

low this VLVD value then the LVDF bit will change to a

high value indicating a low voltage condition. Note that

the LVDF bit is a read-only bit. By polling the LVDF bit in

the PWMC register, the application program can there-

fore determine the presence of a low voltage condition.

Watchdog Timer

The Watchdog Timer is provided to prevent program

malfunctions or sequences from jumping to unknown lo-

cations, due to certain uncontrollable external events

such as electrical noise. It operates by providing a de-

vice reset when the WDT counter overflows. The WDT

clock is supplied by one of two sources selected by con-

figuration option: its own self-contained dedicated inter-

nal WDT oscillator, or the instruction clock which is the

system clock divided by 4. Note that if the WDT configu-

ration option has been disabled, then any instruction re-

lating to its operation will result in no operation.

The internal WDT oscillator has an approximate period

of 65

ms at a supply voltage of 5V. If selected, it is first di-

vided by 256 via an 8-stage counter to give a nominal

period of 17ms. Note that this period can vary with VDD,

temperature and process variations. For longer WDT

time-out periods the WDT prescaler can be utilized. By

writing the required value to bits 0, 1 and 2 of the WDTS

periods can be achieved. With WS0, WS1 and WS2 all

equal to 1, the division ratio is 1:128 which gives a maxi-

mum time-out period of about 2.1s.

A configuration option can select the instruction clock,

which is the system clock divided by 4, as the WDT clock

source instead of the internal WDT oscillator. If the in-

struction clock is used as the clock source, it must be

noted that when the system enters the Power Down

Mode, as the system clock is stopped, then the WDT

clock source will also be stopped. Therefore the WDT

will lose its protecting purposes. In such cases the sys-

tem cannot be restarted by the WDT and can only be re-

started using external signals. For systems that operate

in noisy environments, using the internal WDT oscillator

is therefore the recommended choice.

Under normal program operation, a WDT time-out will

initialise a device reset and set the status bit TO. How-

ever, if the system is in the Power Down Mode, when a

WDT time-out occurs, only the Program Counter and

Stack Pointer will be reset. Three methods can be

adopted to clear the contents of the WDT and the WDT

prescaler. The first is an external hardware reset, which

means a low level on the RES pin, the second is using

the watchdog software instructions and the third is via a

²HALT² instruction.

There are two methods of using software instructions to

clear the Watchdog Timer, one of which must be chosen

by configuration option. The first option is to use the sin-

gle

²CLR WDT² instruction while the second is to use

the two commands

²CLR WDT1² and ²CLR WDT2². For

the first option, a simple execution of

²CLR WDT² will

clear the WDT while for the second option, both

²CLR

WDT1

² and ²CLR WDT2² must both be executed to

successfully clear the WDT. Note that for this second

option, if

²CLR WDT1² is used to clear the WDT, succes-

sive executions of this instruction will have no effect,

only the execution of a

²CLR WDT2² instruction will

clear the WDT. Similarly, after the

²CLR WDT2² instruc-

tion has been executed, only a successive

²CLR WDT1²

instruction can clear the Watchdog Timer.

HT83F02 Datasheet(PDF) 43 Page - Holtek Semiconductor Inc

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