HT56R656 datasheet(76/108 Pages) HOLTEK

76 / 108 page

HT56R62/HT56R65

HT56R642/HT56R644/HT56R654/HT56R656

Rev. 1.00

July 20, 2009

Another reset exists in the form of a Low Voltage Reset,

LVR, where a full reset, similar to the RES reset is imple-

mented in situations where the power supply voltage

falls below a certain threshold.

Reset Functions

There are five ways in which a microcontroller reset can

occur, through events occurring both internally and ex-

ternally:

· Power-on Reset

The most fundamental and unavoidable reset is the

one that occurs after power is first applied to the

microcontroller. As well as ensuring that the Program

Memory begins execution from the first memory ad-

dress, a power-on reset also ensures that certain

other registers are preset to known conditions. All the

I/O port and port control registers will power up in a

high condition ensuring that all pins will be first set to

inputs.

Although the microcontroller has an internal RC reset

function, if the VDD power supply rise time is not fast

enough or does not stabilise quickly at power-on, the

internal reset function may be incapable of providing

proper reset operation. For this reason it is recom-

mended that an external RC network is connected to

the RES pin, whose additional time delay will ensure

that the RES pin remains low for an extended period

to allow the power supply to stabilise. During this time

delay, normal operation of the microcontroller will be

inhibited. After the RES line reaches a certain voltage

value, the reset delay time tRSTD is invoked to provide

an extra delay time after which the microcontroller will

begin normal operation. The abbreviation SST in the

figures stands for System Start-up Timer.

For most applications a resistor connected between

VDD and the RES pin and a capacitor connected be-

tween VSS and the RES pin will provide a suitable ex-

ternal reset circuit. Any wiring connected to the RES

pin should be kept as short as possible to minimise

any stray noise interference.

For applications that operate within an environment

where more noise is present the Reset Circuit shown

is recommended.

More information regarding external reset circuits is

located in Application Note HA0075E on the Holtek

website.

· RES Pin Reset

This type of reset occurs when the microcontroller is

already running and the RES pin is forcefully pulled

low by external hardware such as an external switch.

In this case as in the case of other reset, the Program

Counter will reset to zero and program execution initi-

ated from this point.

R E S

V D D

S S T T i m e - o u t

C h i p R e s e t

0 . 9 V D D

t R S T D

Power-On Reset Timing Chart

R E S

S S T T i m e - o u t

C h i p R e s e t

0 . 9 V D D

0 . 4 V D D

t R S T D

RES Reset Timing Chart

R E S / P A 7

0 . 1 ~ 1 m F

1 0 k W ~

1 0 0 k W

V D D

V S S

0 . 0 1 m F * *

V D D

1 N 4 1 4 8 *

3 0 0 W *

Note:

²*² It is recommended that this component is

added for added ESD protection

²**² It is recommended that this component is

added in environments where power line noise

is significant

External RES Circuit

HT56R656 Datasheet(PDF) 76 Page - Holtek Semiconductor Inc

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