HT68F20 datasheet(46/210 Pages) HOLTEK

46 / 210 page

HT68F20/HT68F30/HT68F40/HT68F50/HT68F60

Rev. 1.00

November 3, 2009

Reset and Initialisation

A reset function is a fundamental part of any

microcontroller ensuring that the device can be set to

some predetermined condition irrespective of outside

parameters. The most important reset condition is after

power is first applied to the microcontroller. In this case,

internal circuitry will ensure that the microcontroller, af-

ter a short delay, will be in a well defined state and ready

to execute the first program instruction. After this

power-on reset, certain important internal registers will

be set to defined states before the program com-

mences. One of these registers is the Program Counter,

which will be reset to zero forcing the microcontroller to

begin program execution from the lowest Program

Memory address.

In addition to the power-on reset, situations may arise

where it is necessary to forcefully apply a reset condition

when the microcontroller is running. One example of this

is where after power has been applied and the

microcontroller is already running, the RES line is force-

fully pulled low. In such a case, known as a normal oper-

ation reset, some of the microcontroller registers remain

unchanged allowing the microcontroller to proceed with

normal operation after the reset line is allowed to return

high.

Another type of reset is when the Watchdog Timer over-

flows and resets the microcontroller. All types of reset

operations result in different register conditions being

setup. Another reset exists in the form of a Low Voltage

Reset, LVR, where a full reset, similar to the RES reset

is implemented 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.

· RES Pin

As the reset pin is shared with PB.0, the reset function

must be selected using a configuration option. Al-

though 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 Enhanced Reset Cir-

cuit shown is recommended.

More information regarding external reset circuits is

located in Application Note HA0075E on the Holtek

website.

P B 0 / R E S

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

R E S

V D D

I n t e r n a l R e s e t

0 . 9 V D D

t R S T D + t S S T

Note: tRSTD is power-on delay, typical time=100ms

Power-On Reset Timing Chart

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