HT95R35 datasheet(41/73 Pages) HOLTEK

41 / 73 page

HT95R35

Rev. 1.00

October 7, 2009

Programming Considerations

By disabling the interrupt enable bits, a requested inter-

rupt can be prevented from being serviced, however,

once an interrupt request flag is set, it will remain in this

condition in the INTC register until the corresponding in-

terrupt is serviced or until the request flag is cleared by a

software instruction.

It is recommended that programs do not use the

²CALL

subroutine

² instruction within the interrupt subroutine. In-

terrupts often occur in an unpredictable manner or need

to be serviced immediately in some applications. If only

one stack is left and the interrupt is not well controlled, the

original control sequence will be damaged once a

²CALL

subroutine

² is executed in the interrupt subroutine.

All of these interrupts have the capability of waking up

the processor when in the Power Down Mode. Only the

Program Counter is pushed onto the stack. If the con-

tents of the register or status register are altered by the

interrupt service program, which may corrupt the de-

sired control sequence, then the contents should be

saved in advance.

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

overflows and resets the microcontroller. All types of re-

set operations result in different register conditions be-

ing setup.

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.

R E S

V D D

S S T T i m e - o u t

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

0 . 9 V D D

t R S T D

Power-On Reset Timing Chart

R E S

V D D

V S S

0 . 1 m F

1 0 0 k W

Basic Reset Circuit

HT95R35 Datasheet(PDF) 41 Page - Holtek Semiconductor Inc

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