HT95R34 datasheet(28/59 Pages) HOLTEK

28 / 59 page

HT95R34

Rev. 1.10

February 18, 2009

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 Enhanced Reset Cir-

cuit 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.

· Low Voltage Reset

- LVR

The microcontroller contains a low voltage reset cir-

cuit in order to monitor the supply voltage of the de-

vice. The LVR function 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 internally. For a valid LVR signal, a low sup-

ply voltage, i.e., a voltage in the range between

0.9V~VLVR must exist for a time greater than that spec-

ified by tLVR in the A.C. characteristics. If the low sup-

ply voltage state does not exceed this value, the LVR

will ignore the low supply voltage and will not perform

a reset function. The actual VLVR value can be se-

lected via configuration options.

· Watchdog Time-out Reset during Normal Operation

The Watchdog time-out Reset during normal opera-

tion is the same as a hardware RES pin reset except

that the Watchdog time-out flag TO will be set to

²1².

· Watchdog Time-out Reset during Power Down

The Watchdog time-out Reset during Power Down is

a little different from other kinds of reset. Most of the

conditions remain unchanged except that the Pro-

gram Counter and the Stack Pointer will be cleared to

²0² and the TO flag will be set to ²1². Refer to the A.C.

Characteristics for tSST details.

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

0 . 1 m F

1 0 0 k W

V D D

V S S

0 . 0 1 m F

1 0 k W

Enhanced Reset Circuit

R E S

V D D

V S S

0 . 1 m F

1 0 0 k W

Basic Reset Circuit

R E S

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

0 . 4 V D D

t R S T D

RES Reset Timing Chart

L V R

S S T T i m e - o u t

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

t R S T D

Low Voltage Reset Timing Chart

W D T T i m e - o u t

S S T T i m e - o u t

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

t R S T D

WDT Time-out Reset during Normal Operation

Timing Chart

W D T T i m e - o u t

S S T T i m e - o u t

t S S T

WDT Time-out Reset during Power Down

Timing Chart

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