Reset Microcontroller

Dear All,

I'm facing a problem when the uC is power up that time it look like uC hang. Sometime after the power is up i need to press the reset switch then only the application can run.

How to overcome this problem when power is up i not need to press the reset switch?

Please kindly advice. Thanks...

It always pays off to use microcontroller supervisory ICs ..
Examples:
LM809, MAX809 .. active L ..
LM810, MAX810 .. active H ..
DS1232, MAX1232 .. active L and H ..

Rgds,
IanP

Dear Ianp,

Thanks for your help again.

I have do some research after received your advice. There is many IC to control the microcontroller RESET pin.

May i know what is the difference if we choose the Reset Threshold is 4.63V and 2.63V?

Thanks for advice.

It only means that the reset signal stays active if the Vcc voltage is below the threshold voltage and still stays active for some [ms] after Vcc rises above the threshold voltage ..

These different voltages are available since newer microcontrollers can operate of different supply voltages, not as it was in the past of only +5Vdc .. see attached table ..

For example - case nr.3 - the reset signal will be generated and maintained if the supply voltage is below or near 80% of the recommended +5V and then when the voltage rises above +4V it will still be active for ≈240ms ..

Rgds,
IanP

Dear Ianp,

Let said the microcontroller Vcc is 5V +/-10% (4.5V~5.5V). So, in this case we should choose 4.38V +/-3% (4.2486V~4.5114V). If the Vcc is lower than 4.5V the reset will active for ≈240ms, am i rite? If yes, how bout if the Vcc is below than 4.25V, what will happen?

In the datasheet said:
The 4.63V and 3.08V options of the LM809/810 use highly accurate circuitry to ensure that the reset threshold occurs only within this range (for 5V and 3.3V
supplies).
So, if we choose 4.63 +/- 3% mean our Vcc must be stable otherwise the reset will alway active?

Thank you,.

Please do understand the tolerance of the threshold: the manufacturer guarantees the threshold voltage is within U[threshold} +/-3% ..
In the 4.38V case it can be anywhere between 4.25 and 4.51 ..
It has only one threshold and almost no hysteresis ..

If the Vcc is lower than 4.5V the reset will active for ≈240ms, am i rite? If yes, how bout if the Vcc is below than 4.25V, what will happen?

Click to expand...

If the supply voltage is at threshold or below, the reset signal is asserted and remains active unless the Vcc rises ..
When supply voltage rises above the threshold voltage the reset signal remains asserted for at least xxx[ms] and at this point the reset is released ..

So, if we choose 4.63 +/- 3% mean our Vcc must be stable otherwise the reset will alway active?

Click to expand...

Theoretically, the Vcc should not fall below 4.76V (4.63+3%), so, yes, it should be more the less stable ..
If it fluctuates you will have the reset signal asserted any time it falls below the threshold ..

See attached picture .. it's marked for low-active reset - high-active will be reversed ..

Rgds,
IanP

Dear Ianp, Thank you. Now then i understand.