[51] 10 uf capacitor on RSET PIN

[debasischhatra]

WHY A RATING OF 10 uf capacitor is connected across reset(pin-9) of 8051 MC?
How a 10uf capacitor and 8.2kOhm resistor taking care to generate minimum 2 machine cycle in Oscillator to reset the Micro CONTROLLER ?

 

[KlausST]

Hi,

I don't see the schematic, but I assume that it is a usual RC combination.
This RC generates a tkme constant of "R x C" in seconds.
What exact time this means for the circuit depends on the initial voktage across the C (I assume 0V) and the thresold level to release the RESET state.

--> read the datasheet

Klaus

 

[paulfjujo]

be carrefull , with a 10µF capacitor ,
you must limit the maximum current going into the Reset input when power Off.

the time constant R*C ... depend of the load connected accross the capacitor (Reset input impedance ) ?
so time is less than R*C
But machine cycle ist very low (any µS) compare to any mS ..

blue curve is with a load of 500K
green curve for a load of 10K

 

[dick_freebird]

What you want, is for the RESET pin LH transition to
lag power supply ramp by enough that uC will be within
predictable-operation supply range before reset releases.

This is somewhat about uC "guts" but also about the
power supply's expected behavior. You could use the
Rec Min supply voltage for the one, but knowing the
(range of) supply behavior wants some thought (or
data). Many supplies have mS-range cold start rise
times and a uS-range reset lag might not be entirely
reliable.

Leaky reset caps are something I've seen render
desktop computers nonfunctional.

 

[KlausST]

Hi,

I strongly recommend to read the datasheet.

I´m not familiar with 8051, but if I remeber right then the RESET with 8051 is high_active.
If this is the case, then the circuit of post#3 is wrong. (it is for low_active RESET)

But I may be mistaken.

Klaus

 

[c_mitra]

resistor taking care to generate minimum 2 machine cycle in Oscillator to reset the Micro CONTROLLER

When power is first applied, the various pins have undefined voltage and this includes the power pins. The microprocessor starts running but various internal parameters are having random values.

Therefore it is important to reset the microcontroller once the voltages on the system has stabilized. The RC circuit at the reset pin serves this purpose.

WHY A RATING OF 10 uf capacitor is connected across reset

Intel can say that; but what is the recommended values (for the RC) ? The exact value of the time corresponding to two machine cycles depends on the crystal (OSC) used in the circuit.