[SOLVED] How does the reset circuit of 8051 work?

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jjeevan007

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hi to all,

can one explain how the reset circuit of 8051 works

here is circuit of 8051



regards
jeevan
 

Re: 8051 Reset operation

The reset line of the 8051 operates in an opposite fashion in comparison to many other microcontrollers, like the PIC, AVR, etc.

During normal operation, the reset pin is normally held low through the use of a weak pull down resistor, 8.2KΩ in this case.

A reset occurs when the capacitor is shorted through the switch which in effect momentarily pulls the reset pin high, resetting the device.

When the switch is released the capacitor begins to charge, once again pull the reset pin low for normal operation.


BigDog
 
Re: [Moved] 8051 Reset operation

why we are using the capacitor here, if we remove the capacitor then also it works properly

what is role of capacitor is here

regards
jeevan
 

Re: [Moved] 8051 Reset operation

jjeevan007 said:
why we are using the capacitor here, if we remove the capacitor then also it works properly

what is role of capacitor is here
Click to expand...

The capacitor is to cause a proper reset when power is first applied. When power is applied, the capacitor begins to charge and holds the reset line high (until it is charged) for long enough to ensure that the 8051 gets a proper start up condition.
 

Re: [Moved] 8051 Reset operation

Er.abdulrehman said:
https://www.google.co.in/url?sa=t&rct=j&q=role+of+capacitor+in+reset+circuit&source=web&cd=6&cad=rja&ved=0CEIQFjAF&url=http%3A%2F%2Fwww.siriusmicro.com%2Fprojects%2Fb1.pdf&ei=_2WOULywIoHQrQfntYHIDg&usg=AFQjCNHLmuM_fByZPtcJcdkMK_Aw0AbTPQ
Click to expand...

I fail to see what relevancy an appnote concerning PIC circuit design has to do with an 8051 reset circuit.

They function in an exact opposite fashion, the PIC is reset when the pin is pulled low, the 8051 is reset when the pin is pulled high.

@jjeevan007

The capacitor serves two functions, it acts a simple switch debounce mechanism and ensuring the reset pin is pulled high for an appropriate period of time to properly initiate a reset.

Once the switch is open the capacitor begins to charge at a rate based on the RC factor, when it reaches a level equivalent to the low logic threshold the device transitions from reset to normal operation.


BigDog
 
Re: [Moved] 8051 Reset operation

For adding up a little detail, calculation of R and C value in reset pin is R x C.
In your circuit, 8.2k x 10uF = 82 ms. This is the time taken by the capacitor to charge completely and keeps the rst pin high...

Correct me if I am wrong...
 
Re: [Moved] 8051 Reset operation

jjeevan007 said:
why we are using the capacitor here, if we remove the capacitor then also it works properly
what is role of capacitor is here
Click to expand...

In "older" 8051-derivatives, the RC reset circuit was a terrible thing, see: **broken link removed**
Some "newer" derivatives use BOR and POR, and they don't need (in most cases) external Rs or Cs, although the pin may still exist .......
However:
Unpredictable behaviour is the most confusing ..... do avoid the use of RC (or C only) reset, please use a proper reset IC.
Click to expand...
That's true !!!

:wink:
IanP
 
Re: [Moved] 8051 Reset operation

got now,

the capacitor is mainly used for power on reset,

correct me if i am wrong,

regards
jjeevan
 

Re: [Moved] 8051 Reset operation

Hello there,
I am newbie in practical field though I studied them 10 years back. I have a 8051 development board with usb only. Unfortunately I also countered the same problem of
Unable to connect at the specified baud rate,Try reducing the baud rate
I tried to reset by shorting the capacitor pins but of no use. Please help me.

With regards
Raju
 

Re: [Moved] 8051 Reset operation

just remove the capacitor from the reset circuit. It works fine.
 

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