Why capacitor & resistor is connected to reset pin in 80

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glenjoy

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Reset Question

I know this might be a silly question, but why is it that in circuits of 8051 micros, there is always a capacitor and resistor connected to the rest pin? WHy not just connect it to ground to run or connect it to Vcc to program.

Thanks.
 

Re: Reset Question

The RC causes a time delay so that all of the registers and state flip flops are fully powered before they are put into a starting condition. You can imagine what would happen if the program address register would start in the middle of the program code and go from there.
 

Re: Reset Question

The RC is for startup reset whenever you apply power, so that CPU is in known state.
 

my dear buddy
microcontroller needs a hi pulse when supplied the power.
this is nessecory because when mcu, gets hi pulse, it resets properly and starts counting from "0"
if it is not reseted properly, program counter may start from any other line but zero.
i hope u understand now
 

Re: Reset Question

When the power to 8051 is turned on, it may not be
very stable. So the RC circuit will keep 8051 reset for
a while until the power is stable. Or it may cause the ROM
code run abnormally. Generally speeking, the reset time
will be several to dozens millisecons. You could fine-tune
the RC to fit your application.

Regards
 

Re: Reset Question

I wish only too add some practical guidance to chose RC values:

From experience (and Intel recomandations) for mostly 8051 based systems, the R and C values must satisfy the folowing condition:

R*C ~ 100ms

( read ~ " aproximately equal")

For example R=10K and C=10uF is a good choise because

10K*10uF=100ms (satify the empiric rule)


Another good choice would be R=22K C=4.7uF etc.
 

Re: Reset Question


Thank you!
 

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