89c55wd power on reset problem

[nikhilele]

89c55wd

i have two boards in one board on power on the controller is not resetting automatically while in other boards it resets automatically.

Reason for this may be:
the board in which reset is not working I have connected 4700uf capacitor for my mobile which is powered through the same board and this mobile needs more current only at startup so i have connected a big capacitor.

my reset ckt is 4.7uf capacitor and 10k resistor.
so what should i increase capacitor or resistor.

 

[wek]

89c55wd examples

None. Drop the RC reset, use a proper reset IC.

wek

 

[nikhilele]

reset 89c55wd

I think it can be done by RC ckt.
Actually the pcb board is ready i cannot add IC to it rather i can add different combination of RC.

 

[memarian]

circuito reset 89c55wd

Dear,

Your RC is OK.I used a 10uf+4.7K (same Time Constant) and it works perfect.i think you sould use a resistor in Parallel of 4700uf Capacitor to discharge it after power down.(in Long time,For example after 3 seconds).

Good luck.

 

[nikhilele]

how much current can a 4700uf cap discharge

Your RC is OK.I used a 10uf+4.7K (same Time Constant) and it works perfect.i think you sould use a resistor in Parallel of 4700uf Capacitor to discharge it after power down.(in Long time,For example after 3 seconds).

I didn't get you why we have to do it like this.

Added after 3 hours 58 minutes:

I donot what is happening in my board when we press reset button it resets without problem.

but when it is switched on through power then the controller starts working after about 20-25secs,

and when we made power on or off in short time then it resets without problem,

this leads to problem related on powering up the board as it has large capacitor it may hinder smooth resetting of the controller.

now i would like to ask as my controller works fine after 20-25sec. is there any harm in it.

 

[memarian]

Dear,

I think your 4700 uf capacitor is parallel with supply.When you turn off supply,and

your 4700uf capacitor has no discharge resistor,your power up reset capacitor

also have charge and when you turn on you power supply no power up reset occures.

i upload a schematic,in this schematic R2 Discharge Power up Reset Capacitor

(and any othr Capcitor that is parallel with supply)after Turn off.

Good Luck

 

[aldevan]

reset ckt

what IC can I use to reset?

 

[memarian]

power on reset microcontroller

Dear,

Check this application note @ atmel Site:

How to Calculate the Capacitor of the Reset Input of a C51 Microcontroller

http://www.atmel.com/dyn/resources/prod_documents/Reset-V13.xls
http://www.atmel.com/dyn/resources/prod_documents/doc4284.pdf

Good Luck.

 

[nikhilele]

circuit using 89c55wd

i think your 4700 uf capacitor is parallel with supply.When you turn off supply,and

your 4700uf capacitor has no discharge resistor,your power up reset capacitor

also have charge and when you turn on you power supply no power up reset occures.

i upload a schematic,in this schematic R2 Discharge Power up Reset Capacitor

(and any othr Capcitor that is parallel with supply)after Turn off.

Good Luck

1k or larger resistor in parallel is already there.
as i am using a led it has 330ohm resistor and led which will discharge.

 

[leonardo0]

Hi

Since i have some experience i can only suggest that you use a proper reset ic .
You can see what is happening with a digital scope in single shot mode .
The r-c problem with reset even if you find the right capacitor and resistor is that your circuit can ocasionaly lock if there are many on-off of the power supply .The capacitor will not discharge properly and the reset wil not function correctly.
There are man techniques but unless you have time to experiment use an reset ic to save yourself time

 

[wek]

You can always squeeze in a 3-legger MAX810 or alike (zillions of similar (beware, positive reset!) are produced).

wek

 

[nikhilele]

can we use monostable multivibrator as reset ic this would be cheap solution.