max232 capacitor value

[abishekmudalur]

hi
i m using uart communication wit pic 16f877a...
i hv used 470nF capacitors in the max232 side....is it ok...???
my system gets reset ,when i m using uart....all other progrms working properly....
i checked the same program in some other kit..its working correctly...
wats the prob....??

 

[hussain_kiet]

you should use 1uF to 10uF capacitors for max232....

 

[abishekmudalur]

my processor get resets....do u think its bcz of this capacitor value...

 

[ckshivaram]

processor reset is not due to capacitor, but you wont get serial output if you use wrong values of cap.... when does the controller reset??? does it happen everytime????? which place of execution of code do you think it gets reset.. which controller are you using????? Does it have an internal watch dog timer??? If so it is getting reset???????????

 

[Jack// ani]

Value of capacitor depends of version of MAX232 chip used. For MAX232CPE its 0.1uF, for other version it is usually between 1uF to 10uF.

Read the datasheet to know which one you're supposed to use.

 

[abishekmudalur]

my controller resets wen serial_out or serial_in occurs.....
i m using 16f877a..
i removed the wdt by "#fuses NOWDT" in the program....
here s my design
one more mistake i made....i wrongly placed 220nF instead of 22pF with crystal osc.....ll it make any difference??

 

[hussain_kiet]

capacitors will not be the reason for reset...
you should check your config bits for WDT, BOR etc...
also the crystal capacitors should be 22pf not 220nF...

 

[abishekmudalur]

but the same prog s executing perfectly in another kit...

 

[ckshivaram]

your WDT is getting reset at some time or It seems you are encountering Brown out reset....

 

[abishekmudalur]

k.....is it needed to change the capacitors from 470nF to 1uF.....
#fuses NOBOR // is it k...

 

[hussain_kiet]

yes you should change the capacitors value...
NOBOR,NOWDT...

 

[abishekmudalur]

thanx....
wat abt crystal capacitors....tat also i need to change???
plz check my diagram.....

 

[Jack// ani]

Yes, 22pF capacitor is necessary, so better get it replaced.

Also, one more piece of suggestion, place 0.1uF across VDD & GND of the MCU to bypass any possible noise in power supply due to oscillation.

 

[akhilmolugu]

Yes, 22pF capacitor is necessary, so better get it replaced.

Also, one more piece of suggestion, place 0.1uF across VDD & GND of the MCU to bypass any possible noise in power supply due to oscillation.

Why only 0.1uF across VDD & GND of the MCU has to be used to bypass any possible noise in power supply due to oscillation? Is there any formulae to calculate the value of capacitor?

 

[amayilsamy]

Connect a capacitor between Vcc and ground of max 232. And check proper ground for micro controller and max232.

 

[nick703]

yes amayilsamy i think u r right so put value on capacitor c3,c4,c5,c6 to 1uF 63v and also put capacitor on max232 vcc means pin no 16 to gnd 100uF 25v .