[SOLVED] 16f877a power on problem

[cheetha]

Assalam-o-alekum

i am using PIC 16f877a, mostly when i turn power on the controller doesnot works giving unknown output at i/o pins. the power needed to be connected and disconnected 3 to 4 times randomly then co0ntroller starts working correctly. plz explain why it is happining and hoe to avoid this situation.

 

[Jinzpaul4u]

Hi there,

Please verify your hardware section fully and power supply section.How much current are you feeding to controller?

Best regards,

 

[nandhu015]

Reset Problem?? Check the reset circuit.

 

[jayanth.devarayanadurga]

Loose connection. It external crystal is used mention capacitor value used in oscillator circuit.

 

[cheetha]

Reset Problem?? Check the reset circuit.

i have just connected MCLR pin to VDD through 10k resistor therefore no reset circuit.

Loose connection. It external crystal is used mention capacitor value used in oscillator circuit.

i have used 4 MHZ crystal with 15pf capacitors

 

[Venkadesh_M]

set the correct configurations bits...........

 

[tpetar]

What can you tell us about power what uC use ?

Check configuration bits.


Best regards,
Peter

 

[bigdogguru]

What are you utilizing to power the device?

A poorly regulated power supply combined with the Brown-Out Reset (BOR) active can cause the device to repeatedly reset, essentially suspending device operation.

I would recommend utilizing a 100nF decoupling capacitor as close as possible between the Vdd and Vss pins, use a well regulated power supply and disable the BOR in the configuration register settings.


BigDog

 

[cheetha]

What are you utilizing to power the device?

i have use 7805 for 5V supply

I would recommend utilizing a 100nF decoupling capacitor as close as possible between the Vdd and Vss pins, use a well regulated power supply and disable the BOR in the configuration register settings.

i have tried it with capacitor but it didnt worked and BOR is disabled already

 

[jayanth.devarayanadurga]

replace 15pf with 22 pf or 27 pf pr 33 pf.

 

[tpetar]

i have use 7805 for 5V supply


i have tried it with capacitor but it didnt worked and BOR is disabled already

What voltage you supply to 7805 voltage regulator and from what source battery, SMPS,... ?

Did you use required capacitors for voltage regulator before and after ? Waht values ?

Did you use 100nF close as possible between pins Vdd/Gnd ?



Best regards,
Peter

:wink:

 

[Venkadesh_M]

If you having any LCD that functions like that means it is the problem of delay, can you tell other elements you having in the circuit..

 

[cheetha]

If you having any LCD that functions like that means it is the problem of delay, can you tell other elements you having in the circuit..

i dont have LCD but 3 digit 7 segment, push buttons and opamps

What voltage you supply to 7805 voltage regulator and from what source battery, SMPS,... ?

Did you use required capacitors for voltage regulator before and after ? Waht values ?

Did you use 100nF close as possible between pins Vdd/Gnd ?

the input to 7805 is from transformer and bridge rectifier
i had used 100nf capacitor at controller power pins

 

[tpetar]

the input to 7805 is from transformer and bridge rectifier
i had used 100nf capacitor at controller power pins

Measure voltage on 7805 input pin with digital voltmeter and post result. Read and write all digits from voltmeter display.

 

[bigdogguru]

the input to 7805 is from transformer and bridge rectifier
i had used 100nf capacitor at controller power pins

Besides the 100nF decoupling/bypass capacitor at the microcontroller Vdd & Vss pins, you'll need to employ capacitors immediately before and immediately after the 7805.



LM78XX/LM78XXA Datasheet 3-Terminal 1A Positive Voltage Regulator

BigDog

 

[kgavionics]

i had the same problem before, and i can confirm it's due to the reset circuit.The solution is simple,you must use a pull up resistor and a push button on the reset pin.Every time to start the circuit,just hit the reset button and everything will work as expected.

 

[cheetha]

attach is the power supply circuit

i had the same problem before, and i can confirm it's due to the reset circuit.The solution is simple,you must use a pull up resistor and a push button on the reset pin.Every time to start the circuit,just hit the reset button and everything will work as expected.

i tried by connecting the pin to ground via hard wire the controller resets but remain hanged untill power is disconnected and then connected again

 

[tpetar]

View attachment 93723

attach is the power supply circuit

i tried by connecting the pin to ground via hard wire the controller resets but remain hanged untill power is disconnected and then connected again

What you trying to do with this circuit? :shock:


Use circuit from post #15.




Best regards,
Peter

 

[Venkadesh_M]

View attachment 93723

attach is the power supply circuit




i tried by connecting the pin to ground via hard wire the controller resets but remain hanged untill power is disconnected and then connected again

Hi
you are varying the output voltage.. surely it will be more than 5V is that you want?.. for 5v the center pin of regulator has to be grounded and like #15, try to measure your circuit output voltage..

 

[cheetha]

Hi
you are varying the output voltage.. surely it will be more than 5V is that you want?.. for 5v the center pin of regulator has to be grounded and like #15, try to measure your circuit output voltage..

yes i am varying output voltage as output of most 7805 is not exactly 5V so to adjust it to 5V i have used this circuit ...