[PIC] vdd to vss capacitor

[wolverine.x6]

hello everyone,here is my first post. I made a simple led blinking circuit for micro-controller test(pic16f876a). But i didnt added the capacitor(0.1uf) between vss and vdd of the circuit. After burning(with seperate pic burner) the led blinking code in pic when i tried to run the circuit it didnt response. I coincidentally shorted the Mclr pin to VDD (shorted the 10k resistor between mclr and vdd) then the circuit start running. Can anyone can explain what exactly was the reason ???
thanls in advance.

 

[betwixt]

The capacitor is necessary to ensure the VDD and VSS pins are at the same signal voltage. Obviously they are at different DC voltages! If any signal is present across VSS and VDD the PIC may behave erraticaly or not work at all. I doubt shorting out the MCLR resistor is what started it running, more likely you disturbed the signals on VDD and it accidentally allowed it to run. For reliable operation you MUST have that capcitor fitted and it should have short wires and be as close to the VSS/VDD pins as possible.

Brian.

 

[hobbyckts]

May be the mclr pin must be pulled high as per the spec that's what you did. Initially as you have not connected anything so it might have got. Clearing continuously .

 

[wolverine.x6]

one thing i forgot to mention that i was running it with a 12 vdc adapter, i have read some threads that ripple is the factor that makes disturbance in circuit, that is a obvious thing but if let suppose for instance ripples were the reason then why shorting the MCLR to VDD force pic to run?
// I doubt shorting out the MCLR resistor is what started it running, more likely you disturbed the signals on VDD// what betwixt suggest was seem to be right , may be MCLR absorbed the noise of adapter supply and filter the VDD.

 

[betwixt]

The MCLR pin draws virtually no current so the value of resistor is relatively unimportant, even a short circuit could be used. MCLR would not absorb noise but I suspect he added capacitance of your hands/tools on the connection may have been enough to start the PIC running. It would be an unreliable method of making it run, the capacitor across VSS/VDD would be the reliable method.

You shouldn't really have ripple on the supplies to a PIC, it will reduce reliability, particularly if you try using the ADC module. Your adapter should really have a fixed voltage regulator in it to ensure a consistent and clean output.

Brian.