PIC16F690 with NE555

[xnihilo]

Well...

I connected portC.4 from a PIC16F690 to a resistor to the BASE lead of a BC182L NPN transistor where the Collector lead is connected to the TRIGGER pin of a NE555 used in monostable mode (this pin is also connected to a 10K weak pull UP resistor).

TRISC.4 is 0 and PORTC.4 is 0 so it will be an output with a logic of 0.

The NE555 is supposed to be triggered when the TRIGGER (pin 2) is pulled low.
As C4 is low because of my TRIS and PORT settings, it should not switch the transistor so the NE555 TRIGGER pin should stay at its pulled up value of 5V.

Actually, when I power up the application, I hear the click of the 5V relay connected to the NE555.

I am wondering what is going on.
When I remove the pic 16F from its socket, the NE555 is not activated, what means that there has to be a positive voltage on the pin C4 when the pic is powered on, just before I can set the TRIS and PORT registers, is that right? Do the I/O pins output a positive voltage when the chip is powered on???
I did not find this in the datasheet or did I miss a line in this datasheet?

[Charles Linquis]

Three questions:

1. Do you have a resistor between the PIC output pin and the base of the transistor?
2. Do you have a pull-down resistor on that output pin?

3. Why do you need the 555 at all? It seems like it would be just as easy to use the PIC as a one-shot to control the transistor to control the relay.

[xnihilo]

Thank you for your reply.

1. Do you have a resistor between the PIC output pin and the base of the transistor?

-> Yes, I call it RB, I calculated the value empiricaly (I have to check on my board if it is a 3.3K or a 4.7K)

2. Do you have a pull-down resistor on that output pin?

-> No I don't...

3. Why do you need the 555 at all? It seems like it would be just as easy to use the PIC as a one-shot to control the transistor to control the relay.

Because I need the MCU to go on doing stuff whle the NE555 is outputing for a specific delay, independently of the MCU. I need the MCU to get rid of the job.

[Charles Linquis]

I would recommend you put a pull-down on the PIC pin. 4.7K or so. That will probably fix your start-up trigger.

And... You could probably find an easy way (such as using a timer interrupt) to allow your PIC to produce a pulse while doing other tasks.

[xnihilo]

Hello.

I thank you for your help. I will try the pull down.

Regqrds.

[Melanie]

The reality is that a PIC I/O's can be indeterminate on Power-On. Set the I/O's as one of the first things you do when you Power-On your PIC. Leaving them set to input (high impedance) may be enough to trip your circuitry.