Input question from a newbe

[Bdlhome]

New to this so be gentle on me. I wanted to solve this on my own, but after lots of reading and experimenting I'm now banging my head on the wall.

I have a prototype board (16F877A based) with 4 buttons that pull RB0-RB3 to ground respectively. I'm having problems reading the input and if it didn't come with a compiled program that worked I would say it needed a pull up resistor.

Can anyone help me with a simple code snippet to say light LED on RD0 when RB0 is grounded.

I starter with the following based on reading.

CMCON = 7
ADCON1 = 7
TRISD = %00000000
TRISB = %11111111

and then was trying to use if portb.0 in a look but no luck.

What an I doing wrong? I have the C source for the program that works but don't appear to be able to replicate the same in PBP.

Thanks,
Brian

[Bdlhome]

Replying to my own question, it looks like pull-up resistors was the problem after all. Adding OPTION_REG = $7F and enabling the internal pull-up resistors appears to have solved my problem

[Heckler]

Barely an hour between complete confusion and WOW! it works!!

Good job, I bet that feels good. I know it does for me when I figure something out... especially after "banging my head on the wall".

congrats!

[mister_e]

We all know what you mean, that's for sure. If your walls are located too far away... here's a portable solution...

Should also grab one of these

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[Bdlhome]

Thanks for the head banging kit, I have a feeling I will got a lot of use out of it while trying to get up to speed.

If you guys don't mind I have another probably very fundamental question, but my basic knowledge of electronics and programming is probably dangerous

For my first Pic project I want to sense the current state of an existing piece of electronics and then be able to control it based on the current state and other inputs that are not part of the existing circuit.

Below is the basics of the existing circuit.



I've measured the current (flowing between 3 and 4) when the switch is closed and it is only a few mA so I think I can happily pull point 4 to ground to emulate the switch being pressed.

For the sensing of the existing circuit and whether the LED is lit, can I just attach point 2 to an input and read a low (using internal pull up resistors)? I didn't know if that would interfere with the existing circuit. For the switch side of things I know I could use a relay or something to completely isolate the two circuits (mine from the existing), but wasn't sure about reading the state of the LED.

The existing hardware isn't cheap, so I don't want to experiment too much.

Any help or advice appreciated.

[HenrikOlsson]

Hi,
Since the external circuit operates on 12V the voltage at (2) will be 12V when the LED is off, if you connect that to an input of your PIC, which operates from 5V, current will flow from +12V, thru the resistor, LED and into the PIC's input pin, up thru the internal clamp diode and into the 5V supply line of the PIC and the LED will turn on (with less intensity since the total voltage is 7V instead of 12V).

Use a high(ish) value resistor between point 4 and your PIC input (say 47k or something). That will limit the current into the PIC to safe level and stop the LED from glowing.


Same thing with (4) really, if the voltage at (4) is 12V when the button is not pressed and you connect your PIC output (which toggles between 0 and 5V) to (4) it's likely that the external circuit will think the button is pressed no matter if the output is high or low. As you say, a relay or transistor switch should do the trick.