Newbie Question

Whizzard9992 · Joined: 13 Apr 2004 Posts: 8

I'm using a PIC16F627A, and I can't get a signal from any pin. Any help would be greatly appreciated. Here's my code. I don't have any I/O pins grounded: only A7 hooked to an LED/RESISTOR.

I have Vss and Vdd at 5.0V regulated. /MCLR = Vdd (*edit).

Here's My code:

languer · Joined: 09 Jan 2004 Posts: 144 Location: USA

I do not really see much wrong with it.

Only things I would point out is that you should have this,

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Guest · Posted: Wed Apr 14, 2004 1:13 am

Additionally use NOLVP option in #fuses statement because as you wrote your port pins are floating.

Whizzard9992 · Joined: 13 Apr 2004 Posts: 8

Thank you all for your input!!!!

I'm a veteran programmer but new to MCUs. I have a book and am drudging my way through it, so please bear with me if my knowledge is like swiss cheese.

I did get it to work, but one comment did spark my curiosity. What is NOLVP? By floating, do you mean thier direction is not specified? I can't find any documentation on it...

Vss = Ground, Vdd = +5.0v, /MCLR = Vdd

My problem was a combination of a number of different things. Firstly, I'm not using a PIC programmer. Instead, I got a Xeltek 3000U. I figured if I was going to drop a gino on a programmer, I would like it to program more than just PICs. I was trying to burn the chip using BINARY instead of INTEL format. Also, I had /MCLR high through a 10K resistor (as it has in my book). Tapping Vdd directly seemed to do the trick. Is the the correct way?

Also, I got it to work without specifying TRISB. I read that this is a depreciated command, so I'm going to avoid it. That's not to say I didn't try it Wink

Thanks again!

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

As far as TRIS, the default is to use standard I/O which should take care of the TRIS registers for you. With Fast I/O or Fixed I/O you have to set data directions yourself.

NOLVP means No Low Voltage Programming. Low voltage programming lets the chip be programmed by voltages in the 0 to 5V range, which is handy if you want to update your code while the chip is in the product. But it also means the program could changed accidentally if pins are left floating. NOLVP means the program can't be changed unless 12V is applied to the programming pin, which makes accidental program corruption much less likely. In a product you should allways tie I/O pins to something. But when experimenting that is not allways convienient.
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DragonPIC · Joined: 11 Nov 2003 Posts: 118

Floating is a term that is used when a pin is left unconnected to any other component. Usually, you only have to worry about this when the pin is left as an input. Inputs should never be left floating even if unused. Either you pull them up or pull them down, preferrably to a state that draws the least current or does not alter the way the rest of your circuit must opperate.

Port B on PIC's can usually be configured with weak (draws little current from VCC) pullups for this reason and just in case the input is being driven by an open collector(drain) output of another device.

You could also just set unused pins to outputs.

Whizzard9992 · Joined: 13 Apr 2004 Posts: 8

Excellent information. This helped a TON.

Thanks!