Can't keep a pin low

[bartman]

Hmmm, can there be a fourth choice?

I swapped PICs with no change
I've triple checked all the connections on the breadboard

That leaves a code problem.

Here is the first part of that:

@ DEVICE pic12F629, INTRC_OSC_NOCLKOUT ' Internal Oscillator
@ DEVICE pic12F629, WDT_ON ' Enable watch dog timer
@ DEVICE pic12F629, PWRT_ON ' Enable power up timer
@ DEVICE pic12F629, MCLR_ON ' Enable MCLR pin

Define OSCCAL_1K 1 ' Oscillator calibration

TRISIO = %11101011 ' turn all I/O to input except GPIO.2 and GPIO.4

CMCON = 7 ' Disable analog comparator

Which is working for all other input and output.

As far as I can tell there is no command that is not allowed. It is all basic stuff like driving a buzzer based on buttons pressed - really nothing fancy. A single input on any of the other lines that does nothing but make a beep seems to be flipping that pin - and that buzzer is on port 2.

I'll keep going through the code.

Thanks.

Bart

I just tried something simple and discovered that when my port 2 goes HIGH, port 4 is going HIGH at the same time, but it will not switch back when port 2 goes LOW again.

Does that suggest anything?

[Luciano]

Hi,

Describe how do you drive the LED and the buzzer.
Post a small schematic with components type and values.

- Voltage on the VDD pin if the PIC.
- Voltage on the emitter of the transistor.
- Type of transistor.
- Type of buzzer.


Also post a complete small program showing the problem.

Best regards,

Luciano

[bartman]

I can give you some of this, but since it's only something I "fiddle with" I don't have anyway right now to provide all info.

I'm running it on 3 volts
Don't have emitter voltage, but it can drive a sound board that runs on 3-4 volts
PNP common type 2N4403
Buzzer is your run of the mill transducer driven from PIN through cap

I don't have a "small" version of the program.

If the line says GPIO.4 = 0 the LED (or the sound board I have on it) works properly.
If I then use GPIO.2 = 1 or I use SOUND GPIO.2 [whatever value] then port 4 goes HIGH instantely.

Switching port 2 back to low does NOT reset port 4 to low so it just works one way.

The idea is that there are three buttons that play different combinations of the SOUND command depending on how they are pressed. With each press there is also a confirmation beep. At the same time there is going to be a sound module working on its own. The confirmation beeps should play and have no effect on the sound module.

The sound module power runs through the transistor which is acting just like a switch that can turn the sound module on or off depending on the value of port 4.

I removed SOUND command that would play the confirmation beep and tried it that way. Without that port 2 address happening the PIC would turn the sound module on or off with the button presses just as it should. That suggested to me that port 2 addressing was causing the problem so I then put a HIGH 2 command in place of the sound command and tried that. Sure enough, the second port 2 went high port 4 did as well.

I will work on a version of the program that is short that I can post here. I'll have to remove a lot to boil it down then I will post that.

Thanks.

Bart

[Ingvar]

Measure the voltage on GPIO.4 when it's low. It MUST be a low voltage(below 1V), if not you will have problems just like this. My guess is that you have a short on the base resistor to the PNP or forgotten it completely.

[bartman]

Success!

It was the resistor issue. I think this is the second time one of those damn things has screwed me up. I also had a 0 and a 1 reversed in part of my code which didn't show up until I got the resistor issue resolved.

So, now my question is this. I have a pre-programmed sound module which I know very little about and won't ever know anymore on. It runs on 3 to 4 volts and probably barely uses any amps at all. It is runs on 50 ma I'd be surprises. It drives a little 1.25" speaker.

I've been using just comman transistors and resistors on the breadboard in an effort to just turn the power on/off to the sound module. That problem now fixed thank God.

But, when I go to put this together for real I want to ensure I'm using suitable stuff that is over-engineered just enough to keep things working as future repair will be pretty much impossible.

So, is there a generic enough way to come up with a suitable transistor and resitor value to do this?

I had originally picked out a BCX5316CT-ND from Digikey as a transistor because it seemed to match the common through hole one I was using for testing. Resistor I was never quite sure on, but was thinking like 330 ohm.

Advice on this issue would be appreciated.

Thanks for enough questions and comments that I could troubleshoot this to a conclusion.

Bart

[Melanie]

>> So, is there a generic enough way to come up with a suitable transistor and resitor value to do this?

How technical do you want to get?

Current in mA through the Collector divided by the gain of the Transistor (see Transistors Datasheet) should give you the current through the Base. Pick a suitable Resistor to fit (5v out from the PIC less 0.7v for the Transistors Emitter/Base junction divded by the previously calculated Base Current). Example 50mA Collector current with a Transistor gain of 150 should give you a Base current of around 333uA. 4.3v/333uA=12.9K Resistor... drop down to next lowest preferred value=12K.

Alternatively fit a 10K Resistor in the Base which should drive most decent small/medium Transistors on the planet.

[bartman]

Probably 10K will do!

But, here are the specs from the transistor I picked.

I'm not sure which of these figures is gain, but I think it is HFE@1c/VCE

min: 100
max 250
mA -150
V -2

In case not, we have VCE(sat)
Max V -.5
@IC (mA) -500
@IB (mA) -50

Also, PIC will be running on 3.7V same as sound board.

Bart