MCLR doesn´t restart the code (include schematics) Why?

Helyos · Joined: 22 Oct 2003 Posts: 22

Hi, that is the problem.
I am using 16f876 and a res of 10K conected from 5v. to MCLR (pull-up).
When I disconect the power suply adn connect it, the chip start correctly.
But if I active MCLR line (low active, I connect 1-2 secconds to ground) the chip doesn´t start the code, why?
Thanks

Ttelmah · Guest

Helyos · Joined: 22 Oct 2003 Posts: 22

Hi, first, thanks for your answer.
I tested all posibilities, I will put here the schematics of my circuit.
I know that has to be an electronic problem, because when I connect the power supply it starts correctly, and when I do a software reset (reset_cpu();) it resets and start again correctly, but if I press the reset button (connect the MCLR line with Ground -with or without a resistance-) it seems that reset (clean the display) but it doesn´t start again the program.
I hope that your opinions can help me. Thanks a lot.

Hans Wedemeyer · Joined: 15 Sep 2003 Posts: 226

This may not be on topic, and don't take offence.

I've seen this style of schematic drawing, it sometimes pops up in application note, and I often wonder who is teaching this style .... ?

OK if that's what you have been taught it's all you know but it sure looks like a mess.

Needles to say I don't like the style.

It does not allow a quick overview of the " function blocks". If you add european logic symbols it would be even worse...

For example:
If you draw a Colpits oscillator this way it can't be possible to identify it at a glance.
A simple tuned circuit would have the coil and capacitor separate and every connection would have to be studied before it became obvious it’s a tuned circuit.

If possible please explain what are the benefits, perhaps the original teacher made some claims about it. I'm seriously interested.

frankb · Joined: 08 Sep 2003 Posts: 7

One thing jumps out. Q1 and Q2 are pinned out wrong AND are connected wrong as drawn. These aren't related to the reset problem but without a thorough H/W design check how many other things might be hooked up wrong? Re-drawing the circuit in a more traditional logical way might shake out a few bugs. The signals have no "flow" the way its presently drawn. Just my 2 cents.

Helyos · Joined: 22 Oct 2003 Posts: 22

Excuse me, I didn´t know that a scheme could bother so much.
This fellow of schemata I seem to good because at a glance I can see an entire, necessary- and- exact- manner circuit, without need of symbols, but it is my opinion.
Thank you very much for your help with the problem with MCLR.
Greetings

frankb · Joined: 08 Sep 2003 Posts: 7

Sorry Helyos Embarassed

. I was just pointing out some errors that could be causing you problems. If you're comfortable with the way you draw schematics then you are welcome to keep doing it that way.
To trouble shoot your reset problem I would recommend doing the following:
1) Remove Q1 and Q2 from the circuit
2) Replace the PIC. It may be damaged depending on how Q1 and Q2 are really hooked up to it.
3) Disconnect everything else connected to the board connectors. (except power)
4) Verify with a scope or meter that the correct pin of the PIC is being pulled low by your reset signal and that the 5 volt power to it remains stable.
5) If this all looks OK and you still can't reset, measure the current being drawn by the circuit. From what I remember seeing on your schematic your current shouldn't be more than about 20 - 30 mA (without LCD connected). If it's more than that something may still be connected wrong.

I hope some of this helps you find the problem. Good luck and I sincerely appologize if I came on too strong Laughing

Helyos · Joined: 22 Oct 2003 Posts: 22

Thank you very much for your time. I answer your questions:
1) I haven´t the transistors conected. In the schematics they seem are wrong (colector, base emisor) but they are well, I put that order for pins reasons (for the layout)
2) I tested the circuit with 3 micros, the same results in three.
3) If I disconnect all I can´t verify that the circuit resets. I tried disconnect the eeprom, the serial/parallel converter, and the same results.
4) I verify with the multimeter and with an osciloscope the signal, and the signal seems correct, from 4,9 v to 0,1 and then became high again.
5) The circuit (with all connected except LCD) current working well is 8,3 mA (with peaks of 14 mA when the buzzer sounds). If I press reset (MCLR to ground) it became 7,1 mA continously and it doesn´t work. :(
What´s wrong??? I´m desperate...

ajt · Joined: 07 Sep 2003 Posts: 110

You might try removing the capacitor on the MCLR line. The PIC has a noise filter on the MCLR line so the cap is unecessary. Possibly the slow rise of the signal is problematic
_________________
Al Testani

Guest · Posted: Sat May 15, 2004 10:04 pm

What is R4 doing ?
One side is connected to MCLR and the other side is BMCLR what does BMCLR do ?????

If you are pulling MCLR low by grounding BMCLR of R4 then it will not reset the uP.

The Cap on MCLR is not needed.

If you just cleaned up your schematci and draw it in a conventional mannor, to boot, read the data sheet and perhaps you can get this thing working.

ajt · Joined: 07 Sep 2003 Posts: 110

Actually R4 alone would not keep the chip from resetting. The data sheet says max low input on MCLR is 0.2Vdd. With C2, however, strange things could happen. R4 and C2 should be eliminated in any case.
_________________
Al Testani

Helyos · Joined: 22 Oct 2003 Posts: 22

The R4 is disconnected and the capacitor is disconnected too. The same results. What´s wrong???

Guest · Posted: Sun May 16, 2004 3:01 pm

Guest · Posted: Sun May 16, 2004 3:21 pm

ajt · Joined: 07 Sep 2003 Posts: 110

I agree.... I think that isolating MCLR totally, connecting a 51K resistor to Vcc (with no cap) and grounding the MCLR end of the resistor will surely reset the processor. Resetting a PIC is not rocket science.
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Al Testani