Power-on reset. Start-up timer.

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- 12th July 2010, 21:20
Lasse1

Power-on reset. Start-up timer.

Hello.
I have a pic16f690 in a project. It works fine at the developement board. But mounted at the card i like to use it with, the program will not start. (It starts if I push the MCLR-pin) Probably this problem is caused by a slow power-up, i have a condensator connected to pwr.
I read in the data cheet that this pic have a couple of featuers who maybee can solve my problem. Power-on-reset, power-up-timer and maybee others. But how do I program it into the pic??
I found this string in this forum: "_PWRT_ON_2L" but I can not find any other code ty try. (It didn´t help).
Can anyone help me?
/ Lasse
- 12th July 2010, 22:10
mackrackit

The MCLR pin needs to be pulled high or shut off.
If you shut it off it can be used as an INPUT.

Add this to your configuration to turn it off

Code:

_MCLRE_OFF

And this is a helpful thread
http://www.picbasic.co.uk/forum/showthread.php?t=543
- 12th July 2010, 23:19
Lasse1

Hi again.
Thank you for your fast answer. It was a helpful link you gave me, I learned a little bit moore. I tried booth "POR" and "PWRT" but unfortunal it didn´t solve the problem. Do you have some other idea? Otherwhise I have to try to solve it the hardwhere-way.
Friendly regards / Lasse
- 12th July 2010, 23:39
mackrackit

Did you try
_MCLRE_OFF
- 13th July 2010, 07:01
Lasse1

Yes. I did.
And to be clear I can say that I am complete sure that my problem depend on power-up. (If I set the PIC in the holder with the pwr-leg bended away, and then connect it after the unit is powered-up, the pic starts correct)
You don´t have any other idea?
Friendly regards / Lasse
- 13th July 2010, 08:04
mackrackit

I am not sure what you mean by pwr-leg?
Can you post how everything is connected using pin numbers in the description or a schematic?
- 13th July 2010, 18:45
Lasse1

Hello.
Sorry about my defenition. I mean the VCC-pin. And I dont think that it is relevant how other pins are connected. I tried different connections and have the same troubble. The thing is that it is one sekund interrupts in the power supply every five sekund, and cause of that it is a condensator connected to the power supply to keep the pic alive. That is working fine, the troubble occurs only when it is powered up, and only when the condensator is connected. This was working fine with a basic stamp but a pic is obviusly more critical.
- 23rd July 2010, 08:29
BrianT

Minimum dV/dT needed

Slowly rising power supplies will trip a PIC up. There is a minimum dV/dT of something like 5 V/uSec needed. RTFM will find it. No amount of resets will recover a PIC that has been powered up slowly.

I had this problem with a current limited power supply that slowly rose in voltage as it had to charge a big cap.

Minimise input storage capacitors, or put them direct across your PSU before the power switch so they can supply a burst current to the PIC circuitry and get the Vdd up within the allowed time.

HTH
BrianT
- 23rd July 2010, 11:58
Dave

Lasse1 , You could also use an Processor supervisor device like MAX690 or something like it. It will hold the processor in reset until the power is stable and above the minimum level. The other thing it can do for you is an external watch dog timer as well as low power supply monitor...

Dave Purola,
N8NTA
- 23rd July 2010, 13:19
BrianT

Supervisor chip unlikely to compensate for slow rise PSU

PIC18F4620 parameter D004 says 0.05V/mSec is the minimum dV/dT to get a reliable startup. A few years ago I had a dodgy system (with a Motorola xxxx supervisor chip) that was intermittent until I reduced the inlet filter capacitors to ensure faster voltage rise at power up.

Your mileage may vary but adding the supervisor did nothing for me.

HTH
BrianT
- 24th July 2010, 00:33
Dave

Brian, Actually, that is exactly what they are made for. I have used them for about the last 5 or so years on all of my products and projects. I have never had a problem with the reset line except when I forget to toggle the watchdog input.....

Dave Purola,
N8NTA
- 26th July 2010, 22:31
Lasse1

Quote:

Originally Posted by Dave

Brian, Actually, that is exactly what they are made for. I have used them for about the last 5 or so years on all of my products and projects. I have never had a problem with the reset line except when I forget to toggle the watchdog input.....

Dave Purola,
N8NTA

Thank you all. I will try a MAX-unit and see. Then I will let you know, here.
/ Lasse