Post by thread:PIC16F877 unreliable start up

Hi Folks, I've been playing around with a sample 16F877 and I am having problems with not getting a reliable start up. I'm trying to run it at 4 MHz (using a crystal and two 15 pF caps). The operation of the oscillator seems to be hit or miss (mostly miss). I've played around with different parts (I've tried a ceramic resonator, different values for the capacitors) and still nothing that is reliable. I have not tried an in-line resistor. Has anybody else seen this type of problem and if you did, what did you do to fix it? Thanx, myke

> I'm trying to run it at 4 MHz (using a > crystal and two 15 pF caps). The operation of the oscillator seems to be > hit or miss (mostly miss). I've played around with different parts (I've > tried a ceramic resonator, different values for the capacitors) and still > nothing that is reliable. I have not tried an in-line resistor. > > Has anybody else seen this type of problem and if you did, what did you do > to fix it? Yes. Exactly the same symptom I had when I was *sure* the config bits were set for XT oscillator, but they were REALLY set for LP oscillator. This is a new part for you? Even money the config is mis-programmed as LP or RC or something, not XT. I hope I'm right, 'cause its easy to fix by re-setting the config bits somewhere in the tool chain. In my case, I had an old #define that was setting the __config bits incorrectly for the part I was using. ------------ Barry King, KA1NLH Engineering Manager NRG Systems "Measuring the Wind's Energy" Hinesburg, Vermont, USA http://www.nrgsystems.com

On Thu, Sep 23, 1999 at 12:59:52AM -0400, Myke Predko wrote: > Hi Folks, > > I've been playing around with a sample 16F877 and I am having problems with > not getting a reliable start up. I'm trying to run it at 4 MHz (using a > crystal and two 15 pF caps). The operation of the oscillator seems to be > hit or miss (mostly miss). I've played around with different parts (I've > tried a ceramic resonator, different values for the capacitors) and still > nothing that is reliable. I have not tried an in-line resistor. > > Has anybody else seen this type of problem and if you did, what did you do > to fix it? I'm not sure if entering the programming mode may affect the oscillator, but is the RB3 pin floating? If yes, tie it to the ground. -- Greetings Wojciech M. Zabolotny http://www.ise.pw.edu.pl/~wzab <--> spam_OUTwzabTakeThisOuTise.pw.edu.pl http://www.gnupg.org Gnu Privacy Guard - protect your mail & data with the FREE cryptographic system

Myke Predko wrote: > > I've been playing around with a sample 16F877 and I am having problems with > not getting a reliable start up. I'm trying to run it at 4 MHz (using a > crystal and two 15 pF caps). The operation of the oscillator seems to be > hit or miss (mostly miss). I've played around with different parts (I've > tried a ceramic resonator, different values for the capacitors) and still > nothing that is reliable. I have not tried an in-line resistor. > > Has anybody else seen this type of problem and if you did, what did you do > to fix it? I plugged my 877 sample directly into my 16C74A based product and have no startup problems. 3.6864 MHz crystal, 15pF caps, no resistors. I suspect that Barry is correct. --Matt

Myke, I'm probably not of much help other than to suggest the obvious which you already know: 1. What's MCLR tied to? 2. Any noise on the supply rails? 3. Is this on a plug-in breadboard? That adds capacitance. 4. Is it a parallel resonance crystal? 5. What Oscillator mode are you using? 6. Is LVP enabled? If so, what's on RB3? I recently upgraded a 16C77 prototype to a 16F877 (using ES samples) and I've had no problems running at 20MHz with 18pf caps at 5Vdd. I'm using a microEngineering Labs PicProto64 board. - Tom At 12:59 AM 9/23/99 -0400, Myke Predko wrote: {Quote hidden}>Hi Folks, > >I've been playing around with a sample 16F877 and I am having problems with >not getting a reliable start up. I'm trying to run it at 4 MHz (using a >crystal and two 15 pF caps). The operation of the oscillator seems to be >hit or miss (mostly miss). I've played around with different parts (I've >tried a ceramic resonator, different values for the capacitors) and still >nothing that is reliable. I have not tried an in-line resistor. > >Has anybody else seen this type of problem and if you did, what did you do >to fix it? > >Thanx, > >myke ------------------------------------------------------------------------ Tom Handley New Age Communications Since '75 before "New Age" and no one around here is waiting for UFOs ;-)

Myke, Also, probably obvious as well, but did you try unbalanced caps? --Bob On Thu, Sep 23, 1999 at 09:42:09AM -0700, Tom Handley wrote: {Quote hidden}> Myke, I'm probably not of much help other than to suggest the obvious > which you already know: > > 1. What's MCLR tied to? > 2. Any noise on the supply rails? > 3. Is this on a plug-in breadboard? That adds capacitance. > 4. Is it a parallel resonance crystal? > 5. What Oscillator mode are you using? > 6. Is LVP enabled? If so, what's on RB3? > > I recently upgraded a 16C77 prototype to a 16F877 (using ES samples) > and I've had no problems running at 20MHz with 18pf caps at 5Vdd. I'm using > a microEngineering Labs PicProto64 board. > > - Tom -- ============================================================ Bob Drzyzgula It's not a problem .....bobKILLspam@spam@drzyzgula.org until something bad happens ============================================================ http://www.drzyzgula.org/bob/electronics/ ============================================================

Hi Wojciech, Thanx for the pointer - you helped me find the problem. In my application (when it did work), I wrote some parameter data to PortB - when I checked it with my Logic Probe, I couldn't understand why it wasn't being driven (I cleared TRISB). I should have looked more closely at the documentation. While it would never boot with RB3 tied to ground, I added the parameter: "_LVP_OFF" To the "__CONFIG" statement and the 16F877 runs fine now. I think this little hint will end up on my web page. Thanx to everyone else who replied. myke {Original Message removed}