Post by thread:[PIC] Power Concerns - Can't get the chip to start

I've been using an olimex board (P18-D) for my PIC development up until today. I'd gotten my project running stably enough that I wanted to put it all on a breadboard and try using it in my car. So, I got it all wired up, moved the chip into the breadboard, and powered the voltage supply setup I was using (simply 5V voltage regulator) off the olimex board's power supply. This started working a bit sketchily, but started up most of the time. I then tried powering it directly off a 12V wall wart. It was still going through the same standard 5V voltage regulator setup, and the chip was getting 4.9V reliably to Vdd as expected, so I would figure it would be fine. However, nothing I can do can get the chip to start up under the new voltage source. I unplug that and plug the olimex board's power supply back in and it works fine. Am I missing something here? The only thing I can think of is that I might need some filtering capacitors or something on the input line...? It doesn't make any sense to me that it would NEVER work, considering it's getting a stable 4.9V on the power line. Help? :) -David

In case anyone's curious, I solved the problem. I did the following two things: First, I added a ton of filtering caps: 220uF, 100uF, 10uF, 4.7uF, and 0.1uF. That helped a good bit. That let me use the non-filtered voltage source and it would boot up 1/3 of the time or so. That was still crappy, so I found the circuit diagram for the olimex board and looked to see what it had that I was missing. I noticed it had a resistor from the MCLR pin to the Vdd pin through a 10k resistor. I looked up in the manual, and that's how you enable POR. Silly me... So, tossed the MCLR pin through an 8.5k resistor (what I had on hand) to Vdd... Boots up every time. I can also remove most of the filtering caps at this point and it runs too, though the 220uF and the 0.1uF seem to be essential. But, in general, that's my lesson for the day - tie up the damn POR pin. :D -David

{Quote hidden}>-----Original Message----- >From: spam_OUTpiclist-bouncesTakeThisOuTmit.edu [.....piclist-bouncesKILLspam@spam@mit.edu] >Sent: 07 October 2005 11:42 >To: Microcontroller discussion list - Public. >Subject: RE: [PIC] Power Concerns - Can't get the chip to >start on my own voltage source > > >In case anyone's curious, I solved the problem. > >I did the following two things: > >First, I added a ton of filtering caps: 220uF, 100uF, 10uF, >4.7uF, and 0.1uF. That helped a good bit. That let me use >the non-filtered voltage source and it would boot up 1/3 of >the time or so. > >That was still crappy, so I found the circuit diagram for the >olimex board and looked to see what it had that I was missing. > I noticed it had a resistor from the MCLR pin to the Vdd pin >through a 10k resistor. I looked up in the manual, and that's >how you enable POR. Silly me... So, tossed the MCLR pin >through an 8.5k resistor (what I had on hand) to Vdd... Boots >up every time. Something sounds screwy to be honest. A 10k resistor from MCLR to Vdd should be perfectly adequate (that's what most of my designs have). The resistor does not "enable" power on reset as such. What it does do is allow in circuit programming, and if combined with a capacitor and another resistor gives a delayed power on reset which can be helpful in some circumstances. If the 10k was open circuit etc. then this would cause you a lot of problems. I would check that you have the brown-out reset, and power up timer enabled in your configuration fuses, and that you have the correct oscillator configuration if you are using an external crystal/resonator. These are very important for reliable start up. Regards Mike ======================================================================= This e-mail is intended for the person it is addressed to only. The information contained in it may be confidential and/or protected by law. If you are not the intended recipient of this message, you must not make any use of this information, or copy or show it to any person. Please contact us immediately to tell us that you have received this e-mail, and return the original to us. Any use, forwarding, printing or copying of this message is strictly prohibited. No part of this message can be considered a request for goods or services. =======================================================================

On Oct 7, 2005, at 3:42 AM, David de Regt wrote: > I can also remove most of the filtering caps at this point and it runs > too, though the 220uF and the 0.1uF seem to be essential. But, in > general, that's my lesson for the day - tie up the damn POR pin. you just had it floating? Yeah, that'd be a problem... Sounds like your wall wart isn't well filtered; some such things, while they put out "DC", apparently contain only a rectifier, which makes them fine for charging NiCd batteries, but not too good for anything else (without adding capacitance yourself.) BillW

One thing I've noticed is that startup takes a LONG time, like 3 seconds, when I'm not using a crystal. When using a crystal, it boots up instantly. The oscillator is set to IntRC in the config byte, and the PWRT, MCLR, BODEN, and CCPMX flags are set (CP and Write enable are off.) I'm wondering if the bootloader is being stupid and just using the IntRC at 32khz instead of immediately bumping it up to 8 MHz. Might be time to try yet another bootloader... -David {Original Message removed}