Post by thread:porta troubles

I discovered a problem with either my circuit or my PIC code, and I'm stumped. Not sure anyone can help "remotely", but on the off chance the symptoms ring any bells for anyone, I thought I'd write it up. One of my porta pins (this is a 16f84) is used to turn on and off power to an external device. porta has some input pins and some output pins, but none of them change from one to the other, so I don't think it's a "read-modify-write" problem. The pin is directly connected to the input of one gate of a 74HC14 (hex schmitt inverter), and the inverter in turn drives the gate of an n-channel MOSFET: so when the porta pin is high, the inverter output is low, turning the FET "on". And vice versa. I'm using bsf and bcf to toggle the pin. The problem is that the pin, once it's been brought high, doesn't seem to want to go back low. I assume it's a software problem, but I've been over the code many, many times and can't find it. And in fact the simulator tells me it's working fine: under simulation, the pin goes low when it should. But in the circuit, once it's on it stays on. Weird. I'm stumped. I can't for the life of me think of any electrical reason the pin should get stuck "on", can anyone else? I've tried several different PICs, and the symptoms are the same, so I don't think I blew up that pin. Besides, when I reset, it goes low, as it should at startup. Any ideas or debugging techniques that might be helpful will be greatly appreciated. Dave Johnson

Dave Johnson wrote: > Any ideas or debugging techniques that might be helpful will be greatly > appreciated. I've had similar problems with a 12C509 over the weekend, and my two debugging techniques are; a) probe the voltage on the pin using body resistance to +5V so as to determine if the pin is open (TRIS set) or being driven. b) drop the whole thing and come back later. -- James Cameron (spam_OUTcameronTakeThisOuTstl.dec.com) OpenVMS, Linux, Firewalls, Software Engineering, CGI, HTTP, X, C, FORTH, COBOL, BASIC, DCL, csh, bash, ksh, sh, Electronics, Microcontrollers, Disability Engineering, Netrek, Bicycles, Pedant, Farming, Home Control, Remote Area Power, Greek Scholar, Tenor Vocalist, Church Sound, Husband. "Specialisation is for insects." -- Robert Heinlein.

> b) drop the whole thing and come back later. Excellent advice!! :-) That, in fact, is just what I did, and a half hour after I got back to it, the problem was found. It was indeed a programming error, but it turns out it wasn't mine at all: it was a bug in the application that talks to my box (the command that toggles that pins wasn't being sent - duh). But if I'd have kept going I'd probably have spent way too much time poking around in my code and my circuit. Dropping the thing for a while is exactly what gave me the perspective to start looking outside the box (literally, in this case). Dave Johnson

At 06:12 PM 3/01/99 -0800, you wrote: Dave, Could it be RA4 which is an open collector?? good luck, Peter Grey {Quote hidden}>I discovered a problem with either my circuit or my PIC code, and I'm >stumped. Not sure anyone can help "remotely", but on the off chance the >symptoms ring any bells for anyone, I thought I'd write it up. > >One of my porta pins (this is a 16f84) is used to turn on and off power >to an external device. porta has some input pins and some output pins, >but none of them change from one to the other, so I don't think it's a >"read-modify-write" problem. > >The pin is directly connected to the input of one gate of a 74HC14 (hex >schmitt inverter), and the inverter in turn drives the gate of an >n-channel MOSFET: so when the porta pin is high, the inverter output is >low, turning the FET "on". And vice versa. > >I'm using bsf and bcf to toggle the pin. The problem is that the pin, >once it's been brought high, doesn't seem to want to go back low. I >assume it's a software problem, but I've been over the code many, many >times and can't find it. And in fact the simulator tells me it's working >fine: under simulation, the pin goes low when it should. But in the >circuit, once it's on it stays on. Weird. I'm stumped. > >I can't for the life of me think of any electrical reason the pin should >get stuck "on", can anyone else? I've tried several different PICs, and >the symptoms are the same, so I don't think I blew up that pin. Besides, >when I reset, it goes low, as it should at startup. > >Any ideas or debugging techniques that might be helpful will be greatly >appreciated. > >Dave Johnson > >

Hi Dave I've had the same problem ones (few weeks ago). I don't remember if the pin on portA was set and won't reset or vice versa. I resolved this problem with this: movlw value andlw(iorlw) value ;mask movwf portA instead of bsf portA,bit bcf portA,bit hope this help Valter Dave Johnson wrote: {Quote hidden}> I discovered a problem with either my circuit or my PIC code, and I'm > stumped. Not sure anyone can help "remotely", but on the off chance the > symptoms ring any bells for anyone, I thought I'd write it up. > > One of my porta pins (this is a 16f84) is used to turn on and off power > to an external device. porta has some input pins and some output pins, > but none of them change from one to the other, so I don't think it's a > "read-modify-write" problem. > > The pin is directly connected to the input of one gate of a 74HC14 (hex > schmitt inverter), and the inverter in turn drives the gate of an > n-channel MOSFET: so when the porta pin is high, the inverter output is > low, turning the FET "on". And vice versa. > > I'm using bsf and bcf to toggle the pin. The problem is that the pin, > once it's been brought high, doesn't seem to want to go back low. I > assume it's a software problem, but I've been over the code many, many > times and can't find it. And in fact the simulator tells me it's working > fine: under simulation, the pin goes low when it should. But in the > circuit, once it's on it stays on. Weird. I'm stumped. > > I can't for the life of me think of any electrical reason the pin should > get stuck "on", can anyone else? I've tried several different PICs, and > the symptoms are the same, so I don't think I blew up that pin. Besides, > when I reset, it goes low, as it should at startup. > > Any ideas or debugging techniques that might be helpful will be greatly > appreciated. > > Dave Johnson

I had the exact same problem when I was trying to drive a mosfet directly from a port pin. Turns out is *was* the read-write-modify problem. I was using a BCF on one pin immediately followed by a BSF on another pin. The first pin wouldn't get pulled low. If I connected a high speed digital scope I could just see a tiny glitch on the pin. Adding a NOP between the instructions cured the problem. I was only running at 4MHz, so I put the problem down to gate capacitance on the mosfet. These days I generaly don't do bit clears and sets directly on ports. I perform bit operations on a shadow register and then copy the shadow register to the port. You *can't* get bitten by this quirk if you do that, although it does take a few more instructions. Regards Mike Rigby-Jones .....mrjonesKILLspam@spam@nortelnetworks.com {Quote hidden}> ---------- > From: Dave Johnson[SMTP:djohnsonKILLspamSIRIUS.COM] > Sent: 04 January 1999 02:12 > To: .....PICLISTKILLspam.....MITVMA.MIT.EDU > Subject: porta troubles > > I discovered a problem with either my circuit or my PIC code, and I'm > stumped. Not sure anyone can help "remotely", but on the off chance the > symptoms ring any bells for anyone, I thought I'd write it up. > > One of my porta pins (this is a 16f84) is used to turn on and off power > to an external device. porta has some input pins and some output pins, > but none of them change from one to the other, so I don't think it's a > "read-modify-write" problem. > > The pin is directly connected to the input of one gate of a 74HC14 (hex > schmitt inverter), and the inverter in turn drives the gate of an > n-channel MOSFET: so when the porta pin is high, the inverter output is > low, turning the FET "on". And vice versa. > > I'm using bsf and bcf to toggle the pin. The problem is that the pin, > once it's been brought high, doesn't seem to want to go back low. I > assume it's a software problem, but I've been over the code many, many > times and can't find it. And in fact the simulator tells me it's working > fine: under simulation, the pin goes low when it should. But in the > circuit, once it's on it stays on. Weird. I'm stumped. > > I can't for the life of me think of any electrical reason the pin should > get stuck "on", can anyone else? I've tried several different PICs, and > the symptoms are the same, so I don't think I blew up that pin. Besides, > when I reset, it goes low, as it should at startup. > > Any ideas or debugging techniques that might be helpful will be greatly > appreciated. > > Dave Johnson >