Phillip Things should be as simple as possible but no simpler Phillip Coiner CTO, GPS Source, Inc. Your source for quality GNSS Networking Solutions and Design Services, Now! -----Original Message----- From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of Bob Axtell Sent: Tuesday, May 23, 2006 2:52 PM To: Microcontroller discussion list - Public. Subject: Re: [PIC] Inputs higher than VDD Phillip wrote: > Hi all > > I'm using a 16F676 to control DC through a GPS RF power divider/splitter. > > First a little background. > > GPS receivers need an antenna preamplifier to over come the line losses at > 1575 GHZ(L1) and 1227 MHz(L2) there soon will be other freqs but that are > all in 1-2 GHz band. > > DC is sent on the center coaxial center conductor up to the preamplifier in > the antenna and RF is sent back the down the coaxial cable to the GPS > receiver. > > > > The RF input of the splitter is connected to a GPS antenna mounted on a > tower with a GPS receiver connected to each of the RF outputs of the > splitter. > > This way you only have to pay a monthly fee for one GPS antenna on the tower > instead of many and the GPS antenna is a low point of failure. > > My pick and choose circuit looks for DC on the inputs and selects one and > only one to provide power to the antenna's preamplifier through the > splitter. > > If you connect two power supplies together they will fight with one "winner" > burning the others out so there can only be one DC path through the splitter > at any give time to the antenna preamplifier. > > The unused DC inputs have a DC load to ground so that the receivers think > they are powering a preamplifier of an antenna and consequently don't throw > antenna faults. > > If one of the GPS receivers keels over and stops providing DC voltage to the > antenna then the next receiver's DC load is switched out and now its power > is routed through the device to the antenna preamplifier while the first > input's path is disconnected form the antenna. > > It there is an antenna fault (over current or under current) then the > antenna DC is switched open/off and all the DC loads are switched out (open > circuit) so now all GPS receivers will throw antenna faults. > > > > The code for this all works fine but I have a couple of issues that are > giving me pause. > > > > There is no external power available so I steal some of the antenna current > to power my the control and gain block circuitry of the splitter. > > > > Most GPS receivers use 5V DC to bias the antenna preamplifier but some can > output as much as 12V DC so the logic inputs of my PIC can be anywhere from > 5V to 12V. > I think it would be better to provide a separate supply. [Phillip says] I agree it would be better but this is not an option. There is no power on the mast. I have to use what is on the coaxial cable provided by the GPS receivers. > I used a low drop out regulator to power the PIC at 3.3v and the RF > amplifier circuitry (the device can be operated as a passive splitter so the > RF amplifier is optional) > > So I put a series resistor and a 3V zeiner to ground on each of the DC > inputs to the PIC to keep the inputs below VDD that was fine till I had to > program the thing. > > The zeiner kept the programming lines at 3V and this was no good. > > So now there is a series resistor a zeiner to ground and another series > resistor on the input pins so the ICD2 can pull the programming lines higher > than 3 volts during programming. > The ICD2 is debugging or programming? In production, I'd use another programmer, the ICD2 is much too cranky to use as a production programmer. If you are debugging only, you can debug at 3.3V OK. [Phillip says] I agree the ICD2 is okay for development but it is not a production programmer it is very temperamental. My technician hates it with good reason. [Phillip says] > The ICD2 programming connector is connected directly to the device and the > LDO regulator is not hurt by taking VDD i.e. its output pin or VDD of the > PIC to 5 volts. (whilst the LDO regulator has no input power of course) > > The trouble is that according to the data sheet my 3V zeiner can go as high > as 3.5 when in breakdown. > > If the PIC's VDD is at 3.3V and the input pin is at 3.5V due to the 3V > zeiner heating...is this bad?....how bad? > Why not use a 2.5V Zener? It will never go above 3.3, and is high enough to express the correct logic level. Better is to install schottky diodes to VDD at the chip input, so that when the value tried to go above VDD, the tiny current is routed into the VDD supply. Careful here: some types of VDD supplies may not work correctly this way. [Phillip says] I will have to go to the lower voltage zeiner no matter the cost. If I have a schottky to VDD and my input goes to 12V I have 12 volts through a schottky to the output of my 3.3 regulator. But I also thought that the MCLR line had to go to 12V in order to program the device?? I need to use the RA3 line as an input so I can't clamp it to VDD for that reason either. > I can't really find anything on the data sheet and I was hoping that someone > out there could tell me about any latch up or other issues.. (I could lower > the zeiner to 2.7 but I'd rather not unless I have to.) > > Those low voltage zeners are expensive, right? [Phillip says] I'm not sure about the cost of the lower voltage zeiner but it looks like I'm buying no matter what. I just found the spec in the data sheet it says Voltage on all other pins with respect to VSS -.3V to (VDD + .3V) now 3.5 is less than 3.3V + .3V = 3.6V But if my 12V supply is a tiny bit over 12V then I reckon it will be too close to the wire. > > > Does anyone out there have any experience (good or bad) with taking inputs > a "little" higher than VDD? > > Bad, bad ideal. The PIC will either lock up of become unable to be reset properly. > > > > > > > The other issue is that I'm I am doing twenty units for evaluation i.e. > environmental lighting strikes etc. > use those cute ESD suppressors in 0805 by Cooper-Bussman. VERY low capacitance (.5pF). > Out of the twenty 4 did not verify after programming. I'll bet. [Phillip says] We solder all kinds of other products at these stations with no problems. We have just switched over to non lead solder and we had to up the temperature on our irons. I reckon I could add the ESD suppressors next to the PIC inputs. It just might be the better part of valor. > > > They showed incorrect data at a specific location (all four showed the same > location?????) > > Sometimes I can reprogram and they pass other times they might fail ten > times in a row. > > If I replace the PIC everything is fine? > > How could I get four devices out of twenty that are bad? > They are not bad, they simply are never properly reset. [Phillip says] [Phillip says] I'm not sure what you meant here can you elaborate? > I can't imagine this is a quality issue. > > All the parts appear to have the same date/batch codes according to my > technician. > > Any guesses to what is going on here? > > > Get rid of the external power sources. [Phillip says] I'm not sure what you meant here either can you elaborate? Mucho!!! thanks Bob --Bob > > > > > Phillip > > Things should be as simple as possible but no simpler > > > > > > > > Phillip Coiner > > CTO, GPS Source, Inc. > > > > > > Your source for quality GNSS Networking Solutions and Design Services, Now! > > > > > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist