This is a multi-part message in MIME format. --===============0994702745== Content-Type: multipart/related; boundary="----=_NextPart_000_0029_01C67E5F.27930FE0" This is a multi-part message in MIME format. ------=_NextPart_000_0029_01C67E5F.27930FE0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit 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 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 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? 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.) Does anyone out there have any experience (good or bad) with taking inputs a "little" higher than VDD? The other issue is that I'm I am doing twenty units for evaluation i.e. environmental lighting strikes etc. Out of the twenty 4 did not verify after programming. 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? 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? Phillip Things should be as simple as possible but no simpler Phillip Coiner CTO, GPS Source, Inc. 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