Short circuit on the SX28 power leads
TDDodds
Posts: 4
I have had good success in using the SX18 chips with leads in a couple of my projects. Since Parallax stopped making these, I have recently been setting up to use the SX20·& SX28 surface mount chips. When programming the SX28, I have found that about 10% of the time the chip goes to a direct short when I plug-in or remove the SX-Key for programming.·It's so bad that it will fry a hole in the top of the chip if I provide enough current from the power supply.
I have checked the wiring very carefully and it is OK, and anyway·I see the same problem·with a chip connected only to +5 volts on 3 & 4 and ground on 1 & 14. The condition of MCLR doesn't make any difference. I thought it might have something to do with the SX-Key, so I substituted an 80 ohm resistor to get the same current in parallel with a 40 mfd. cap·to get a glitch-kick, and when I bridge that across the chip's power leads it also causes it to go to a permanent·short! I tried a parallel 1mfd capacitor to try to eliminate a·voltage glitch, but the problem remains.
Has anybody else seen this - and is there a fix? ·It is obviously some kind of power glitch/voltage problem, but I am amazed that·anything can cause the chip to internally short annd·burn up.
I have checked the wiring very carefully and it is OK, and anyway·I see the same problem·with a chip connected only to +5 volts on 3 & 4 and ground on 1 & 14. The condition of MCLR doesn't make any difference. I thought it might have something to do with the SX-Key, so I substituted an 80 ohm resistor to get the same current in parallel with a 40 mfd. cap·to get a glitch-kick, and when I bridge that across the chip's power leads it also causes it to go to a permanent·short! I tried a parallel 1mfd capacitor to try to eliminate a·voltage glitch, but the problem remains.
Has anybody else seen this - and is there a fix? ·It is obviously some kind of power glitch/voltage problem, but I am amazed that·anything can cause the chip to internally short annd·burn up.
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It sounds like you have had the same experience I have. I also fried a couple of chips and and SX-Key. I
I also·wasn't too concerned initially, as I figured that it was bad practice to attach a Key with the power on. My problem now is that I put·my first three prototype boards using the SX28 into service a couple of weeks ago, and one was returned today with the SX28 burned to a crisp for no apparent reason - it looks exactly like the ones I fried while programming.
Since I have determined with my resistor test that a power glitch will cause the chip to go to a short, even when it is not being programmed, there is·a problem here that I need to resolve before I can ship any more product.
Tom