Well, the nasty question is which 3 of the 6 boards are failing, yours or the assemblers
Can you check the crystal to see if its oscillating? Maybe you have the wrong load caps on this time around?
Are the voltage levels right? Can you probe them with an oscilloscope to check for signal quality? (though I'd spend more time on the oscillator question)
Are there any tantalum caps in your design (and if so, are they backwards)?
WOW! The most wildly improbably thing just happened! I was measuring the current one of these boards draws, and after I did that I didn't bother to unhook it or even turn off the power supply. I got preoccupied with something else. Suddenly, I noticed that the LEDs on my board were blinking erratically. I think this is the board I tried to burn a test app into that makes those LEDs blink.That gave me a hint the crystal was an issue. So, I did what any experimenter would do. I poked at it. That made things change. The LEDs would stop blinking, or change their rate. I hooked up my programmer and I could read the device ID!So I whipped out my soldering iron and tried to reheat the crystal (its thru hole). No joy. So then I replaced it with a 20 MHZ crystal (I don't have any 16MHz on hand) and wala! It seems my board is repaired! Later today I'll go out and get some 16 MHz crystals and see if that's the issue with the other board I have here as well.So then, that brings me to the next question. Why did this happen? The crystal I am using is thru hole. The one I installed on the board I made that failed, I installed with a soldering iron. The assembler used a wave soldering machine for all his. It is a DigiKey part number: 887-1244-ND which is a 16 MHz, 18pf thru hole crystal made by TXC Corporation. Are these knows to be easily damaged by heat, or for being particularly fragile?
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