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My project's run too fast

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Post the schematic for your thermometer so we can have a look. I edited my post after discovering that the PIC16F84 doesn't have an A/D converter.
 
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I have post at the begining but here is again.
 

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Well, the temperature sensor you're using is digital so that rules out A/D conversion and reference-voltage issues.

I would verify that the 18B20's supply voltage is within spec for both your supplies (3.0-5.5V). Otherwise, it's probably a software issue. Maybe if the crystal is running fast it's causing the PIC to request readings from the 18B20 faster than it's rated for.

EDIT: One more thing... is your 5V supply stable, and providing enough current? Maybe it's got ripple or sag that's affecting your circuits. Try putting a 100uF cap across the 5V supply input, and a .1uF cap across Vdd/Vss on the PIC as close to the PIC as possible.
 
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Well, the temperature sensor you're using is digital so that rules out A/D conversion and reference-voltage issues.

I would verify that the 18B20's supply voltage is within spec for both your supplies (3.0-5.5V). Otherwise, it's probably a software issue. Maybe if the crystal is running fast it's causing the PIC to request readings from the 18B20 faster than it's rated for.

EDIT: One more thing... is your 5V supply stable, and providing enough current? Maybe it's got ripple or sag that's affecting your circuits. Try putting a 100uF cap across the 5V supply input, and a .1uF cap across Vdd/Vss on the PIC as close to the PIC as possible.
My supply is stable,but I dont anderstend why at 5v thermometer wrongs,in 3.7v works fine?
 
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