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Voltage Limiter in DS1621 pc thermometer


gamelektro

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i made the project DS1621 pc thermometer
http://www.electronics-lab.com/projects/pc/010/index.html

There is something i want to ask?

1. about voltage limiters (R1, DZ2, R2, DZ1)  on the SCL clock input and SDA data input.
how  to look for the value for resistor and dioda zener for that project, i mean 4700 ohm and 5 volt dioda zener, what formula used in searching the value for that resistor and dioda zener. ? how to count it ? until we get the value for resistor and dioda zener?.

2.i'm learning the source code, but there is something i don't understand.

temperature = (temperature_int * 256 + temperature_frac) / 128 * 5 / 10
    If temperature_int >= 128 Then
        temperature = temperature - 256

i don't understand about the code, does anyone can help me to explain this code and explain the formula code? Please help.

i'm sorry about the english sentence
i'm doing my best to write it.
Thank You
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