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docs/library/machine.I2C: Add a note about I2C pull-up resistors.
Quite regularly users complain about unexpected behavior of I2C, calling it a bug, when in fact the trouble is caused by missing pull-up resistors. So this commit adds a note to the documentation, in the slim hope that people will find and read it.
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‎docs/library/machine.I2C.rst‎

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@@ -20,6 +20,17 @@ Software I2C is implemented by bit-banging and can be used on any pin but is not
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as efficient. These classes have the same methods available and differ primarily
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in the way they are constructed.
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.. Note::
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The I2C bus requires pull-up circuitry on both SDA and SCL for it's operation.
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Usually these are resistors in the range of 1 - 10 kOhm, connected from each SDA/SCL
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to Vcc. Without these, the behaviour is undefined and may range from blocking,
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unexpected watchdog reset to just wrong values. Often, this pull-up circuitry
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is built-in already to the MCU board or sensor breakout boards, but there is
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no rule for that. So please check in case of trouble. See also this excellent
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`learning guide <https://learn.adafruit.com/working-with-i2c-devices/pull-up-resistors>`_
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by Adafruit about I2C wiring.
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Example usage::
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from machine import I2C

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