Previous to this patch the i2c.scan() method would do up to 100 probes per
I2C address, to detect the devices on the bus. This repeated probing was a
relic from when the code was copied from the accelerometer initialisation,
which requires to do repeated probes while waiting for the accelerometer
chip to turn on.
But I2C devices shouldn't need more than 1 probe to detect their presence,
and the generic software I2C implementation uses 1 probe successfully. So
this patch changes the implementation to use 1 probe per address, which
significantly speeds up the scan operation.
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