Converting an analog voltage like a MAP sensor reading to a binary number requires an analog to digital convertor. There are a bunch of analog lines that come into the ADC module through a multiplexer... basically a switch that reroutes the voltages to the output line depending on what MUX lines are set.

Converting to a digital value takes time, so there is usually an operation that tells the chip to start reading, then it will come back and read the digital value.

-- AlexHarford - 30 Jun 2006