From The Apple Wiki

The AKM AK8973 is a hall effect magnetic sensor for use as an electronic compass. It features a built-in 8-bit ADC, pre-amplifier, DAC sensor, EEPROM, temperature sensor, and digital output. This ICWorks Arranger Report is a set of high magnification images and annotations of the analog and select digital portions of the device.

It is delivered with software that allows the client to organize the annotations into functional schematics. In iOS case, the app is called Compass