A compass is a navigational instrument which is sensitive to the magnetic field of the earth. A typical compass has a magnetic strip which aligns itself with magnetic north, and from this, orientation can be determined.

A digital compass is not free to move the same way that an analogue compass is, as it is typically fixed to a circuit board. Consequently, an output corresponds to the orientation of a specific reference point of the compass.

The CMPS03 digital compass output data is acessible in two manners. First is as a digital query, conducted in the same manner as an EPROM query. To read the compass output this way, pins 2 and 3 can be connect to a micro- processor for a direct digital reading. The second manner is as a pulse width modulated (PWM) signal. If the PWM signal is to be used, pins 2 and 3 should be pulled to 5 volts with 47K pull up resistors. The purpose of a pull up resistor is to establish a default value on a pin if it is not to be used.

The PWM signal is output on pin 4. The bearing of the device in degrees corresponds to milliseconds the output is high, minus 1. The output thus will be high from 1ms (0 degrees) to 36.99 ms (359.9 degrees). Following this, the signal goes low unconditionally for 65ms.

The output of the PWM signal can be easily interpreted by the BASIC stamp microprocessor. The BASIC stamp contains a function pulsin in which returns the number of stamp time steps the input signal is high. The stamp time step is fixed, thus the value returned by pulsin may be scaled to determine the number of milliseconds the pulse is high. The time step for a 2SX basic stamp is 8 microseconds, fast enough to deliver the necessary time resolution. The digital compass is the final navigational electronic sensor. Together with the accelerometer and gyroscope a complete navigational sensing system can be constructed.

• Magnetoresistance