Hi-Res Joystick Software

This project was jointly created by John Kowalski, Robert Gault and Nickolas Marentes

How the ADC works

The real work when converting any analogue voltage to a digital reading with the Color Computer is actually done using the DAC (Digital-to-Analogue) converter. To understand how the ADC (Analogue-to-Digital) circuit works, we must understand how the DAC works.

In simple terms, the DAC accepts data from the computer and converts it to an analogue voltage. Although the computer's data bus is 8-bit, the input to the DAC is only 6-bits. This represents a value from 0 to 63 or 64 individual voltage steps as output from the DAC. This is also how the 64 levels of volume is derived when the DAC output is diverted and used for sound generation.

In order to get a digital reading from the joystick port, the output of the DAC is fed into an input of a voltage comparator. At the same time, the voltage from the joystick or mouse is fed into a second input of the voltage comparator. This voltage comparator compares the two input voltages and sets an output high or low to indicate the difference. Software running on the Color Computer can read the comparator output and determine if the DAC voltage needs to be raised or lowered to attempt to match the joystick port voltage. This test is repeated until it locates the DAC voltage value which coincides with the incoming voltage. Through a process of successive approximation, an accurate ADC value can be found.

This is the first stage of reading the joystick port that derives the normal 6-bit joystick port reading.

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