Potentiometer setting detection by measuring the ratio of RC time constants

An analog to digital conversion apparatus for use with a microprocessor device having at least two tristate output lines and an interrupt input comprising:

• a potentiometer having a first fixed terminal connected to a first of said tristate output lines, a second fixed terminal connected to said interrupt input and a variable terminal connected to a second of said tristate output lines, the resistance between said variable terminal and either of said first and second fixed terminals being operator adjustable;
• a capacitor having a first terminal connected to said interrupt input, and a second terminal connected to a reference voltage;
• a discharge means connected to said capacitor and controlled by said microprocessor device for discharging said capacitor;
• and
• the microprocessor device being programmed for
• measuring the reference time constant of the resistance of said potentiometer between said first and second fixed terminals and said capacitor by discharging said capacitor via said discharge means, charging said capacitor through said first fixed terminal of said potentiometer by applying a digital "1" output to said first tristate output line and applying a high impedance output to the other of said at least two tristate output lines and counting the number of predetermined time intervals required for the voltage across said capacitor to reach a predetermined threshold voltage at said interrupt input, thereby producing a first count,
• measuring the variable time constant of the resistance of said potentiometer between said variable terminal and said second fixed terminal and said capacitor by discharging said capacitor via said discharge means, charging said capacitor through said variable terminal of said potentiometer by applying a digital "1" output to said second tristate output line and applying a high impedance output to the other of said at least two tristate output lines and counting the number of predetermined time intervals required for the voltage across said capacitor to reach said predetermined threshold voltage at said interrupt input,
• measuring the reference time constant of the resistance of said potentiometer between said first and second fixed terminals and said capacitor a second time by discharging said capacitor via said discharge means, charging said capacitor through said first fixed terminal of said potentiometer by applying a digital "1" output to said first tristate output line and applying a high impedance output to the other of said at least two tristate output lines and counting the number of predetermined time intervals required for the voltage across said capacitor to reach said predetermined threshold voltage at said interrupt input, thereby producing a second count,
• comparing said first count and said second count,
• if said first count and said second count differ by more than a predetermined amount, again measuring the reference time constant of the resistance of said potentiometer between said first and second fixed terminals and said capacitor thereby producing another first count, again measuring the variable time constant of the resistance of said potentiometer between said variable terminal and said second fixed terminal and said capacitor, again measuring the reference time constant of the resistance of said potentiometer between said first and second fixed terminals and said capacitor thereby producing another second count, and again comparing said first count and said second count, and
• if said first count and said second count do not differ by more than said predetermined amount computing the digital ratio of said count of predetermined time intervals of said variable time constant and said count of predetermined time intervals of said reference time constant, said digital ratio being the digital representation of the operator adjusting of said potentiometer.

A method of forming a signal representing an operator adjustment of a potentiometer having two fixed terminals and a variable terminal, comprising the steps of:

• measuring the reference time constant of the resistance between the fixed terminals of the potentiometer and a capacitor, by discharging said capacitor, charging said capacitor through said fixed terminals of the potentiometer and counting the number of predetermined time intervals for the voltage across said capacitor to reach a predetermined threshold voltage, thereby producing a first count;
• measuring the variable time constant of the resistance of the potentiometer between a fixed terminal and the variable terminal and said capacitor, by discharging said capacitor, charging said capacitor through said variable terminal of the potentiometer and counting the number of predetermined time intervals for the voltage across said capacitor to reach said predetermined threshold voltage;
• again measuring the reference time constant of the resistance between the fixed terminals of the potentiometer and said capacitor, by discharging said capacitor, charging said capacitor through said fixed terminals of the potentiometer and counting the number of predetermined time intervals for the voltage across said capacitor to reach said predetermined threshold voltage thereby producing a second count;
• comparing said first count and said second count,
• if said first count differs from said second count by more than a predetermined amount then again measuring the reference time constant of the resistance between said fixed terminals of the potentiometer and said capacitor, thereby producing a new first count, again measuring the variable time constant of the resistance between said fixed terminal and said variable terminal of the potentiometer and said capacitor, and again measuring the reference time constant of the resistance between said fixed terminals of the potentiometer and said capacitor, thereby producing a new second count, and then again comparing said first count and said second count, and
• if said first count does not differ from said second count by more than said predetermined amount, computing the digital ratio of said count of the number of predetermined time intervals of said variable time constant and said count of the number of predetermined time intervals of said reference time constant, said digital ratio being the digital representation of the operator adjustment of the potentiometer.