External constant specification in a digital electronic system

Claim 2

A digital constant specification system in a digital electronic system, said digital constant system comprising: a precision fixed resistor having a predetermined reference resistance between first and second terminals; a constant specifying fixed resistor having a constant specifying resistance between first and second terminals; a capacitor having a first terminal connected to said second terminal of said precision fixed resistor and said second terminal of said constant specifying resistor and a second terminal connected to a predetermined reference voltage; a discharge means connected to said capacitor for discharging said capacitor; a first time constant measuring means for measuring the reference time constant of said precision fixed resistor by discharging said capacitor via said discharge means, charging said capacitor through said first terminal of said precision fixed resistor and counting the number of predetermined time intervals required for the voltage across said capacitor to reach a predetermined threshold voltage; a second time constant measuring means for measuring the constant specifying time constant of said constant specifying resistor by discharging said capacitor via said discharge means, charging said capacitor through said first terminal of said constant specifying resistor and counting the number of predetermined time intervals required for the voltage across said capacitor to reach a predetermined threshold voltage; a measurement sequence means connected to said first time constant measurement means and said second time constant measurement means for producing a sequence of measurements by causing said first time constant measurement means to measure said reference time constant a first time thereby producing a first count, then causing said second time constant measurement means to measure said constant specifying time constant, then causing said first time constant measurement means to measure said reference time constant a second time thereby producing a second count, and for comparing said first count and said second count and repeating said sequence of measurements if said first count and said second count differ by more than a predetermined amount; and a ratio means for computing the digital ratio of the count of predetermined time intervals of said reference time constant and the count of predetermined time intervals of said constant specifying time constant, said ratio being said

Claim 3

A digital constant specification system for use with a microprocessor device having at least two tristate output lines and an interrupt input, said digital constant specification system comprising: a precision fixed resistor having a predetermined reference resistance between first and second terminals, said first terminal being connected to a first tristate output line; a constant specifying fixed resistor having a constant specifying resistance between first and second terminals, said first terminal connected to a second tristate output; a capacitor having a first terminal connected to said interrupt input, said second terminal of said precision fixed resistor and said second terminal of said constant specifying resistor, 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 to measure the reference time constant of the reference resistance of said precision fixed resistor and said capacitor by discharging said capacitor via said discharge means, charging said capacitor through said first terminal of said precision fixed resistor 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, measure the constant specifying time constant of the constant specifying resistance of said constant specifying fixed resistor and said capacitor by discharging said capacitor via said discharge means, charging said capacitor through said first terminal of said constant specifying resistor 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 a predetermined threshold voltage at said interrupt input, measure the reference time constant of the reference resistance of said precision fixed resistor and said capacitor again by discharging said capacitor via said discharge means, charging said capacitor through said first terminal of said precision fixed resistor 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 of said interrupt input, thereby producing a second count, compare said first count and said second count, if said first count and said second count differs by more than a predetermined amount, set said first count equal to said second count, again measure the constant specifying time constant of the constant specifying resistance of said constant specifying fixed resistor and said capacitor, again measure the reference time constant of the reference resistance of said precision fixed resistor and said capacitor thereby producing a second count, and again compare 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 compute the digital ratio of said reference time constant and said constant specifying time constant, said digital ratio being said digital constant.

Claim 4

A digital constant specification system for use with a microprocessor device having at least two tristate output lines and an interrupt input, said digital constant specification system comprising: a precision fixed resistor having a predetermined reference resistance between first and second terminals, said first terminal being connected to a first tristate output line; a constant specifying fixed resistor having a constant specifying resistance between first and second terminals, said first terminal connected to a second tristate output; a capacitor having a first terminal connected to said interrupt input, said second terminal of said precision fixed resistor and said second terminal of said constant specifying resistor, 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 to measure the reference time constant of the reference resistance of said precision fixed resistor and said capacitor by discharging said capacitor via said discharge means, charging said capacitor through said first terminal of said precision fixed resistor 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, measure the constant specifying time constant of the constant specifying resistance of said constant specifying fixed resistor and said capacitor by discharging said capacitor via said discharge means, charging said capacitor through said first terminal of said constant specifying resistor 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 a predetermined threshold voltage at said interrupt input, measure the reference time constant of the reference resistance of said precision fixed resistor and said capacitor again by discharging said capacitor via said discharge means, charging said capacitor through said first terminal of said precision fixed resistor 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 of said interrupt input, thereby producing a second count, compare said first count and said second count, if said first count and said second count differs by more than a predetermined amount, set said first count equal to said second count, again measure the constant specifying time constant of the constant specifying resistance of said constant specifying fixed resistor and said capacitor, again measure the reference time constant of the reference resistance of said precision fixed resistor and said capacitor thereby producing a second count, and again compare 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 compute the digital ratio of said reference time constant and said constant specifying time constant, said digital ratio being said digital constant.

Claim 5

The digital constant specification system claimed in claim 2, wherein: said microprocessor device is further programmed to count the number of predetermined time intervals required for the voltage across said capacitor to reach a predetermined threshold voltage at said interrupt input by incrementing a memory register at the rate of operation of said microprocessor under closed loop program operation until an interrupt input is received.