Relaxation oscillator based keypad decoder
AssigneeTexas Instruments Incorporated
InventorStephen J. Fedigan
Life of this patent
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Abstract
The keypad interface element of this invention uses a relaxation oscillator and a digital keypad processor having a counter/timer to decode specific keys. The RC portion of the relaxation oscillator includes a resistance ladder and a set of momentary on pushbutton switches disposed change resistance dependent upon which key is pressed. This causes the relaxation oscillator to produce an output signal having a corresponding frequency. The counter/timer of the digital keypad processor produces a count corresponding to the oscillator frequency. The digital keypad processor latches and holds a binary number specifically identifying the depressed key. A state machine in the digital keypad processor provides transient-free, noise immune keypad decoding.
Claims7
A method of keypad decoding employing a relaxation oscillator with a resistor/capacitor time constant comprising the steps of:
- disposing pushbutton key switches relative to the resistor to change a resistance value of the resistor to unique resistance on a key press;
- detecting an oscillation frequency of the relaxation oscillator including
- counting clock pulses between consecutive cycles of the relaxation oscillator,
- detecting when a current count differs from a prior count for more than a predetermined number of oscillation frequency count cycles,
- if the current count differs from the prior count for more than the predetermined number of oscillation frequency count cycles, then detecting when a variance in the current count is less than a predetermined variance threshold for more than a predetermined number of oscillation frequency count cycles, and
- if the variance in the current count is less than the predetermined variance threshold for more than the predetermined number of oscillation frequency count cycles, then setting the prior count to the current count;
- and
- determining a key pressed from the oscillation frequency.
A data entry apparatus comprising:
- a relaxation oscillator including
- an inverter having input and output,
- a capacitor connected between the input of said inverter input and ground,
- a resistor ladder connected between said input and said output of said inverter, and
- a set of momentary on pushbutton key switches connected to said resistor ladder disposed to change a resistance of said resistor ladder upon activation of a pushbutton key switch;
- and
- a digital keypad processor coupled to said relaxation oscillator generating a digital key code corresponding to one or more pushbutton key switches pressed based upon relaxation oscillator frequency including
- a counter receiving clock pulses for counting between consecutive cycles of said relaxation oscillator,
- a current period register storing a last count,
- a measured period register storing last determined relaxation oscillator count,
- a comparator triggered when said last count differs from said last determined relaxation oscillator count, and
- a state machine triggered by said comparator changing said last determined relaxation oscillator count to said last count if last count differs from said last determined relaxation oscillator count for longer than a predetermined period of time.
Dependencies are inferred from the claim text.