Adaptive stylus frontend to identify and update zero force offset pressure in an active stylus

1. An adaptive stylus frontend to identify and update zero force offset pressure codes in an active stylus, the adaptive stylus frontend comprising:

a pressure sensor to sense force applied by a user at stylus tip and convert sensed force to analog signal;

a high gain instrumentation amplifier configured to receive the analog signal and provide the required gain to the analog signal;

an analog-to-digital converter (ADC) configured to convert the analog signal to digital codes at a predetermined sampling frequency; and

an adaptive offset controller configured to determine a hover state and an inking state,

wherein the hover state is determined by:

calculating a first mean and a first standard deviation of the digital codes;

comparing the first standard deviation to a predetermined standard deviation threshold value that is determined based on the noise tolerance of the stylus through trial tests, and if the first standard deviation is less than the predetermined standard deviation threshold value, setting the first mean value as hover_to_tip threshold value; and

comparing the digital codes to the hover_to_tip threshold value;

wherein the inking state is determined by:

entering the inking state if value of the digital codes is greater than the hover_to_tip threshold value;

calculating a second mean and a second standard deviation if the digital codes is greater than a tip_to_hover threshold value;

comparing the second standard deviation to a second predetermined standard deviation threshold value;

starting a timer if the second standard deviation is less than the second predetermined standard deviation threshold value; and

resetting the timer if the second standard deviation value is greater than the second predetermined standard deviation threshold value and continue to be in inking state; and

wherein the controller adaptively identifies, and update zero force offset pressure codes, wherein zero force offset pressure code is the ADC output value when no external force is applied at stylus tip.

2. The adaptive stylus frontend as claimed in claim 1 , wherein the adaptive stylus frontend is coupled to a pressure sensor to sense force applied by a user at the stylus tip and convert the sensed force to the analog signal.

3. The adaptive stylus frontend as claimed in claim 1 , wherein the zero force offset pressure comprises the pressure sensor offset introduced by a pressure sensor, the amplifier offset introduced by the amplifier, and the offset introduced by the analog to digital converter.

4. The adaptive stylus frontend as claimed in claim 1 , wherein the adaptive identification and updating of the zero force offset pressure codes enable the stylus to track the changes in the zero force offset pressure with time, temperature, or any other physical factors, and allows the stylus to instantaneously transition between a hover state and an inking state accurately.

5. The adaptive stylus frontend as claimed in claim 1 , wherein the stylus transitions to hover state when the digital code is less than tip_to_hover threshold value or setting second mean value as hover_to_tip threshold value and transitioning to hover state on expiry of the timer.

6. A method to adaptively identify and update zero force offset pressure codes in an active stylus, the method comprising:

converting analog signals at output of a pressure sensor to digital codes, wherein the analog signals correspond to force applied by a user at stylus tip;

determining a hover state by the stylus, wherein determining the hover state comprises:

calculating a first mean and a first standard deviation of the digital codes;

comparing the standard deviation to a predetermined standard deviation threshold value and if the first standard deviation is less than the predetermined standard deviation threshold value, setting first mean value as hover_to_tip threshold value; and

comparing the digital codes to the hover_to_tip threshold value; and

determining an inking state by the stylus, wherein determining the inking state comprises:

entering the inking state if value of the digital codes is greater than the hover_to_tip threshold value;

calculating a second mean and a second standard deviation if the digital codes is greater than a tip_to_hover threshold value;

comparing the second standard deviation to a second predetermined standard deviation threshold value;

starting a timer if the standard deviation is less than the second predetermined standard deviation threshold value;

resetting the timer if the second standard deviation value is greater than the second predetermined standard deviation threshold value; and

transitioning to hover state when the digital code is less than tip_to_hover threshold value or setting second mean value as hover_to_tip threshold value and transitioning to hover state on expiry of the timer.

7. The method as claimed in claim 6 , comprising recalculating the first mean and the first standard deviation if the first standard deviation is greater than the predetermined standard deviation value.

8. The method as claimed in claim 6 , comprising remaining in the hover state if value of the digital codes are less than the hover_to_tip threshold value.

9. The method as claimed in claim 6 , comprising remaining in the inking state until the digital code is less than tip_to_hover threshold value or setting second mean value as hover_to_tip threshold value and transitioning to hover state on expiry of the timer.