System and method for noise reduction in touch screen system

• US 6,624,835 B2
• Filed: 01/09/2001
• Issued: 09/23/2003
• Est. Priority Date: 01/09/2001
• Status: Active Grant

• Current Assignee: Advanced Micro Devices, Inc.
• Original Assignee: National Semiconductor Corporation (Texas Instruments, Inc.)
• Inventors: Willig, Randy C.
• Primary Examiner(s): Saras, Steven
• Assistant Examiner(s): Alphonse, Fritz
• Application Number:
  US09/757,739
• Publication Number:
  US 20020089491A1
• Time in Patent Office: 987 Days
• Field of Search: 345/173, 345/98, 345/174, 345/179, 178/19.01, 178/19.02, 178/19.03, 178/20.01
• US Class Current: 347/173
• CPC Class Codes: G06F 3/0412 Digitisers structurally integrated in a display
• CPC Class Codes: G06F 3/04182 Filtering of noise external to the device and not generated by digitiser components
• CPC Class Codes: G06F 3/04184 Synchronisation with the driving of the display or the backlighting unit to avoid interferences generated internally
• CPC Class Codes: G06F 3/045 using resistive elements, e.g. a single continuous surface or two parallel surfaces put in contact

Abstract

There is disclosed a system and method for noise reduction in touch screen systems. A touch screen system comprises an analog to digital converter capable of converting to digital signals analog signals from at least one analog resistive film in a touch screen. The analog signal is indicative of a location on the touch screen. The invention comprises an apparatus for reducing noise in a touch screen comprising a microcontroller coupled to said analog to digital converter. The microcontroller receives a synchronization trigger signal, and in response to receiving the synchronization trigger signal, waits to receive digital signals from the analog to digital converter until the expiration of a period of time in which noise associated with the synchronization trigger signal has occurred. A method for reducing common mode noise is disclosed that actively senses and removes the ground return offset from the analog signals of the touch screen. Common mode noise is reduced by removing common mode noise from touch screen data signals by holding the touch screen data signals in a sample and hold circuit.

Claims

1. For use in a touch screen system of the type comprising an analog to digital converter capable of converting to digital signals analog signals from at least one analog resistive film in a touch screen indicative of a location within said touch screen, an apparatus for reducing noise in said touch screen comprising:

a microcontroller coupled to said analog to digital converter, said microcontroller capable of receiving a synchronization trigger signal, and in response to receiving said synchronization trigger signal, capable of waiting to receive said digital signals from said analog to digital converter until the expiration of a period of time in which noise associated with said synchronization trigger signal has occurred.

2. The apparatus as claimed in claim 1 wherein said microcontroller receives said synchronization trigger signal from a liquid display controller.

3. The apparatus as claimed in claim 1 wherein said microcontroller receives said synchronization trigger signal from signal conditioning circuitry.

4. The apparatus as claimed in claim 1 wherein said noise comprises an impulse noise spike that occurs within a predetermined time period after the leading edge of said synchronization trigger signal.

5. The apparatus as claimed in claim 1 wherein said noise comprises a compressed squarewave noise component that occurs within a predetermined time period after the leading edge of said synchronization trigger signal.

6. The apparatus as claimed in claim 1 wherein said microcontroller is capable of:

waiting to receive said digital signals from said analog to digital converter until the expiration of a first period of time in which noise associated with a first synchronization trigger signal has occurred;

recording a first value that represents a location in said touch screen;

waiting to receive said digital signals from said analog to digital converter until the expiration of a second period of time in which noise associated with a second synchronization trigger signal has occurred;

recording a second value that represents said location in said touch screen; and

averaging said first and second values to obtain a value of said location with reduced noise.

7. The apparatus as claimed in claim 6 wherein said noise is reduced from a noise level of plus or minus ten pixels to a noise level no greater than plus or minus one pixel.

8. The apparatus as claimed in claim 1 wherein said microcontroller waits a predetermined time period after receiving the leading edge of said synchronization trigger signal to receive said digital signals from said analog to digital converter.

9. The apparatus as claimed in claim 1 wherein said microcontroller is capable of:

sampling signals from said analog to digital converter;

detecting noise pulses in said sampled signals;

recording the time of arrival of each noise pulse;

detecting a preselected number, N, of noise pulses;

determining the shortest noise pulse interval;

determining whether said shortest noise pulse interval is greater than the time to sample both an X plane signal and a Y plane signal;

sampling both said X plane signal and said Y plane signal on each noise pulse trigger if said shortest noise pulse interval is greater than the time to sample both an X plane signal and a Y plane signal;

determining whether said shortest noise pulse interval is greater than the time to sample only one of the plane signals if said shortest noise pulse interval is not greater than the time to sample both an X plane signal and a Y plane signal;

sampling only one of the plane signals on each noise pulse trigger if said shortest noise pulse interval is greater than the time to sample only one of the plane signals; and

triggering said microcontroller on each occurrence of a noise pulse if said shortest noise pulse interval is not greater than the time to sample only one of the plane signals.

10. The apparatus as claimed in claim 1 wherein said microcontroller is capable of:

sampling signals from said analog to digital converter;

detecting a noise signal in said sampled signals;

recording the time of arrival of each sample of noise signal;

detecting a preselected number, N, of samples of noise signal;

determining the frequency of said noise signal from said N samples of noise signal;

determining the amplitude of said noise signal from said N samples of noise signal;

determining whether said noise signal has a constant amplitude and waveshape;

sampling an X plane value and a Y plane value at the same time in a noise cycle if said noise signal has a constant amplitude and waveshape; and

sampling an X plane value and a Y plane value between noise pulses in said noise cycle if said noise signal does not have a constant amplitude and waveshape.

11. The apparatus as claimed in claim 1 wherein said microcontroller is capable of:

simultaneously sending data signals from all channels of said touch screen to a sample/hold circuit; and

removing said common mode noise from said data signals by holding said data signals in said sample/hold circuit.

12. For use in a touch screen system of the type comprising an analog to digital converter capable of converting to digital signals analog signals from at least one analog resistive film in a touch screen indicative of a location within said touch screen, an apparatus for reducing noise in said touch screen comprising:

a microcontroller coupled to said analog to digital converter, said microcontroller capable of receiving a noise trigger signal, and in response to receiving said noise trigger signal, capable of waiting to receive said digital signals from said analog to digital converter until the expiration of a period of time in which noise associated with said noise trigger signal has occurred.

13. The apparatus as claimed in claim 12 wherein said microcontroller receives said noise trigger signal from signal conditioning circuitry.

14. The apparatus as claimed in claim 12 wherein said microcontroller is capable of:

waiting to receive said noise trigger signal;

waiting for a noise signal to settle after said noise trigger signal has been received;

sending a control signal to drive a first axis to obtain a first axis measurement;

waiting for settling delay on a first axis signal;

recording a first value of said first axis measurement that represents a location in said touch screen;

sending a control signal to drive a second axis to obtain a second axis measurement;

waiting for settling delay on a second axis signal;

recording a second value of said second axis measurement that represents said location in said touch screen; and

using said first value of said first axis measurement and said second value of said second axis measurement to obtain a value of said location with reduced noise.

15. A touch screen system comprising:

a display device;

at least one analog resistive film within a touch screen placed over said display device, said at least one analog resistive film capable of creating analog signals indicative of a location within said touch screen;

an analog to digital converter capable of converting to digital signals said analog signals from said at least one analog resistive film, said touch screen system comprising an apparatus for reducing noise in said touch screen comprising;

a microcontroller coupled to said analog to digital converter, said microcontroller capable of receiving a synchronization trigger signal, and in response to receiving said synchronization trigger signal, capable of waiting to receive said digital signals from said analog to digital converter until the expiration of a period of time in which noise associated with said synchronization trigger signal has occurred.

16. The touch screen system as claimed in claim 15 wherein said microcontroller is capable of:

waiting to receive said digital signals from said analog to digital converter until the expiration of a first period of time in which noise associated with a first synchronization trigger signal has occurred;

recording a first value that represents a location in said touch screen;

waiting to receive said digital signals from said analog to digital converter until the expiration of a second period of time in which noise associated with a second synchronization trigger signal has occurred;

recording a second value that represents said location in said touch screen; and

averaging said first and second values to obtain a value of said location with reduced noise.

17. The touch screen system as claimed in claim 16 wherein said noise is reduced from a noise level of plus or minus ten pixels to a noise level no greater than plus or minus one pixel.

18. The touch screen system as claimed in claim 15 wherein said microcontroller waits a predetermined time period after receiving the leading edge of said synchronization trigger signal to receive said digital signals from said analog to digital converter.

19. The touch screen system as claimed in claim 15 wherein said microcontroller is capable of:

sampling signals from said analog to digital converter;

detecting noise pulses in said sampled signals;

recording the time of arrival of each noise pulse;

detecting a preselected number, N, of noise pulses;

determining the shortest noise pulse interval;

determining whether said shortest noise pulse interval is greater than the time to sample both an X plane signal and a Y plane signal;

sampling both said X plane signal and said Y plane signal on each noise pulse trigger if said shortest noise pulse interval is greater than the time to sample both an X plane signal and a Y plane signal;

determining whether said shortest noise pulse interval is greater than the time to sample only one of the plane signals if said shortest noise pulse interval is not greater than the time to sample both an X plane signal and a Y plane signal;

sampling only one of the plane signals on each noise pulse trigger if said shortest noise pulse interval is greater than the time to sample only one of the plane signals; and

triggering said microcontroller on each occurrence of a noise pulse if said shortest noise pulse interval is not greater than the time to sample only one of the plane signals.

20. The touch screen system as claimed in claim 15 wherein said microcontroller is capable of:

sampling signals from said analog to digital converter;

detecting a noise signal in said sampled signals;

recording the time of arrival of each sample of noise signal;

detecting a preselected number, N, of samples of noise signal;

determining the frequency of said noise signal from said N samples of noise signal;

determining the amplitude of said noise signal from said N samples of noise signal;

determining whether said noise signal has a constant amplitude and waveshape;

sampling an X plane value and a Y plane value at the same time in a noise cycle if said noise signal has a constant amplitude and waveshape; and

sampling an X plane value and a Y plane value between noise pulses in said noise cycle if said noise signal does not have a constant amplitude and waveshape.

21. The touch screen system as claimed in claim 15 wherein said microcontroller is capable of:

simultaneously sending data signals from all channels of said touch screen to a sample and hold circuit; and

removing said common mode noise from said data signals by holding said data signals in said sample and hold circuit.

22. A touch screen system comprising:

a display device;

at least one analog resistive film within a touch screen placed over said display device, said at least one analog resistive film capable of creating analog signals indicative of a location within said touch screen;

an analog to digital converter capable of converting to digital signals said analog signals from said at least one analog resistive film, said touch screen system comprising an apparatus for reducing noise in said touch screen comprising;

a microcontroller coupled to said analog to digital converter, said microcontroller capable of receiving a noise trigger signal, and in response to receiving said noise trigger signal, capable of waiting to receive said digital signals from said analog to digital converter until the expiration of a period of time in which noise associated with said noise trigger signal has occurred.

23. The touch screen system as claimed in claim 22 wherein said microcontroller is capable of:

waiting to receive said noise trigger signal;

waiting for a noise signal to settle after said noise trigger signal has been received;

sending a control signal to drive a first axis to obtain a first axis measurement;

waiting for settling delay on a first axis signal;

recording a first value of said first axis measurement that represents a location in said touch screen;

sending a control signal to drive a second axis to obtain a second axis measurement;

waiting for settling delay on a second axis signal;

recording a second value of said second axis measurement that represents said location in said touch screen; and

using said first value of said first axis measurement and said second value of said second axis measurement to obtain a value of said location with reduced noise.

24. For use in a touch screen system of the type comprising an analog to digital converter capable of converting to digital signals analog signals from at least one analog resistive film in a touch screen indicative of a location within said touch screen, a method for reducing noise in said touch screen comprising the steps of:

receiving a synchronization trigger signal in a microcontroller coupled to said digital to analog converter;

waiting until the expiration of a period of time in which noise associated with said synchronization trigger signal has occurred; and

receiving said digital signals in said microcontroller.

25. The method as claimed in claim 24 further comprising the steps of:

sending a first synchronization signal to a microcontroller coupled to said analog to digital converter;

waiting to receive said digital signals from said analog to digital converter in said microcontroller until the expiration of a first period of time in which noise associated with said first synchronization trigger signal has occurred;

recording in said microcontroller a first value that represents a location in said touch screen;

sending a second synchronization signal to said microcontroller;

waiting to receive said digital signals from said analog to digital converter in said microcontroller until the expiration of a second period of time in which noise associated with said second synchronization trigger signal has occurred;

recording in said microcontroller a second value that represents said location in said touch screen; and

averaging said first and second values to obtain a value of said location with reduced noise.

26. The method as claimed in claim 24 further comprising the steps of:

waiting to receive a noise trigger signal in said microcontroller;

waiting for a noise signal to settle after said noise trigger signal has been received;

sending a control signal from said microcontroller to drive a first axis to obtain a first axis measurement;

waiting for settling delay on a first axis signal;

recording a first value of said first axis measurement that represents a location in said touch screen;

sending a control signal from said microcontroller to drive a second axis to obtain a second axis measurement;

waiting for settling delay on a second axis signal;

recording a second value of said second axis measurement that represents said location in said touch screen; and

using said first value of said first axis measurement and said second value of said second axis measurement to obtain a value of said location with reduced noise.

27. The method as claimed in claim 24 further comprising the steps of:

sampling signals from said analog to digital converter in said microcontroller;

detecting noise pulses in said sampled signals;

recording the time of arrival of each noise pulse;

detecting a preselected number, N, of noise pulses;

determining the shortest noise pulse interval;

determining whether said shortest noise pulse interval is greater than the time to sample both an X plane signal and a Y plane signal;

sampling both said X plane signal and said Y plane signal on each noise pulse trigger if said shortest noise pulse interval is greater than the time to sample both an X plane signal and a Y plane signal;

determining whether said shortest noise pulse interval is greater than the time to sample only one of the plane signals if said shortest noise pulse interval is not greater than the time to sample both an X plane signal and a Y plane signal;

sampling only one of the plane signals on each noise pulse trigger if said shortest noise pulse interval is greater than the time to sample only one of the plane signals; and

triggering said microcontroller on each occurrence of a noise pulse if said shortest noise pulse interval is not greater than the time to sample only one of the plane signals.

28. The method as claimed in claim 24 further comprising the steps of:

sampling signals from said analog to digital converter in said microcontroller;

detecting a noise signal in said sampled signals;

recording the time of arrival of each sample of noise signal;

detecting a preselected number, N, of samples of noise signal;

determining the frequency of said noise signal from said N samples of noise signal;

determining the amplitude of said noise signal from said N samples of noise signal;

determining whether said noise signal has a constant amplitude and waveshape;

sampling an X plane value and a Y plane value at the same time in a noise cycle if said noise signal has a constant amplitude and waveshape; and

sampling an X plane value and a Y plane value between noise pulses in said noise cycle if said noise signal does not have a constant amplitude and waveshape.

29. The method as claimed in claim 24 further comprising the steps of:

simultaneously sending data signals from all channels of said touch screen to a sample and hold circuit; and

removing said common mode noise from said data signals by holding said data signals in said sample and hold circuit.

30. The method as claimed in claim 24 further comprising a method for reducing common mode noise in said touch screen, said method comprising the steps of:

actively sensing the ground return offset of said touch screen; and

removing said ground return offset from said analog signals of said touch screen.