Touch-on-Metal Keypad with N-1 Key Scan Averaging

• US 20160226521A1
• Filed: 02/03/2016
• Published: 08/04/2016
• Est. Priority Date: 02/03/2015
• Status: Active Grant

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: Liu, Dongtai
• Granted Patent: US 10,505,564 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: H03K 17/962 Capacitive touch switches
• CPC Class Codes: H03K 17/9622 using a plurality of detectors, e.g. keyboard
• CPC Class Codes: H03K 17/975 using a capacitive movable element
• CPC Class Codes: H03K 17/98 having a plurality of control members, e.g. keyboard
• CPC Class Codes: H03K 2217/94026 Automatic threshold calibration; e.g. threshold automatically adapts to ambient conditions or follows variation of input
• CPC Class Codes: H03M 11/20 Dynamic coding, i.e. by key scanning H03M11/26 takes precedence

Abstract

For touch-on-metal (ToM) apparatus, an N-1 key scan averaging (with baseline drift compensation) technique, including: (a) during successive key-scan periods, scanning each N key-touch sense circuits, to determine a respective touch-key sense signal, including a key-press sense signal, determine a respective touch-key baseline value based on the touch-key sense signal independent of any key-press condition; and (b) during each key-scan period, for each touch key averaging the touch-key baseline values for the other N-1 touch-keys to generate a respective N-1 key-scan average value, comparing the touch-key sense signal to a threshold value based on the respective N-1 key-scan average value, and signaling a key-press condition if at least one touch-key output signal is greater in magnitude than the respective N-1 key-scan average value by the threshold value. Touch-key baseline values can be determined by incrementing the touch-key baseline value if the touch-key sense signal is greater than the touch-key baseline value, decrementing the touch-key baseline value if the touch-key sense signal is less than the touch-key baseline value, and generating the normalized touch-key baseline value by subtracting the touch-key baseline value from the touch-key sense signal.

Claims

1. A method suitable for use with a touch-on-metal N-key input apparatus including a metal fascia defining N touch-key areas, overlying a key-matrix of N key-touch sensor circuits associated with a respective touch-key area, and each key-touch sensor circuit is configured to provide a touch-key sense signal, including a key-press sense signal in response to depressing the metal fascia above the key-touch circuit, the method comprising

during successive key-scan periods, scanning each N key-touch sense circuits, to

determine a respective touch-key sense signal, including a key-press sense signal,

determine a respective touch-key baseline value based on the touch-key sense signal independent of any key-press condition;

and

during each key-scan period, for each touch key

averaging the touch-key baseline values for the other N-1 touch-keys to generate a respective N-1 key-scan average value,

comparing the touch-key sense signal to a threshold value based on the respective N-1 key-scan average value, and

signaling a key-press condition if at least one touch-key output signal is greater in magnitude than the respective N-1 key-scan average value by the threshold value.

2. The method of claim 1, wherein, during each key-scan period, for each touch-key, generating a respective touch-key baseline value comprises generating a normalized touch-key baseline value, with suppressed offset.

3. The method of claim 2, wherein generating the normalized touch-key baseline value for each touch-key comprises:

incrementing the touch-key baseline value if the touch-key sense signal is greater than the touch-key baseline value;

decrementing the touch-key baseline value if the touch-key sense signal is less than the touch-key baseline value; and

generating the normalized touch-key baseline value by subtracting the touch-key baseline value from the touch-key sense signal.

4. The method of claim 1, further comprising signaling a multi-touch condition for multiple key-press conditions.