Abstract:
A capacitive touch sensor and LED driver device achieves a reduction in pin count by multiplexing LED drive functionality and capacitive sense functionality on each input/output pin. A control circuit switches between LED drive mode and capacitive sense mode at a frequency of approximately 200 Hz, although other switching frequencies can be used. A bias driver functions as a current sink for LEDs in LED drive mode and can also be used to drive a bias voltage to the LEDs during capacitive sense mode to improve noise immunity.

Description:
RELATED APPLICATION DATA 
     This application claims priority pursuant to 35 U.S.C. §119(e) to U.S. provisional patent application Ser. No. 61/434,321 filed Jan. 19, 2011, the subject matter of which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to multiple-capacitive-sensor controllers, and more particularly, to multiple-capacitive-sensor controllers containing general purpose input-output ports that can be shared by an LED driver and a capacitive touch detector. 
     2. Description of Related Art 
     Multiple-capacitive-sensor controllers are well known in the art and are an integral part of a sensor that uses the properties of capacitors to detect physical distance from the user or touches by the user. In addition to being a pure sense device, most multiple-capacitive-sensor controllers also contain input-output ports that can be used to control Light Emitting Diodes (LED). 
     Touch sensors such as capacitive buttons are often used in devices to allow the user to control the devices by approaching or touching them. LED drivers are often used to facilitate a response to the proximity of the user or touches by the user. Therefore, it would reduce the number of components and system cost if the multiple-capacitive-sensor controllers could also be used to control the LEDs. 
       FIG. 1  depicts a simplified circuit diagram of a system containing a multiple-capacitive-sensor controller  101  that is typical of the prior art. The multiple-capacitive-sensor controller  101  contains multiple I/O ports. Each I/O port can be configured as either a capacitive sensor pin or an LED driver. A capacitive sensor pin  102 , after configuration, will have a capacitive sensor  104  connected to a pad  106 . Pad  106  is then connected to a capacitive element  108 . When a user interacts with capacitive element  108  through physical means such as approaching or touching it, the capacitive value of capacitive element  108  will change. Capacitive sensor  104  detects the capacitance change, allowing the system to react to the user interaction accordingly. An LED driver pin  110 , after configuration, will have an LED Driver  112  connected to pad  116 , which is connected to LED  114 . This allows the system to use the capacitive sensor controller to control the LEDs. 
     However, the single sensor controller solution still requires one I/O port per function. Thus, it is not the most efficient solution in terms of system area and cost. For example, the prior art multiple capacitive sensor controller will need to have 10 I/Os if the system needs 5 capacitive inputs and 5 LED drivers. Accordingly, being able to use a single I/O for both capacitive input and LED driver functionalities reduces the I/O port requirement of the sensor controller. Using the present invention, a multiple capacitive sensor controller that needs to handle 5 capacitive inputs and 5 LED drivers will only need 6 I/O ports: 5 I/O ports to handle the capacitive inputs and LED drivers and 1 bias driver port. 
     SUMMARY OF THE INVENTION 
     An embodiment of an electronic touch sensor device in accordance with the present invention comprises a control circuit, at least one light emitting diode (LED) having a first electrode and a second electrode, and at least one input/output circuit operatively connected to the control circuit. The input/output circuit includes a capacitive sense circuit that is configured to measure a change in capacitance when an object approaches the touch sensor device. The input/output circuit also includes an LED driver circuit configured to drive the first electrode of the LED. The control circuit is configured to selectively disable and enable the capacitive sense circuit and the LED drive circuit such that when the capacitive sense circuit is disabled, the LED drive circuit is enabled, and vice versa. The embodiment of the electronic touch sensor device further includes a bias driver circuit configured to drive the second electrode of the LED. 
     In some embodiments of an electronic touch sensor device in accordance with the present invention, there are N input/output circuits configured to drive N LEDs, where N is an integer greater than one. In such an embodiment, the bias driver circuit is configured to drive the second electrode of all N LEDs. 
     In some embodiments, the control circuit may comprise an application specific integrated circuit (ASIC). In other embodiments, the control circuit may comprise a field-programmable gate array (FPGA), any other type of logic circuit, or a microprocessor or similar device. 
     In some embodiments of a touch sensor in accordance with the present invention, the control circuit is configured to switch between the LED drive mode and the capacitive sense mode on a periodic basis. In some embodiments, the switching frequency is set to be approximately 200 Hz. However, other switching frequencies are possible and would similarly fall within the scope and spirit of the present invention. 
     In some embodiments of a touch sensor in accordance with the present invention, the control signal is configured to drive a pulse-width-modulated (PWM) signal to each LED driver circuit such that the brightness of each LED driven by each LED driver circuit can be controlled by adjusting the duty cycle of the PWM waveform. 
     In some embodiments of a touch sensor in accordance with the present invention, the bias driver circuit is configured such that when the LED drivers are disabled and the capacitive sensors are enabled, the bias driver applies a voltage to the second electrode of each LED that substantially matches the voltage at the first electrode of each LED. This reduces the susceptibility of the capacitive sense circuit to noise. 
     While particular embodiments of an electronic touch sensor device have been described, one of ordinary skill in the art studying the present specification and description of the invention will become aware of other variations and applications of the invention that will also fall within the scope and spirit of the present invention. The invention is described in detail below with reference to the appended sheets of drawings which are first described briefly below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a simplified circuit diagram of a system containing a multiple-capacitive-sensor controller typical of the prior art; 
         FIG. 2  depicts a circuit diagram of a system containing a multiple-capacitive-sensor controller in accordance with an exemplary embodiment of the present invention; 
         FIG. 3  depicts a simplified representation of an I/O port during LED drive operation in accordance with an exemplary embodiment of the present invention; 
         FIG. 4  depicts a simplified representation of an I/O port during capacitive sensing operation; 
         FIG. 5  depicts a simple circuit diagram of a control circuit in accordance with an exemplary embodiment of the present invention; and 
         FIG. 6  depicts a voltage waveform and timing diagram associated with an embodiment of a multiple-capacitive-sensor controller in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the present invention includes an apparatus and method of using a single I/O port as a capacitive input and LED driver.  FIG. 2  depicts a simplified circuit diagram of a multiple-touch-sensor controller  201  in accordance with an embodiment of the present invention. The multiple-touch-sensor controller  201  contains a bias driver  205 , a control circuit  219  and n number of I/O ports  221 . Each I/O port  221  contains both an LED driver  207  and a capacitive sensor  209 . The output of LED driver  207  is connected to the input of capacitive sensor  209  and pad  211 . Pad  211  is further connected to control circuit  219 , capacitive element  215  and the anode of LED  213 . The cathode of LED  213  is then connected to bias driver  205  through pad  217 . Control circuit  219  controls the LED drivers, capacitive sensors and bias driver based on the functionality needed, as described below. 
       FIG. 3  depicts a simplified representation of an I/O port during LED drive operation in accordance with an embodiment of the present invention. In this operation, control circuit  301  disables capacitive sensor  309  by driving capacitive sensor enable signal  321  to a logical low while simultaneously enabling LED driver  307  by driving LED driver enable signal  319  to a logical high. The LED driver enable signal  319  can also be controlled by a low-high pulse-width modulation (PWM), which is well known in the art, to control the LED intensity. The bias driver  305  is tied to a fixed voltage, such as ground, to allow for current flow through LED  313 . 
       FIG. 4  depicts a simplified representation of an I/O port during a capacitive sensing operation. In this operation, control circuit  401  enables capacitive sensor  409  by driving capacitive sensor enable signal  421  to a logical high while simultaneously disabling LED driver  407  by driving LED driver enable signal  419  to a logical low. When a user interacts with capacitive element  415 , the capacitance value of capacitive element  415  will change, resulting in a change in voltage level on pad  411 . Capacitive sensor  409  detects the voltage level on pad  411 , allowing the system to react to the user interaction. The control circuit also feeds the voltage level to bias driver  405 . This allows bias driver  405  to track the voltage on pad  411  and drives the same voltage to the cathode of LED  413 , resulting in a constant, near zero voltage across LED  413 . 
     In comparison with prior art capacitive sensor controllers, the current invention allows an LED and a capacitive element to share an I/O port because the control circuit can control when to enable or disable the capacitive sensing and LED drive operations. Such a control circuit can be implemented using an Application Specific Integrated Circuit (ASIC), programmable logic such as a Field Programmable Gate Array (FPGA), a microprocessor, or similar device known in the art.  FIG. 5  depicts one embodiment of a multiple-capacitive-sensor controller in accordance with the present invention, wherein a time-multiplexed control circuit enables capacitive sensing for a period of time to detect possible inputs, after which it disables capacitive sensing and enables LED drive operation. In this embodiment, counter  504  uses a fixed frequency clock signal  502  to determine time. State machine  506  uses the timing signal generated by counter  504  to determine whether the I/O ports should be in capacitive sensing or LED drive operation. State machine  506  controls bias driver mux select signal  518 , bias driver out signal  510 , capacitive sensor enable signal  514  and LED driver enable signal  516  accordingly. It should be recognized by one of ordinary skill in the art that the state machine described above is only one possible implementation of the invention. Other implementations, including software controllers, digital controllers, or analog controllers are also possible and would similarly fall within the scope and spirit of the present invention. 
     In accordance with one embodiment of the present invention,  FIG. 6  depicts an exemplary voltage level and timing diagram of the various signals during a complete timing cycle, spanning one capacitive sensing and LED drive operation. During LED drive operation  601 , the capacitive sensor is disabled, as reflected by waveform  605 . The LED driver is enabled, as reflected by waveform  621 . During this time, the voltage level at pad  211  is controlled by the LED driver, as reflected by waveform  613 . The bias driver mux is set to select ground, as reflected by waveform  609 . This ties the output of the bias driver to ground, as reflected by waveform  617 , allowing current to flow through the LED. During capacitive sensing operation  603 , the capacitive sensor is enabled, as reflected by waveform  607 . The LED driver is disabled, as reflected by waveform  623 . When a user approaches or touches capacitive element  215 , the capacitance of capacitive element  315  changes, as reflected by waveform  615 . The voltage level on pad  211  also changes, as reflected by waveform  614 . The bias driver mux is set to select the pad voltage level, as reflected by waveform  611 . Therefore, the bias driver outputs the same voltage level that appeared at pad  211 , as reflected by waveform  619 , resulting in a constant, near zero voltage across LED  215 . In a preferred embodiment of a multiple-capacitive-sensor controller in accordance with the present invention, a complete timing cycle, comprising capacitive sense mode and LED drive mode combined, has a duration of approximately 5 milliseconds (200 Hz). Of course, other operating frequencies are also possible and would fall within the scope and spirit of the present invention. At a frequency of 200 Hz or above, the capacitive read operation occurs with sufficient frequency to properly detect user inputs, and each capacitive read operation is short enough to ensure that the LED off time is unnoticeable to the user. 
     In addition to reducing system area and cost, the present invention also improves noise immunity and reduces system current consumption. With reference with  FIG. 2 , the LED  213  is electrically connected to capacitive sensor  209  and acts as an antenna. This allows other, adjacent pins and power supplies to inject electrical noise onto the capacitive sensor, creating false input detection. In an embodiment of the present invention, the bias driver ensures that the LED&#39;s anode and cathode are driven to the same voltage as the capacitive sensor input during the capacitive sense operation, eliminating noise injection from other, adjacent pins and power supplies through the LED. Nulling the voltage across the LED&#39;s anode and cathode also neutralizes the LED&#39;s parasitic parameters such as capacitance and leakage current, which, in turn, eliminates charge flow through the LED, increasing noise isolation from other pins. 
     Although a particular embodiment of a multiple touch sensor controller was discussed here, other embodiments and implementations are possible and would also fall within the scope and spirit of the present invention. Similarly, the control circuit discussed above with reference to  FIG. 5  can be easily adapted by one of ordinary skill in the art to handle other scenarios, such as an event driven scenario. For example, in an alternative embodiment, the system has a standby mode during which the LED is turned off. Accordingly, the control circuit is designed such that when in standby mode, the capacitive sensors are enabled and the LED drivers are disabled. The control circuit will continuously monitor the capacitive sensors and only enables the LED drivers when a user input is detected. 
     The invention is further defined by the following claims.