Abstract:
An on-chip detection circuit automatically detects when an external switch has been activated. When the device is initialized, the detection circuit measures the operating current and coverts the information into an analog voltage. The analog voltage is then processed through an on-chip analog-to-digital converter and the digitized result is stored as a reference value. To sense the open and close action of the off-chip mechanical switch, the device then takes a sample of the operating current periodically, digitizes this information and compares the sampled value to the reference value. A change in value larger than some predetermined level indicates the external switch has been activated.

Description:
FIELD  
         [0001]    The present application relates to on-chip detection circuits, and more particularly to providing an on-chip detection circuit to detect the action of an off-chip switch.  
         BACKGROUND  
         [0002]    In wireless communication systems, a mobile station may be comprised of a handset including a microphone and speaker which are used to receive and place calls. Handsets typically require the user to hold the handset close to the user&#39;s head so the microphone and speaker are positioned close to the user&#39;s mouth and ear, respectively. However, there are times in which the user may desire to use the handset in a handsfree mode, such as when walking or using the handset for an extended period of time. In these circumstances, a headset may be used which connects to the handset and allows the user to operate the handset in a handsfree mode. Additionally, the headset allows the user to operate in a handsfree mode while maintaining privacy between the user and the handset. In addition to a mobile station, other portable devices such as personal digital assistants (PDA), portable music players and similar devices may take advantage of the use of a headset.  
           [0003]    Wireless communication systems typically comprise a plurality of base stations and mobile stations that communicate using an over-the-air communication protocol using physical layer technologies such as Code Division Multiple Access (CDMA) technology. As more functionality is added to wireless handsets and other devices, it may be desirable to include a control switch with a headset to allow a user to operate or activate features in the device from the headset. For example, a user could close the control switch to activate a mute function, control playback of an audio file, launch a web browser, or dial a number. Of course, the control switch may activate any number of features, and is not limited to the features listed here.  
           [0004]    For the control switch to be functional, a detection circuit may be used. Detection circuits are designed to effectively sense a change in operating current trigged by the opening and closing action of the switch on the headset. Currently detection is done off-chip, requiring extra off-chip components, which increases the cost and complexity of the portable device. Alternatively, a current-to-voltage conversion may be performed and a sample-and-hold circuit may used to store the information. However, this technique performs all functions in the analog domain, and thus requires a high quality operational amplifier, comparator, and low leakage sample-and-hold capacitor. These devices are large and expensive, and thus not desirable. Because the activity time is at least once every 25 ms, it is difficult to design an accurate and power efficient sample-and-hold circuit for the application.  
           [0005]    What is needed is a system that detects the open and close action of a mechanical switch and performs the functions in the digital domain. By determining the action of the mechanical switch in the digital domain, a more compact, accurate, and efficient design may be obtained. The system should also be capable of being incorporated on-chip to minimize the complexity and cost of the solution.  
         SUMMARY  
         [0006]    An on-chip detection circuit automatically detects when an external switch has been activated. When a device is initialized, the detection circuit measures the operating current and coverts the information into an analog voltage. The analog voltage is then processed through an on-chip analog-to-digital converter and the digitized result is stored as a reference value. To sense the open and close action of the off-chip mechanical switch, the device then takes a sample of the operating current periodically, digitizes this information and compares the sampled value to the reference value. A change in value larger than some predetermined level indicates the external switch has been activated.  
           [0007]    Another embodiment includes a device having a current monitor adapted to sense the current of an external circuit having a switch. The sensed current is converted to a digital value which is supplied to a comparator. The comparator compares the digital value to a reference value and determines the switch has been actuated if the digital value differs from the reference value by at least a predetermined amount.  
           [0008]    These and other features and advantages of the embodiments will become more apparent upon reading the following detailed description and upon reference to the accompanying drawings. 
       
    
    
     DESCRIPTION OF DRAWINGS  
       [0009]    [0009]FIG. 1 illustrates components of a wireless communication system including a headset and a mobile station.  
         [0010]    [0010]FIG. 2 is a circuit diagram of a detection circuit which may be used in a portable device.  
         [0011]    [0011]FIG. 3 illustrates a process for determining the open and close action of an off-chip switch. 
     
    
     DETAILED DESCRIPTION  
       [0012]    [0012]FIG. 1 illustrates components of an exemplary wireless communication system  100 . The wireless communication system  100  includes a device such as a mobile station  105 , or handset. The mobile station  105  is used in the wireless communication system  100  to communicate with a base station (not shown). This communication may use an over-the-air communication protocol using physical layer technologies such as Code Division Multiple Access (CDMA) technology. Although the present embodiment is shown with a mobile station  105 , it can be appreciated that any device may be used that needs to detect the activation of an external switch. The mobile station  105  includes a speaker  110  which allows a user to hear sounds and a microphone  115  which detects the user&#39;s voice. Typically, the user holds the mobile station  105  to the user&#39;s head so that the speaker  110  is proximate the ear and the microphone  115  is proximate the mouth. The mobile station  105  may also include a port  120  designed to accept a plug  175  from an external headset  150 .  
         [0013]    The external headset  150  may be used in with an external device such as the mobile station  105 . The external headset  150  includes an earpiece  155 , a microphone  160 , a connector cord  165 , a switch  170 , and the plug  175 . The headset  150  may be used when the user desires to operate the device in a handsfree mode. To use the headset, the plug  175  is inserted in the port  120  of the mobile station  105 . When the headset  150  is connected, the signals typically directed to the speaker  110  are sent to the earpiece  155 , and the microphone  115  is disconnected while the microphone  160  is connected. Signals are transferred between the mobile station  105  and the headset  150  via the connector cord  165 . The mobile station  105  may contain hardware, software, and/or firmware that detects the presence of the headset  150  and modifies any signals accordingly.  
         [0014]    The headset  150  may contain an external switch  170  which may be manually activated by the user. Activation of the switch may invoke features in the device from the headset, such as a mute function, control playback of an audio file, launch a web browser, or dial a number. In order to invoke those features, the mobile station  105  needs to detect when the switch has been closed. The present embodiment includes a detection circuit which detects the action of the mechanical switch  170  and performs the detection functions in the digital domain. By determining the action of the mechanical switch  170  in the digital domain, a compact, accurate, and efficient design is obtained.  
         [0015]    [0015]FIG. 2 is a circuit diagram of a detection circuit  200  which may be used in a portable device. The detection circuit includes external circuits  205 ,  210 , an analog portion  215 , and a digital portion  220 . The external circuits  205 ,  210  include one or more headsets  150  having external switches  170  which may connect to the mobile station  105 . The detection circuit  200  may be used with a plurality of external circuits, however for purposes of the description, only a first external circuit  205  will be discussed. The external circuit  205  includes the headset  150  and the external switch  170  (SW ext ). The headset  150  and external switch  170  are connected to capacitor C 1 , and then to the CODEC. The external circuit  205  also includes an external resistor R ext . The value of the external resistor R ext  is selected by the manufacturer, and its value directly affects the magnitude of the current jump when the external switch  170  is closed. In one embodiment, the value of the external resistor R ext  may range from 1.69 kΩ to 4KΩ.  
         [0016]    The external circuit  205  is connected to the analog portion  215  of the detection circuit  200  through a pin called MicBias. A signal swing of approximately 229 mVpp may be expected on the AC coupled input node to the CODEC transmission path, and the current resulting from such a swing has to be treated as noise by the detection circuit  200 , so that the detection circuit can reduce the chance of a false positive indication in the detection circuit  200 .  
         [0017]    The analog portion  215  of the detection circuit  200  includes a MicBias voltage generator  225 , a current mirror consisting of transistors M 2  and M 3 , and a resistor R 1 . The MicBias voltage generator  225  may be used to power the external circuit  205 . Although a 1.8V input voltage is shown, it can be appreciated other voltage levels may be used. The transistor M 2  in the current mirror may be of a wide-swing design to ensure the transistor M 2  can handle a maximum current level even with a reduced on-chip analog power supply level. The current sensed by the transistor M 2  is the current from the external circuit. As the external switch  170  is closed, the current from the external circuit increases, and this increase in current is mirrored to transistor M 3 . The current mirror consisting of transistors M 2  and M 3  should be accurate, but not one-to-one. This design reduces the current on transistor M 3 , thus saving power. The current from the transistor M 3  flows through the resistor R 1  to generate the test voltage V test . The temperature and process variation of the on-chip resistor R 1  should be considered while determining the voltage level for V test . The voltage V test  should be designed so that even in the worst condition, the current mirror transistor M 3  remains in saturation.  
         [0018]    The test voltage V test  is supplied to the digital portion  220  of the detection circuit  200 . The digital portion  220  of the detection circuit includes a multiplexer  230 , an analog-to-digital converter (ADC)  235 , and a mobile station modem  240  including a store section  245  and a compare section  250 . The multiplexer  230  receives the test voltage V test  and may selectively provide the test voltage V test  to the ADC  235 . In one embodiment, the multiplexer  230  has eight inputs, although any size multiplexer  230  may be used. The ADC  235  converts the analog test voltage V test  to a digital voltage value. The multiplexer  230  may include a switch (not shown) which controls the transfer of the digital value to the mobile station modem  240  for processing. The switch in the multiplexer  230  may be controlled by the mobile station modem  240 , which may continuously trigger the switch through control line sw_ctrl at a periodic rate (e.g. every 25 ms). Triggering the switch periodically ensures the digital test value is continuously provided to the mobile station modem for comparison. The digital value may be transferred to the mobile station modem  240  serially using, for example, a serial bus interface. Alternatively, the digital value may be transferred to the mobile station modem  240  in parallel.  
         [0019]    The mobile station modem  240  includes the store section  245  which contains the reference value. The compare section  250  is used to compare the digital value to the reference value. If the digital value exceeds the reference value, then the mobile station modem  240  determines the external switch  170  was activated causing a current jump. The mobile station modem  240  may then generate a signal indicating activation of the external switch  170 . The mobile station modem  240  may be programmed so that the digital value should exceed the reference value by a predetermined amount before the switch activation is detected. This may reduce the risk of noise in the system causing a false indication of switch activation.  
         [0020]    [0020]FIG. 3 illustrates a process  300  for determining the open and close action of the off-chip switch  170 . The process  300  begins in a START block  305 . Proceeding to block  310 , the process  300  detects the presence of external circuitry such as the headset  150  and generates a reference value. The reference value is generated once the external circuitry is detected with the switch  170  in the open position. The reference value is stored for later comparison.  
         [0021]    Proceeding to block  315 , the process  300  triggers an internal switch and senses the current from the external circuitry. The current may be from one external circuit or a plurality of external circuits. After the current is sensed, the process  300  proceeds to block  320 , where the current is converted to a digital value. In one embodiment shown above in FIG. 2, the sensed current is mirrored to a test point. At the test point, the test voltage is determined and provided to an ADC where the test voltage is converted to a digital value.  
         [0022]    Proceeding to block  325 , the digital value is then compared to the reference voltage. If the digital voltage is the same as the reference voltage, the detection circuit concludes the switch has not been activated and proceeds along the NO branch to block  335 . Returning to block  325 , if the digital value is greater than the reference value, the detection circuit concludes the switch has been activated and proceeds along the YES branch to block  330 . To ensure noise in the system does not inadvertently signal a switch activation, the digital value may need to exceed the reference value by a predetermined level before a switch activation is indicated.  
         [0023]    In block  330 , the activation circuit generates a signal indicating the switch  170  has been activated. This signal may be used by the mobile station  105  or other device as necessary.  
         [0024]    Returning to block  335 , after the digital value is compared to the reference value, the process  300  repeats the check at a periodic rate (such as every  25  ms). In one embodiment, this is accomplished by checking to see if the rate timer has expired. If the timer has not expired, the process  300  loops along the NO branch. Once the timer expires, the process  300  proceeds along the YES branch back to block  315  where the internal switch is again triggered to obtain a new current value. The process  300  continues to run as long as the headset  150  is connected to the mobile station  105 .  
         [0025]    Although the present device has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. Such changes and modifications are to be understood as being included within the scope of the present device as defined by the appended claims.