Abstract:
An apparatus and method for detecting the closure of one of several switches using a single input line of a detector is disclosed. The input line is also used to carry an audio-frequency signal when a recording switch is closed. The switches are connected to a network of resistors that cooperate with a pull-up resistor to form a voltage divider. The pull-up resistor is connected to the input line. The switches are closed to selectively switch resistors in the network out of the voltage divider to change the voltage on the input line. A detector reads the voltage on the input line and compares it with a set of predetermined values to determine which switch is closed. If a recording switch is closed, a microphone connected to the network of resistors converts sound to an audio-frequency signal onto the input line.

Description:
FIELD OF INVENTION 
     This invention relates to a method and apparatus for detecting closure of one of several switches using one input pin. More particularly, this invention relates to detecting the closure of one of several switches using one input pin of a microprocessor and subsequently using the same input pin to receive audio frequency signals if a particular recording switch is detected to be closed. 
     BACKGROUND 
     In the prior art, usually more than one input line of a microprocessor or microcontroller is required for the detection of closure of one of two or more switches. Generally, a keypad having n 2  buttons or switches arranged in an n×n matrix will require 2N number of input/output lines for decoding switch closures on the keypad. For example a two-by-two keypad requires four input and output (I/O) lines for determining a key press and a three-by-three keypad requires six I/O lines. Such a matrix configuration works if the required number of I/O lines is available. However it is not always possible to provide the required number of I/O lines due to either space or cost constraints. 
     Examples of devices where cost and size are important design considerations are portable music playback devices such as CD players and more recently palm-sized personal computer (P/PC). As these devices are meant to be carried around, it is highly desirable to reduce the size of these devices. Reducing the number of I/O lines required is one way of achieving a smaller device. 
     Another factor which dictates that less I/O lines be used in such devices is the types of standard jacks available for connecting a pair of earphones to the device. Standard jacks are either of the three or four-pole types. A three-pole jack has three sections or poles, two of these poles are used for the left and right sound channels and the last pole for a ground (GND) connection. Such a three-pole jack suffices for connecting a pair of stereo earphones to the device. To support anything more than the pair of stereo earphones, more connections than what a three-pole jack can support are required. 
     For example, 4-pole jacks have become a standard part when remote controls are introduced. When used to play music, these devices are usually carried on the waist or in a bag rendering the control buttons on the device difficult to access. To increase usability, manufacturers of these devices have introduced a remote control which is attached to the wires connecting the earphones to the device. This remote control is small in size and has a few control buttons—play, stop, forward and reverse. When a user puts on the earphones, the remote control is conveniently located at chest level and therefore allows the user easy access. 
     With a recording device such as a P/PC, it will also be convenient to locate a microphone on the remote control. With the microphone added onto the remote control, more connections are required to the device. These connections would require jacks to have more than four poles, preferably five poles. However, five-poles jacks are currently non-standard and would have to be specially manufactured. Cost involved in tooling such a non-standard jack is prohibitive. One possible solution is to use two separate standard jacks to support the combination of the remote control, the earphones and the microphone. However, such a solution would impede the usability of the device and would require more space on the device than would a single jack. 
     From the foregoing, the prior art therefore has a need for an innovative method and apparatus for detecting the closure of one of a plurality of switches using one input pin. The method also uses the same input pin to receive an audio frequency signal after detecting that a particular recording switch is closed. With this apparatus, a standard four-pole jack will be sufficient for connecting a remote control, a pair of earphones and a microphone to an audio recording and playback device. It is advantageous to use such a four-pole jack as the device would then be able to operate with a pair of earphones that is attached to a standard three-pole jack. 
     SUMMARY 
     In accordance with a preferred embodiment, an apparatus for detecting the closure of one of several switches using a single input line of a detector has a voltage generator and a comparator. The voltage generator is configured to be responsive to a switch closure for generating a predetermined output voltage on the input line. The detector has a comparator that receives the output voltage on the input line and determines which one of the several switches is closed. 
     Preferably, the comparator has an analog-to-digital converter (ADC) and a microprocessor. The ADC converts the output voltage on the input line to a digital voltage value. The microprocessor compares the digital voltage value with a set of predetermined values to determine which switch is closed. Preferably, the voltage generator is a voltage divider that has a first resistance connected to a second resistance at a control node. The control node is connected to the input line. In one embodiment, the second resistance is derived from a network of resistors connected in series. The switches are connected to these resistors to selectively switch resistors out of the second resistance to change the second resistance. With the change in the second resistance, the voltage at the control node also changes. 
     Optionally, the apparatus further includes a microphone for converting sound to an audio-frequency signal onto the input line. This microphone is connected to the network of resistors to form the second resistance. The apparatus also includes a third resistance that can be connected across the first resistance to produce a lower overall resistance across the first resistance. This lower resistance across the first resistance raises the voltage on the input line to appropriately bias the microphone. 
     The voltage divider and the microphone are preferably contained in a remote control housing separate from the detector. The microphone is connected to the detector using a cable which includes the input line. 
     In another aspect of the present invention, a remote control and earphones set that forms part of the earlier described apparatus has a four-pole jack, a remote control and a pair of earphones. The set is connected to a device that includes the detector using the four-pole jack. The remote control is coupled to the four-pole jack. The remote control includes the voltage divider described above. The four-pole jack carries audio signals from the device for playing back on the earphones. Preferably, the remote control is directly coupled to the four-pole jack, and the earphones are coupled to the four-pole jack via the remote control. 
     Also in accordance with the preferred embodiment, the present invention provides a method of detecting the closure of one of several switches using the apparatus described above. The method involves generating a predetermined voltage on the input line when a switch is closed. The method further converts the predetermined voltage received on the input line to a digital voltage value. The method also measures the voltage of a battery used to power the apparatus. After measuring the battery voltage, the method selects a set of predetermined voltages that corresponds to the battery voltage. The method then compares the digital voltage value with the selected set of predetermined voltages to determine which switch is closed. When a recording switch is detected to be closed, the method changes the voltage on the input line to appropriately bias the microphone such that an audio-frequency signal is received substantially undistorted on the input line. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention will be better understood with reference to the following drawing, in which: 
         FIG. 1  is a block diagram of palm-sized personal computer (P/PC) having a device, a remote control and a pair of earphones. 
         FIG. 2  is a block diagram similar to  FIG. 1  showing the electronic hardware in the remote control and the device according to a preferred embodiment of the present invention. The remote control is shown connected to the device at a control node. The remote control has several switches and a microphone for audio recording. 
         FIG. 3  is a graph of the voltage at the control node during a recording session using the P/PC in  FIG. 1 . 
         FIG. 4  is a sequence of steps for decoding the closure of the switches in  FIG. 2  and for adjusting the voltage at the control node when it is detected that the switch associated with a recording function is closed. 
         FIG. 5  is an alternative embodiment of the remote control of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Hereafter, a preferred embodiment of the present invention will be described in the context of a palm-sized personal computer (P/PC) equipped with a remote control. However, it is to be understood that the invention is equally applicable to other types of computing devices or electronic devices wherein a single input/output (I/O) pin is used for detecting depression or closure of one of several buttons or switches and for the receiving of audio frequency signals from a microphone if it is detected that a recording switch is closed. 
       FIG. 1  is a block diagram of a P/PC system  2  which is capable of recording and playing back audio frequency signals which are digitally stored in a suitable format such as the MP 3  format. The system  2  includes a P/PC device  4 , a pair of stereo earphones  6  and a remote control  8 . The remote control is electrically coupled to a four-pole jack  10  using a cable  12 . The stereo earphones  6  are preferably electrically coupled to the four-pole jack  10  via the remote control  8 . When connected to a corresponding four-pole socket on the device  4 , the four-pole jack carries a left-channel and a right-channel audio signal from the device  4  for playback on the earphones  6 , a ground (GND) signal and a control signal for input into the device  4 . The length of the cable  12  is chosen such that when a user wears the earphones  6 , the remote control  8  is approximately at the chest level of the user. Such a positioning of the remote control  8  allows the user to easily access the remote control  8 . On the remote control  8  are five buttons or switches S 1 –S 5  ( FIG. 2 ) for performing functions common to audio recording and playback devices. The switches are designated RECORD, PLAY, STOP, FORWARD and BACKWARD. 
       FIG. 2  is a schematic diagram showing the connections between the remote control  8  and the device  4 . According to the preferred embodiment of the invention, a network of five resistive elements such as resistors R 1 –R 5  are connected in series in the remote control  8 . This series of resistors R 1 –R 5  are preferably connected in series with a microphone  12 . A free end of the microphone  12  is connected to a pole of the four-pole jack  10 . On the device end of this connection is a pull-up resistive element, such as a resistor R 6 . Resistor R 6  is connected to a battery for powering the device  4 . This pull-up resistor R 6  together with the microphone  12  and resistors R 1 –R 5  form a voltage divider. The connection point between the microphone  12  and the pull-up resistor R is designated a control node  14 . The voltage divider acts as a voltage generator for generating a control voltage on this control node  14 . Five switches S 1 –S 5  are electrically coupled to the series of resistors R 1 –R 5  such that closure of the switches is able to cause predetermined voltages to be generated at the control node  14 . The switches S 1 –S 5  are preferably of the bi-state push button type which is normally open and held down to be closed. In the preferred embodiment, such a switch connection can be realized by connecting one end of each switch S 1 –S 5  to GND and the other end of each switch to interconnection nodes between the resistors R 1 –R 5 . When all the switches S 1 –S 5  are open, none of the resistor R 1 –R 5  is connected to GND. The resistor R 6  in the device  4  pulls the control node  14  to the battery voltage. When the switch S 5  is closed, resistors R 1 –R 4  are excluded from the voltage divider to leave only resistor R 5  in the voltage divider. This resistor R 5  in the voltage divider will result in a predetermined minimum voltage to be dropped or generated at the control node  14 . Similarly, when only the switch S 4  is closed, resistors R 1 –R 3  are excluded and only resistors R 4  and R 5  are left in the voltage divider. The voltage of the control node  14  changes similarly with the closures of the other switches S 1 –S 3 . If only the switch S 1  is closed, all the resistors R 1 –R 5  will form part of the voltage divider to cause a predetermined maximum voltage to drop at the control node  14 . 
     If more than one switch is closed in such a configuration of the switches S 1 –S 5 , the switch which leaves the least number of resistors R 1 –R 5  to form the voltage divider will be detected to be a dominant switch. For example, if switches S 3  and S 5  are closed simultaneously, only resistor R 5  remains in the voltage divider since switch S 5  connects the resistor R 5  to GND, excluding resistors R 1 –R 4  from the voltage divider even though the switch S 3  is also closed. Such a configuration of the switches and resistors allows the more dominant switches to be designated as switches for activating more important functions. In this preferred embodiment, the switch S 5  is designated for activating a recording function. This recording function is viewed as most important and should pre-empt the other functions if activated. Similarly, it is preferable that the other switches S 4 –S 1  are designated for activating a playback, stop, forward and backward function in descending order of priority. 
     An interface to the control node  14  in the device  4  for detecting switch closures is next described. On the device side, the control node  14  is electrically coupled via an I/O line to an I/O pin  16  of a SH 3  microprocessor  17  available from Hitachi. This I/O pin  16  serves two purposes—an interrupt and an input to an analog-to-digital converter (ADC)  18  on the microprocessor  17 . There are several design objectives to be met when designing the electronic hardware of the present invention. The first objective is to have an ability to bring the microprocessor  17  out of a power saving mode such as the light-sleep and deep-sleep mode on the SH 3  microprocessor  17  when one of the switches S 1 –S 5  is closed. Given that the closure of the switch S 1  generates the highest voltage at the control node  14 , the value of resistors R 1 –R 7  have to be selected to ensure that the voltage generated at the control node  14  is sufficiently low to interrupt the microprocessor  17  to cause it to run an interrupt service routine or interrupt handler. The voltage at the control node  14  should be less than half of the battery level to be able to cause an interrupt in the microprocessor  17 . This voltage is dependent on the semiconductor technology that is used to build the microprocessor. 
     Another design objective is that the device  4  should be able to support a standard pair of stereo earphones that comes with a standard three-pole jack. When such a three-pole jack is inserted into a socket for receiving a standard four-pole jack, the GND pole will connect the GND and CONTROL poles thereby shorting the control line to GND. In such a situation, only the resistor R 6  in the voltage divider is connected across the battery. This resistor R 6  is therefore selected to be of a high resistance to reduce current drain of the battery when the three-pole jack is inserted in the socket for a four-pole jack. 
     After the value of resistor R 6  is determined, the values of the resistors R 1 –R 5  are determined next. The total resistance of the microphone and the resistors R 1 –R 5 , are previously explained, must result in a voltage of less than half the battery voltage at the control node  14 . In the preferred embodiment, this voltage at the control node is chosen to be approximately 1V. It should be noted that when a three-pole jack is inserted into the socket for receiving a four-pole jack, the control node would be shorted to GND to be at 0V. Therefore, any switch closure should generate a minimum control voltage that is appreciably different from GND. In this preferred embodiment, the lowest voltage generated when a switch is closed for a device powered by a fully charged battery of 3.3V is about 0.2V. The values of resistors R 1 –R 5  are chosen such that the voltages generated at the control node fall in the range of 0.2V to 1.0V. Preferably, the generated voltages are sufficiently spaced in the range to cater for tolerances in the values of the resistors. The control voltages for a fully charged battery at 3.3V are given in the table below. 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Closure of Switch 
                 Voltage at control node 
               
               
                   
                   
               
             
             
               
                   
                 None 
                 3.3 V 
               
               
                   
                 S5 (RECORD) 
                 0.2 V 
               
               
                   
                 S4 (PLAY) 
                 0.4 V 
               
               
                   
                 S3 (STOP) 
                 0.6 V 
               
               
                   
                 S2 (FORWARD) 
                 0.8 V 
               
               
                   
                 S1 (BACKWARD) 
                 1.0 V 
               
               
                   
                   
               
             
          
         
       
     
     These voltage values are stored in the memory (not shown) in the device  4  and used for subsequent comparison with a measured voltage at the control node for determining which switch is closed. Since the battery voltage drops with usage, more than one set of these voltages that correspond to different battery voltages may be required. Take as an example when the battery level has dropped to 2.8V. At such a battery level, closure of the switch S 1  will result in a voltage of approximately 0.8V to be dropped at the control node  14 . If the microprocessor  17  relies only on a single set of voltage values (as shown in the table above) to determine which switch is closed, it would erroneously determine that the switch S 2  is closed instead of the switch S 1 . Therefore when the battery voltage is at 2.8V, a different set of predetermined voltages is required for determination of switch closure. 
     Connected across the resistor R 6  is a resistor R 7  in series with a switch such as a MOSFET  15 . When the switch S 5  is closed during a recording session, the voltage at the control node is held at about 0.2V. This biasing voltage is not sufficient to allow maximum swing of an audio-frequency signal received from the microphone  12 . A negative-going portion of the audio-frequency signal will be clipped. Such clipping of the signal results in undesirable distortion when the signal is reproduced. It is therefore important to raise the voltage at the control node  14  to a level which allows the audio-frequency signal to attain its maximum swing in both directions of the bias voltage. In the preferred embodiment, the biasing voltage is raised to about half of the battery voltage by switching the resistor R 7  across R 6  to reduce the overall resistance across resistor R 6 . The MOSFET  15  is switched on and off to connect and disconnect resistor R 7  from resistor R 6 . The gate of the MOSFET  15  is connected to an output pin of the microprocessor to allow the microprocessor  17  to switch the MOSFET  15  on and off. 
       FIG. 3  is a graph showing the voltage at the control node  14  during a recording session. This graph is best described in conjunction with a description of a sequence of steps for detecting switch closures that is implemented in an interrupt handler.  FIG. 4  is a flowchart of the sequence  20  of steps. The sequence  20  begins in a START step  22  when a switch is closed to cause an interrupt to occur in the microprocessor. The microprocessor  17  executes the interrupt handler associated with the I/O pin to which the control node  14  is connected. Before the switch is closed, the control node is pulled high by the resistor R 6  and the control voltage is at the battery voltage. This voltage at the control node  14  is indicated by arrow A in  FIG. 3 . After the START step  22 , the sequence  20  proceeds to a CONTROL VOLTAGE AT ZERO VOLTAGE? step  24 , where a comparator compares the analog control voltage with predetermined values to determine which switch is closed. In this preferred embodiment, the microprocessor performs a comparator function by converting the analog control node voltage to a digital value and determining if that value is close to or equal to zero volt. An on-chip analog-to-digital converter (ADC) in the microprocessor preferably performs this conversion. If it is determined in this step that the control voltage is close to or equal to zero volt, the sequence  20  proceeds to a DISCONNECT R 7  step  26 . In this step  26 , the microprocessor  17  switches the MOSFET  15  off to disconnect resistor R 7  from resistor R 6  to allow resistor R 6  to limit the current drain from the battery as previously described. The sequence  20  then proceeds to an END step  27 , where the microprocessor may optionally return to a power saving mode. 
     If however it is determined in the CONTROL VOLTAGE AT ZERO VOLT? step  24  that the control voltage is not close to or equal to zero volt, the sequence  20  proceeds to a MEASURE BATTERY VOLTAGE step  28 , where the microprocessor determines the battery voltage. The sequence then proceeds to a DECODE SWITCH CLOSURE step  30 , where the microprocessor compares the digital voltage value against an appropriate set of voltages corresponding to the measured battery voltage to determine which of the several switches S 1 –S 5  are closed. The sequence next proceeds to a RECORD SWITCH CLOSED? step  32 , where the microprocessor determines if it is the switch S 5  that is closed. If it is determined that it is not the switch S 5  that is closed, the microprocessor proceeds to a PERFORM OTHER TASKS step  34 . In this step, the interrupt handler calls the appropriate software routine that implements the activated function associated with a highest priority closed switch to perform the function. 
     If it is determined in the RECORD SWITCH CLOSED? step  32  that the switch S 5  is closed—when the voltage at the control node is about 0.2V—the sequence  20  proceeds to a CONNECT R 7  steps  36 . An arrow B in  FIG. 3  indicates this control node voltage when switch S 5  is closed. In the CONNECT R 7  step  36 , the microprocessor turns the MOSFET  15  on to connect resistor R 7  across resistor R 6 . This connection changes the resistance across resistor R 6  in the voltage divider and as a result, raises the control node voltage to about half of the battery voltage. An arrow C in  FIG. 3  indicates this raised voltage at the control node  14 . The sequence then proceeds to a BEGIN RECORDING step  38 . In this step, a user can begin to speak into the microphone  12 . The microphone converts the speech into an audio-frequency signal. This signal is appropriately biased to allow maximum swing of the signal in either direction of the bias voltage. As the peak-to-peak voltage of this audio-frequency signal is small, it is preferable that the signal be amplified using an audio pre-amplifier  40  ( FIG. 2 ). The amplified signal at the output of the pre-amplifier  40  is then subsequently digitally coded by a CODEC  42  ( FIG. 2 ). The coded values from the CODEC  42  are then stored in the memory of the device  4  for subsequent retrieval. 
     The sequence  20  next proceeds to a RECORD SWITCH RELEASED? STEP  44 . In this step, the microprocessor  17  determines if the closed switch S 5  is released by either polling the I/O pin or by interrupt means associated with the pin. If it is determined that the closed switch S 5  is not released, the sequence loops around this step  44 . If however it is determined that the closed switch S 5  is released, the sequence proceeds to an END RECORDING step  46 , where the interrupt handler terminates the recording session. When the closed switch S 5  is released, the voltage at the control node is also pulled up to the battery voltage by the resistor R 6 . The sequence finally terminates in the END step  27 , where the microprocessor can optionally disconnect resistor R 7  from across resistor R 6  by turning the MOSFET off. 
     While the present invention has been shown and described with reference to the foregoing operational principles and preferred embodiment, it will be apparent to those skilled in the art that other changes in form and detail may be made. As an example, the microphone  12  may be connected in parallel to the resistor R 1 –R 5  instead of the series connection in the preferred embodiment.  FIG. 5  shows a microphone  12  connected in parallel to the resistors R 1 –R 5 . 
     The switch S 5  in this alternative embodiment is of a double-pole-single-throw type. When actuated, this switch S 5  connects the resistors R 5  and the microphone  12  to a ground signal in the device  4 . When connected in this manner, the microphone  12  does not affect the voltage at the microprocessor pin when the switches S 1 –S 4  are actuated. Such a configuration allows a higher tolerance in the resistance values of the resistors R 1 –R 5 . As a result, switch closure can be more accurately determined.