Abstract:
A method of controlling a terminal of an integrated circuit includes determining a frequency ratio between a frequency of a signal and a frequency of another signal received by an integrated circuit. A selected signal appearing at a selected terminal of the integrated circuit is selectively interpreted in accordance with an operating mode when the frequency ratio is below a selected value and in accordance with another operating mode when the frequency of the signal is above a selected value.

Description:
FIELD OF INVENTION 
   The present invention relates in general to integrated circuits and in particular to circuits and methods for reducing pin count in multiple-mode integrated circuit devices. 
   BACKGROUND OF INVENTION 
   Reducing the number of pins required by an integrated circuit is normally one important factor in reducing overall packaged integrated circuit device size and cost. Additionally, integrated circuit devices with smaller numbers of pins typically assist in reducing the design complexity at the higher levels of the system application. At the same time, any reduction in the number of pins cannot unduly limit the input/output capability of the integrated circuit nor unreasonably constrain the range of operations available to the end user. This situation is particularly true in regard to integrated circuit devices that support multiple modes, such as digital audio devices which can be utilized in either pulse code modulation (PCM) or Direct Stream Digital (DSD) applications and thereby support different application options in a single package. 
   A pulse code modulated (PCM) audio system typically utilizes three clocks and a single stream of PCM—encoded serial audio data (SDATA). Specifically, an external master clock ( EMCK ) signal controls the overall timing of the processing operations, a serial or bit clock ( SCLK ) signal times the transfer of the individual bits of serial PCM audio data, and a left-right clock ( LRCK ) signal differentiates between left and right stereo data samples in the PCM data stream. On the other hand, the Direct Stream Digital (DSD) protocol, used to record audio under the Sony/Philips Super Audio Compact Disk (SACD) standard, is based on two channels of one-bit audio data ( DSDA  and  DSDB ) and a single serial clock ( DSD   —   CLK ) signal. The DSD protocol also utilizes the external master clock  EMCK . Therefore, in order to accommodate both modes in a single flexible and efficient integrated circuit device, an input/output scheme must be developed which addresses the differences between the DSD and PCM protocols with a minimum number of pins. 
   One current approach to providing the required input/output capability in DSD—PCM multiple-mode devices utilizes two independent sets of pins, one set for exchanging DSD protocol data and clock signals, and another set for exchanging PCM data and clock signals. This technique, however, is contrary to the goal of reducing the number of pins on the packaged device and/or overall package size. Another conventional approach is to share some pins for both the DSD and PCM modes, and dedicating other pins for supporting only one mode or the other. For example, one or more of the pins required for exchanging PCM mode clock signals might be also used for exchanging one channel of data in the DSD mode. However, this scheme normally requires additional internal and external control circuitry and /or one or more mode control pins for configuring the data and clock pins to support the selected operating mode. 
   Consequently, new techniques are required for supporting multiple-mode integrated circuits with a minimum number of pins. In particular, such techniques should not require the dedication of one or more available pins for mode configuration purposes nor require substantial additional control circuitry. 
   SUMMARY OF INVENTION 
   The principles of the present invention advantageously provide efficient techniques for minimizing the number of terminals on an integrated circuit. According to one particular embodiment, a method is disclosed for controlling a terminal of an integrated circuit and includes determining a frequency ratio between a selected signal received by the integrated circuit and another selected signal received by the integrated circuit. A selected signal appearing at a selected terminal of the integrated circuit is selectively interpreted in accordance with an operating mode when the frequency ratio is below a selected value and in accordance with another operating mode when the frequency of the signal is above a selected value. 
   Advantageously, the principles of the present invention allow a signal applied to a selected terminal of an integrated circuit to control the operation of one or more other pins of that integrated circuit. In turn, pins can be shared between different operating modes and the overall number of pins required to fully support the integrated circuit is advantageously reduced. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a diagram of a representative audio system application of a digital to analog converter (DAC) subsystem according to the principles of the present invention; 
       FIG. 2  is a block diagram of an exemplary DAC subsystem embodying the principles of the present invention and suitable for use in the representative system of  FIG. 1 ; and 
       FIG. 3  is a block diagram of clock generation and pin mode control circuitry suitable for use in applications such as the DAC subsystem shown in  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The principles of the present invention and their advantages are best understood by referring to the illustrated embodiment depicted in  FIGS. 1–3  of the drawings, in which like numbers designate like parts. 
     FIG. 1  is a diagram of a representative audio system  100  including a dual-mode pulse code modulation (PCM)—Direct Stream Digital (DSD) digital to analog converter (DAC)  101  subsystem according to the principles of the present invention. In this example, DAC subsystem  101  forms part of an audio component  102 , such as a compact disk (CD) player, digital audio tape (DAT) player, or digital versatile disk (DVD) unit. A digital media drive  103  recovers the digital data, for example, one-bit DSD audio from a Sony/Philips Super Audio Compact Disk (SACD), or multiple-bit PCM audio data from a traditional compact disk (CD). In either case, the recovered audio data, along with corresponding clocks and control signals, are passed to DAC subsystem  101 , as discussed further below. The resulting analog audio data output from DAC subsystem  101  undergoes further processing in analog/audio processing block  104  prior to amplification in audio amplification block  105 . Audio amplification block  105  then drives a set of conventional speakers  106   a  and  106   b.    
   In the PCM mode, multi-bit PCM—encoded audio data are received from media drive  103  by DAC subsystem  101  serially through an  SDATA/DSDA  pin timed by a serial clock ( SCLK ) signal received through the  SCLK/DSDB  pin. Left and right channel stereo data received through the  SDATA/DSDA  pin are alternately processed in response to a left-right clock ( LRCK ) signal, which is normally at the audio sampling rate, received through a corresponding  LRCK/DSD   —   CLK  pin. In system  100 , an external master clock ( EMCK ) signal is received by DAC subsystem  101  from digital media drive  103  through an  EMCK  pin. 
   In the DSD mode, two-channels of one-bit audio data,  DSDA  and  DSDB , are received by DAC subsystem  101  through the  SDATA/DSDA  and  SCLK/DSDB  pins, respectively. The DSD clock signal  DSDA   —   CLK  times the transfer of the  DSDA  and  DSDB  audio data and is received through the  LRCK/DSD   —   CLK  pin in the DSD mode. In alternate embodiments, the signal-pin mapping varies depending on the given integrated circuit design. For example, in one alternate embodiment, the  SCLK/DSDB  pin receives DSD data DSDA in the DSD mode and the SDATA/DSDA pin receives DSD data DSDB data in the DSD mode. 
     FIG. 2  is a high-level functional block diagram of a representative embodiment of DAC subsystem  101  shown in  FIG. 1 . DAC subsystem  101  includes serial interface, clock signal generator, and pin mode control block  201 , which provides the interface with media drive  103 , also of  FIG. 1 , through the  SDATA/DSDA, SCLK/DSDB, LRCK/DSD   —   CLK , and EMCK pins. As discussed in further detail below, serial interface, clock generator, and pin mode control block  201  includes detection circuitry which monitors the frequency of the clock signal presented at the  LRCK/DSD   —   CLK  pin, determines whether that frequency corresponds to either the frequency of the  LRCK  clock signal associated with the PCM mode or the frequency of the  DSD   —   CLK  clock signal associated with the DSD mode and then directs the signals at the  SDATA/DSDA  and  SCLK/DSDB  pins to be accordingly interpreted for PCM or DSD mode operations. 
   The illustrated embodiment of DAC subsystem  101  shown in  FIG. 2  processes two channels of either PCM or DSD audio data, with each audio channel passing through a corresponding digital interpolation filter  202   a – 202   b , delta-sigma DAC  203   a – 203   b , and analog output filter  204   a – 204   b . In the DSD mode, the data path including delta-sigma DAC  203   a , and analog output filter  204   a , processes the audio data  DSDA  received through the  SDATA/DSDA  pin, while the data path including delta-sigma DAC  203   b , and analog output filter  204   b  processes the audio data  DSDB  received through the  SCLK/DSDB  pin. In the illustrated embodiment, interpolation filters  202   a  and  202   b  are not utilized in DSD mode, as DSD data directly received from the SACD medium from media drive  103  of  FIG. 1 , are at a sufficiently high sampling rate without interpolation. In other words, in the DSD mode, the audio data are not passed through the interpolator. In the PCM mode, the data path including interpolation filter  202   a , delta-sigma DAC  203   a , and analog output filter  204   a , process left channel audio from the PCM stream received at the  SDATA/DSDA  pin in response to the  LRCK  clock signal received at the  LRCK/DSD   —   CLK  pin. Similarly, the data path including interpolation filter  202   b , delta-sigma DAC  203   b , and analog output filter  204   b  alternately process right channel data in the PCM stream received at the  SDATA/DSDA  pin in the PCM mode in response to LRCK signal. As stated earlier, DAC subsystem  101  receives the data serially through  SDATA/DSDA  pin timed by  SCLK  signal received through the  SCLK/DSDB  pin. 
   Generally, digital interpolation filters  202   a – 202   b  increase the sample rate of the corresponding data stream, as required in the PCM mode. Delta-sigma DACs  203   a  and  203   b  perform noise shaping on the digital data and generate corresponding analog data streams. Analog filters  204   a  and  204   b  perform low-pass filtering to remove noise above the audio passband. 
     FIG. 3  is a block diagram of representative clock signal generation—PCM/DSD mode detection circuitry  300  suitable for use in serial interface, clock generator, and pin mode control block  201  of  FIG. 2 . Clock generation/detection circuitry  300  includes a clock signal generator  301 , which receives an external clock signal, such as the external master clock ( EMCK ) signal shown in  FIG. 1 , and generates one or more internal clock signals, such as an internal master clock ( IMCK ) signal shown in  FIG. 3 . Generally, clock generator  301  generates the  IMCK  signal with a predetermined relationship with the  EMCK  signal under the control of control data  MCLK   —   DIV . In PCM audio applications, the  IMCK  signal has a frequency at least twice the  SCLK  signal frequency and has a selected oversampling ratio with respect to the frequency of the  LRCK  signal, for example 256×, 128×or 64×. Furthermore, the  IMCK  signal has an absolute frequency range dictated by the operating characteristics of the device-internal circuitry. 
   Control data  MCLK   —   DIV  (internal to mode detection circuitry  300 ) are generated by clock signal ratio and PCM/DSD detector  302  under the control of finite state machine  303 . Generally, while clock generator circuitry  301  cycles through the possible values of the  IMCK  signal, clock ratio detector  302  compares the externally generated  LRCK  signal, at the input sample rate, against each current  IMCK  provided through a clock buffer tree  304 . In turn, clock ratio detector  302  steps through values of the  MCLK   —   DiV  data until the desired  IMCK  to  LRCK  frequency ratio is obtained. 
   Absolute rate detector  305  monitors the absolute frequency of the  IMCK  signal and allows finite state machine  303  to maintain the absolute frequency of the  IMCK  signal within predetermined limits while the  IMCK  signal is varied to achieve the proper  IMCK  clock signal to  LRCK  signal frequency ratio. A clock retimer  306  retimes the  IMCK  signal as required for internal noise management. 
   As previously indicated, in the PCM mode, the  LRCK/DSD   —   CLK  pin receives the  LRCK  signal and during the DSD mode, the  LRCK/DSD CLK  pin receives the  DSD   —   CLK  signal. Typically, the frequency of the  LCLK  signal utilized in PCM audio systems to differentiate between multiple-bit samples of left and right channel data input received through the  SDATA  pin is smaller than the frequency of the  DSD   —   CLK  signal used in DSD audio systems to clock one-bit data samples through the  DSDA  and  DSDB  pins. For example, in current PCM audio systems, the frequency of the  LCLK  signal is at the audio sample rate, which is normally 192 kHz or less. For an associated  MCLK  signal having a frequency of 12.288 MHz, the  LRCK  signal frequency to  MCLK  signal frequency ratio for a 192 kHz clock signal is sixty-four (64). In contrast, in an SACD system operating on DSD data, the  DSD   —   CLK  signal frequency is typically 2.8 MHz. Therefore, in the DSD mode, the MCLK signal frequency to  DSD   —   CLK  clock signal frequency ratio is much smaller, in this case 12.288 MHz to 2.8 MHz, or approximately four (4) to one (1). 
   According to the principles of the present invention, clock signal ratio and PCM/DSD mode detect block  302  counts the number of periods of the  EMCK  signal received at the  EMCK  pin per period of the current signal presented at the  LRCK/DSD   —   CLK  pin. If the number of the periods of the  MCLK  signal per signal period detected at the  LRCK/DSD   —   CLK  pin is small, such as thirty-two (32) or less, then the signal received at the  LRCK/DSD   —   CLK  pin is treated as the  DSD   —   CLK  clock signal and DAC subsystem  101  of  FIG. 1  operates in the DSD mode. In this case, the signals received at the  SDATA/DSDA  and  SCLK/DSDB  pins are accordingly treated as the  DSDA  and  DSDB  data signals, respectively. On the other hand, if the number of  MCLK  signal periods per period of the signal received at the  LRCK/DSD   —   CLK  pin is greater than thirty-two (32), then the clock signal received at the  LRCK/DSD   —   CLK  pin is treated as the  LRCK  clock signal, the signal received at the  SDATA/DSDA  pin is treated as PCM  SDATA  data, and the signal received at the  SCLK/DSDB  pin is treated as the PCM  SCLK  clock signal. In this case, DAC subsystem  101  operates in the PCM mode. 
   In additional embodiments, detection of the current mode and the corresponding interpretation of the pin inputs is performed by observing the relationship between signals other than those received at the  LRCK/DSD   —   CLK  and  EMCK  pins. For example, in one particular alternate embodiment, the signals appearing at the  SCLK/DSDB  and  LRCK/DSD   —   CLK  pins are compared in frequency, and from the resulting frequency ratio, a determination is made as to whether the integrated circuit is in the PCM or DSD mode currently. 
   In sum, application of the principles of the present invention advantageously allow for a multiple-mode integrated circuit to be supported by a reduced or minimized number of pins. In turn, the overall device size becomes smaller, less expensive, and easier to utilize at the system level. In particular, these principles provide an efficient way of detecting the current operating mode by observing the characteristics of corresponding input signals received at selected input/output pins. Depending on the characteristics of the detected input signals, the integrated circuit enters the proper operating mode and the signals received at all of the corresponding pins are appropriately treated according to that mode. 
   While a particular embodiment of the invention has been shown and described, changes and modifications may be made therein without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.