Abstract:
The present invention provides a method and apparatus to define and sustain such a physical level by connecting the output through a transmission gate to an input pin. For a certain state of the output, one level of an input may be fed through to the output to generate an output voltage level. In the preferred embodiment of the present invention, a chip select signal {overscore (CS)} is used to define a low level logic signal. An control logic selectively switches a high level logic signal voltage (e.g., V+supply voltage) or the low level logic signal voltage ({overscore (CS)}) to produce an output digital logic signal. In a further embodiment of the present invention, separate logic level signals IN H  and IN L  may be selectively switched by control logic to generate an output logic level signal independent of supply voltages V+ and V−.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The subject matter of the present application is related to that in U.S. patent application Ser. No. 09/466,835, filed Dec. 20, 1999, entitled “Techniques for Improving Signal to Noise Ratio in a Digital Filter using Spread Zeros” (Nanda), and U.S. patent application Ser. No. 09/521,675 filed concurrently herewith, entitled “Single Wire Interface for Analog-to-digital Converter” (Pastorello et al.) both of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to the field of semiconductor devices. In particular, the present invention relates to an apparatus for controlling the voltage level of a logic output in relation to the logic input voltage level. 
     BACKGROUND OF THE INVENTION 
     For a chip working in a multiple supply environment, the physical voltage level of an output pin may be different from every supply to the chip. For example, an analog-to-digital converter may be supplied by a so-called dual supply power supply with a V+of +3 Volts and a V− of −2 Volts. Alternately, other supply voltages may be used, such as a V+ of +5 Volts and a V− of 0 Volts. 
     Digital signal levels within such circuitry may have voltage levels different than supply voltage levels. Thus, in the examples given above, a high logic level may be 3 Volts and a low logic level may be 0 Volts. Logic level voltages may or may not correspond to input supply voltages. 
     As a result, there is no simple technique for generating appropriate and consistent voltage levels for output digital signals on such a chip. In addition, if a chip is designed to work with various voltage supply levels, generating consistent and accurate logic level signals relative to supply voltage levels may be somewhat difficult. 
     One intuitive approach to solving these problems is to provide an additional input reference voltage signal or signals through a corresponding separate input pin or pins representing one or more logic levels. However, such an approach requires one or more extra input pins for such reference voltage signals. 
     In semiconductor chip design, it is desirable to reduce the number of input, output, or voltage supply pins in order to reduce package size and reduce cost. Thus, using an additional pin or pins for reference voltage signals may not be an acceptable solution. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus to define and sustain such a physical level by connecting the output through a transmission gate to an input pin. For a certain state of the output, one level of an input may be fed through to the output to generate an output voltage level. 
     In the preferred embodiment of the present invention, a chip select signal {overscore (CS)} is used to define a low level logic signal. An control logic selectively switches a high level logic signal voltage (e.g., V+ supply voltage) or the low level logic signal voltage ({overscore (CS)}) to produce an output digital logic signal. 
     In a further embodiment of the present invention, separate logic level signals IN H  and IN L  may be selectively switched by a control logic to generate an output logic level signal independent of supply voltages V+ and V−. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a single channel differential input analog-to-digital converter. 
     FIG. 2 is a simplified schematic of some components of the single channel differential input analog-to-digital converter in an is alternative embodiment of the present invention. 
     FIG. 3 is a simplified schematic of the preferred embodiment of an output level generating circuit of the present invention. 
     FIG. 4 is a graph illustrating the relationship between logic threshold levels, supply voltage levels, and chip select signal {overscore (CS)}. 
     FIG. 5 is a graph illustrating the relationship between output logic threshold levels, supply voltage levels, and chip select signal {overscore (CS)}. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a block diagram of a single channel differential input analog-to-digital converter 100. In the diagram of FIG. 1, the rectangular border represents the boundary of the chip, while the white circles on that border represent input, output, or supply pins for the chip. FIG. 1 is taken from the CRYSTAL CS5510/11/12/13 data sheet, DS337PP2, September 1999, incorporated herein by reference in its entirety. 
     Both the analog differential signal inputs AIN− and AIN+ as well as reference input VREF are buffered through respective buffers  110  and  120  to reduce input current requirements. The analog outputs of buffers  110  and  120  are fed to a differential  4 th order delta-sigma modulator  130  which converts the analog input to produce a digital data stream to digital filter  140 . Such delta-sigma modulators are discussed, for example, in Leung et al., U.S. Pat. No. 5,719, 573, incorporated herein by reference. 
     Digital filter  140  may comprise, for example, a spread zero filter to convert single bits to a multiple bit representation, reducing the word rate in the process. An example of such a spread zero filter is disclosed, for example, in copending application Ser. No. 09/466,835 entitled “Techniques for Improving Signal to Noise Ratio in a Digital Filter using Spread Zeros” (Nanda) filed on Dec. 20, 1999 and incorporated herein by reference. 
     Output from digital filter  140  may then be fed to output control logic  150 . Output control logic  150  may output digital data through a simple serial output line SDO in response to chip select signal {overscore (CS)}. When chip select signal {overscore (CS)} goes low, output control logic  150  outputs digital data. When signal {overscore (CS)} goes high, the output SDO may be tri-stated. 
     Depending upon application, either SCLK, {overscore (CS)} or a combination of signals may be used to control data output. For example, in some applications, signal {overscore (CS)} may be held low, enabling the chip at all times and data output controlled by activating or deactivating signal SCLK. In other applications (particularly where signal SCLK is used as a system source clock) signal SCLK may be continually activated and data flow controlled by toggling signal {overscore (CS)}. In yet other applications, control of both signals {overscore (CS)} and SCLK may be used to control data flow from single channel differential input analog-to-digital converter  100 . 
     Signal SCLK is a clock signal used to shift data out from output control logic  150 . Clock generator  160  may be used to generate a clock signal for single channel differential input analog-to-digital converter  100  based upon either a signal from oscillator  170  or from signal SCLK. This internal clock signal may be used, for example, to clock differential  4   th  order delta-sigma modulator  130 , as well as clocking data within the device. 
     As illustrated in FIG. 1, oscillator  170  may be used as a source clock signal in one version of single channel differential input analog-to-digital converter  100  (e.g., part numbers CS5511/13) whereas SCLK may be used as a source clock signal in another version of single channel differential input analog-to-digital converter  100  (e.g., part numbers CS5510/12). Different versions of the same chip may be enabled as a so-called bond-wire option The actual semiconductor chip for all four parts (CS5510/11/12/13) may be substantially or completely identical. However, depending on how the chip is wire-bonded to the die (packaging) may determine whether the resultant device uses SCLK or internal oscillator  170  as a clock source. A corresponding pad within the chip may be tied to supply voltage or ground to enable one mode or the other. 
     Supply voltages V+ and V− may take respective ones of a number of predetermined voltage levels, depending upon application. For example, in one application, V+ may be +5 Volts and V− may be ground. In another application, V+ may be +3 Volts and V− may take on a value of −2 Volts. In still another application, V+ may be +2.5 Volts and V− may be −2.5 Volts. Other voltage levels are possible within the spirit and scope of the present invention. In general, there may be a predetermined spread between these voltage levels (e.g., 5 Volts difference). 
     Output logic levels for serial data SDO, on the other hand, may have high and low logic levels independent of supply voltages V+ and V−. For example, in one environment, a low logic level may be defined as 2 Volts, whereas a high logic level may be defined as 5 Volts. In another application, low logic level may be defined as 0 Volts, and a high logic level as 5 Volts. In yet another case, the low logic level may be 1 Volt and the high logic level may be 4 Volts. All of these possible logic levels may or may not correspond to input supply voltage levels. Thus, it is desirable to be able to generate correct output logic levels which is consistent regardless of supply voltage levels. 
     In the example of high logic level of 4 Volts and low logic level of 1 Volt, neither of the example power supply voltage levels (+5 Volts/0 Volts or +3 Volts/−2 Volts) may supply the correct logic levels. 
     FIG. 3 is a simplified schematic of a preferred embodiment of an output level generating circuit of the present invention. In the embodiment of FIG. 3, a desired output level for a high logic level signal may be equal to supply voltage V+ (e.g., 5 Volts) whereas the low level logic signal may take some other value unequal to either of the supply voltages V+ or V−. 
     Signal {overscore (CS)} may take one of two values. When signal {overscore (CS)} is high, the device is de-selected (hence the term “chip select”) and the output of signal SDO may be tri-stated. When signal {overscore (CS)} is low, the device is selected (enabled). In the application of FIG. 3, the low value for signal {overscore (CS)} may be utilized as the low value for output data for signal SDO. 
     Control logic  330  represents an abstraction of the output control logic  150  of FIG.  1 . As may be readily appreciated by one of ordinary skill in the art, the present invention may be applied to circuits other than analog-to-digital converters. Thus, control logic  320  may represent output control logic  150  of FIG. 1, or some other type of digital signal generator. 
     Control logic  330  may generate a high level logic signal for serial data output SDO by activating P-type MOSFET  310 . When P-type MOSFET  310  is activated, serial data output signal SDO becomes equal to voltage V+, minus any voltage drop across P-type MOSFET  310  (e.g., 0.6 Volts). The resulting voltage is defined as the high logic level signal in this instance. A P-type MOSFET is selected in this particular embodiment, as the relatively high voltage from V+ results in a low voltage drop (e.g., 0.6 Volts) across p-type MOSFET  310  (as opposed to an n-type MOSFET). 
     Control logic  330  may generate a low level logic signal for serial data output SDO by activating T-gate  320 . When T-gate  320  is activated, serial data output signal SDO becomes equal to the voltage of signal {overscore (CS)} (chip select), minus any voltage drop (e.g., 0.6 Volts) across T-gate  320 . The resulting voltage is defined as the low logic level signal in this instance. 
     A T-gate is selected in this particular embodiment, as the voltage levels of signal {overscore (CS)} may vary from application to application. If {overscore (CS)} has a relatively high voltage level, a p-type device may be appropriate to provide a low voltage drop (e.g., 0.6 Volts). When {overscore (CS)} has a relatively low voltage level, an n-type device may be appropriate to prove a low voltage drop. Thus, selecting a p-Type MOSFET or n-Type MOSFET may or may not be suitable, depending on the voltage range of {overscore (CS)}. A T-gate, on the other hand, will provide a low voltage drop for all applications. 
     A T-gate may also be used in place of P-type MOSFET  310  without departing from the spirit and scope of the present invention. However, since voltage V+ in this application will always be relatively high, a p-type MOSFET will be more than suitable and moreover less expensive (in terms of transistor count) to implement. 
     FIG. 2 is a simplified schematic of some components of the single channel differential input analog-to-digital converter in an alternative embodiment of the present invention. Again, as in the diagram of FIG. 1, the rectangular border  200  represents the boundary of the chip, while the white circles on that border represent input, output, or supply pins for the chip. 
     Whereas the embodiment of FIG. 3 utilized only one input value ({overscore (CS)}) to generate an output logic level, the apparatus of FIG. 2 generalizes the basic concept to a situation where two input signal values are used to generate both high and low logic level signals. Signal IN L  may represent a low logic level signal generated by voltage source  210  (e.g., {overscore (CS)}) Signal IN H  may represent a high level logic signal generated by voltage source  220 . Signals IN L  and IN H  may have voltage levels different from supply voltage V+ and V−. 
     Control Logic  250  may be analogous to output control logic  150  of FIG. 1, or may a general logic signal generating circuit. T-gates  230  and  240  may be used to selectively switch input signals IN H  and IN L , respectively as output signal OUT. As in FIG. 3, one or both of T-gates  230  and  240  may be substituted by an appropriate N-type or P-type MOSFET, if the expected voltage range of IN H  or IN L  allows such an economy in design. 
     Note also that the embodiment of FIG. 2 is a generalization of the invention for a situation where both high and low logic levels are not defined by V+ and V−. In a similar manner to the embodiment of FIG. 3, a further alternative embodiment may be provided where a low level logic signal is defined by supply voltage V− and the high level logic signal defined by an input voltage 
     FIGS. 4 and 5 illustrate the potential relationships between logic levels and supply voltages, as well as logic level thresholds for the preferred embodiment of the present invention as illustrated in FIG.  3 . FIG. 4 is a graph illustrating the relationship between input logic threshold levels, supply voltage levels, and chip select signal {overscore (CS)}. In FIG. 4, signal waveform  410  represents a digital logic signal having high and low logic levels as well as transitions between high and low logic levels. Such input signal waveforms may include, for example, chip select signal {overscore (CS)} and data clocking signal SCLK. 
     As illustrated in FIG. 4, the high and low logic levels may be independent of supply voltages V+ and V−. In particular, a low logic level may be considered equivalent to {overscore (CS)}, as this signal is a low level logic signal generated from the environment in which the device is operating. Threshold values for input signals are shown as V IH  and V IL , which are determined as a function of supply voltage levels. An input above V IH  will be interpreted as a high logic level. If an input signal is below V IL , it will be interpreted as a low logic level signal. 
     In the example of FIG. 4, V IH  may be set as supply voltage V+ minus a constant value of 0.45 Volts. V IL  may be set as half of the difference between supply voltages V+ and V−, plus a constant 0.6 Volts, minus supply voltage V−. Table I illustrates the resultant input threshold voltage limits for a number of example supply voltages V+ and V−. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 V+ 
                 V− 
                 V IH   
                 V IL   
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 +5 
                   
                 0 
                   
                 4.55 
                 V 
                 3.1 
                 V 
               
               
                   
                 +2.5 
                   
                 −2.5 
                   
                 2.05 
                 V 
                 0.6 
                 V 
               
               
                   
                 +3 
                 V 
                 −2 
                 V 
                 2.55 
                 V 
                 1.1 
                 V 
               
               
                   
                 +4 
                 V 
                 −1 
                 V 
                 3.55 
                   
                 2.1 
                 V 
               
               
                   
                   
               
             
          
         
       
     
     Note that regardless of supply voltages V+ and V−, the threshold levels may be automatically adjusted to compensate for variations in supply voltage. Thus, regardless of logic levels within the operating environment (within a reasonable range), the device will properly interpret logic high and low levels based upon input thresholds calculated from supply voltage levels. The formula values illustrated in FIG. 4 are selected to allow proper high and low level logic level determination. 
     FIG. 5 is a graph illustrating the relationship between output logic threshold levels, supply voltage levels, input logic level V IL , and chip select signal {overscore (CS)}. As in FIG. 4, signal waveform  510  represents a digital logic signal having high and low logic levels as well as transitions between high and low logic levels. 
     As illustrated in FIG. 5, the high and low logic levels may be independent of supply voltages V+ and V−. In particular, a low logic level may be considered roughly equivalent to {overscore (CS)}, as this signal is a low level logic signal generated from the environment in which the device is operating. Threshold values for output signals are illustrated as V OH  and V OL . In this instance, V OH  is dependent on V+ and V OL  a function of {overscore (CS)} (See FIG.  3 ). An output above V OH  will be interpreted as a high logic level. If an output signal is below V OL , it will be interpreted as a low logic level signal. 
     In the example of FIG. 5, V OH  may be set as supply voltage V+ minus a constant value of 0.6 Volts. V OL  may be set equal to the value of {overscore (CS)} plus a constant 0.6 Volts. Table II illustrates the resultant output threshold voltage limits for a number of example supply voltages V+ and V−. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE II 
               
               
                   
                   
               
               
                   
                 V+ 
                 V− 
                 V OH   
                 V OL   
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 +5 
                   
                 0 
                   
                 4.4 
                   
                 {overscore (CS)} + 0.6 V 
               
               
                   
                 +2.5 
                   
                 −2.5 
                   
                 1.9 
                 V 
                 {overscore (CS)} + 0.6 V 
               
               
                   
                 +3 
                 V 
                 −2 
                 V 
                 2.4 
                 V 
                 {overscore (CS)} + 0.6 V 
               
               
                   
                 +4 
                 V 
                 −1 
                 V 
                 3.4 
                   
                 {overscore (CS)} + 0.6 V 
               
               
                   
                   
               
             
          
         
       
     
     Note that regardless of supply voltage V+, the high logic level threshold V OH  may be automatically adjusted to compensate for variations in supply voltage. On the other hand, output supply voltage threshold level V OL  fluctuates as a function of signal {overscore (CS)}. In the formulas of FIG. 4, the voltage constant 0.6 Volts represents the voltage drop across a MOSFET or T-gate. 
     It should also be noted that the equations set forth in FIG.  4  and Table II may also be suitably altered for an embodiment where both high and low level logic signals are referenced from system sources IN H  and IN L  as in FIG.  2 . In such an embodiment, threshold voltage levels for both high and low levels of the logic signal may be made a function of input values IN H  and IN L . 
     While the preferred embodiment and various alternative embodiments of the invention have been disclosed and described in detail herein, it may be apparent to those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope thereof. 
     For example, while illustrated in the context of an analog-to-digital converter, the present invention. may also be applied to other circuits where one or more of input or output logic levels may be independent of supply voltage levels V+ and V−.