Abstract:
The serial interface operable, for example, to facilitate high speed differential data transfer between integrated circuits provides level shifting of an incoming data signal using a switched capacitor technique which level shifts the common mode voltage with minimal attenuation and minimal reduction of bandwidths. The serial interface also includes a DC offset correction loop of the input data receiver path. The level shifting circuit operates by sensing the incoming common mode voltage of a differential data signal with a resistor divider and sampling the difference between the measured input common mode voltage and desired input differential voltages generated by a differential DAC in the DC offset correction loop on two small capacitors. The small capacitors are switched across larger in-signal-path capacitors cyclically, so that over time a charge will build up to give the desired level shift to shift the common mode voltage of the incoming signal to the level tolerable by low voltage high speed transistors in the receiving integrated circuit.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is generally directed to a system and method for level translation in a serial data interface. More specifically, the present invention is directed to a system and method for providing voltage level translation in a serial data interface with minimal signal attenuation and bandwidth reduction. The subject system and method provide for translation of common mode voltage to enable safe differential serial data transfer between integrated circuits, even in high speed environments, without adverse effect upon the transferred signal. 
     With the many advances in the electronics industry, high speed differential serial data transfer between integrated circuits has become commonplace. Many of those same advances, however, require ever more stringent control over various operational parameters, not the least of which is the level of common mode voltage that certain low voltage devices may safely tolerate. Sensitivities to common mode voltage, the average voltage level of differentially paired signals, have heightened considerably with semiconductor processing technologies increasingly yielding thinner oxide devices. 
     Serial communication involves the process of sending data one bit at a time sequentially, over a communication channel or computer bus. Among other things, serial interfaces minimize the number of input/output pins on integrated circuits (IC&#39;s) and reduce the wiring interconnects required, thereby decreasing the cost of IC&#39;s. Examples of such low-cost serial interfaces used to reduce the number of pins in IC&#39;s, include SPI, I 2 C, and 1-Wire links. 
     A number of standards have been generated over the years to define data rates and signal levels carried by these serial links (buses). Such serial interface standards require DC coupling between integrated circuits to allow for indeterminately long strings of 1&#39;s and 0&#39;s. Notably, some of the standards reflect earlier technologies requiring higher supply voltages (for example, 1.8V, 2.5V, or 3.3V) than may be acceptable for certain technologies today. The thinner oxide transistor devices available now, especially for high speed applications, offer gate oxide thickness in the 90 nm, 65 nm, 45 nm range and below. These devices can safely tolerate common mode voltages Vcm up to about 1 V, which is significantly less than the Vcm levels in signals generated by earlier integrated circuit technologies. Interface measures are necessary to ensure that the Vcm levels applied to receiving circuits containing such thin oxide transistor devices are translated to sufficiently low levels that they do not cause harm. 
     Several approaches have been taken in the industry to address this problem of common mode voltage incompatibility. One often used approach is the inclusion of an AC coupling capacitor in each differential signal line, so that the non-varying portion of an incoming signal on the line is filtered out. An obvious drawback of this approach is that the coupling capacitors prevent the passage of DC voltage levels. An additional drawback is that it requires at least two external components for each pair of differential serial lines. Since the number of serial lines on an integrated circuit may be quite high, the number of external components may also become quite high, excessively so. 
     Another known approach to addressing the problem of common mode voltage incompatibility is to include in the receiver integrated circuit an input stage containing thicker oxide transistors having higher voltage tolerance. Such high voltage transistors, however, tend to be considerably slower than the thinner oxide transistors required for high speed applications. Receiver circuits integrated with these higher voltage input stages may not even operate when their serial buses receive very high speed data streams in the range, for instance, of 10 Gbit/sec. An additional drawback of this approach is that receiver circuits with the thicker oxide transistors are powered by higher supply voltages which lead to greater power dissipation. 
     Yet another known approach to addressing the common mode voltage incompatibility problem is to level shift the incoming signal toward ground using resistive means. This, however, adds undue power dissipation by the resistive elements. Another drawback is the bandwidth reduction due to the resistive elements interacting with the parasitics of the given load capacitance. Furthermore, the shifted voltage for the incoming voltage is determined necessarily by the input common mode voltage and for that reason cannot be well controlled. Finally, the resistive level shifting of the incoming common mode voltage means that the desired signal level is necessarily attenuated at input (by the resistive elements&#39; voltage divider ratio). Such signal attenuation is undesirable, especially in case of a long line, since the signal may already be greatly degraded as it is by the line channel. 
     There is therefore a need for an interface approach whereby compatibility between an incoming high level common mode voltage signal and a receiver stage with heightened sensitivity to such high voltage levels may be preserved, with minimal attenuation of the incoming signal and minimal reduction in signal bandwidth. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an interface for preserving compatibility between an incoming signal and a receiver stage with heightened sensitivity to the voltage levels of the incoming signal, with minimal attenuation of the incoming signal and minimal reduction in signal bandwidth. 
     It is another object of the present invention to provide an on-chip serial interface for high speed differential data transfer between circuits which accomplishes a level shifting of the incoming common mode voltage to a level tolerable to highly sensitive devices of the receiving circuit with minimal attenuation of the incoming signal and minimal reduction in signal bandwidth. 
     It is a further object of the present invention to provide a serial interface operable to reduce a common mode voltage of a differential input data signal to a level tolerable to thin oxide transistor devices of the receiving circuit by selective actuation of a switched capacitive segment disposed in each differential signal path. 
     These and other objects are attained in a system formed in accordance with the present invention for voltage level translation in a serial data interface between transmitting and receiving circuits. The system includes an interface unit defining at least one signal path to the receiving circuit for an incoming signal transmitted by the transmitting circuit. The system also includes a voltage sensing unit sensing an incoming voltage level of the incoming signal in the signal path, as well as at least one switched capacitive segment disposed in the signal path. The switched capacitive segment is selectively switched between alternative operational configurations to translate the incoming signal in voltage to a preselected compatible level for the receiving circuit. 
     The switched capacitive segment includes a first capacitive device disposed in the signal path, and a second capacitive device. The second capacitive device in one operational configuration is coupled in parallel to the first capacitive device, and in the other operational configuration is decoupled from the first capacitive device to be coupled between a first reference voltage node at the incoming signal level and a second voltage reference node offset by a predetermined voltage shift therefrom. The incoming signal is thereby translated in voltage by the predetermined voltage shift for safe transmission to the receiving circuit. 
     In certain exemplary embodiments of the present invention, the system&#39;s interface unit defines a differential pair of signal paths, and the system further comprises a pair of the switched capacitive segments, each disposed in one of the differentially paired signal paths. In certain applications of such exemplary embodiments, the second voltage reference node of each switched capacitive segment is coupled to a correction signal generated by a differential offset correction circuit. The predetermined voltage shift applied in each said signal path is then selectively set responsive to the differential offset correction circuit. 
     A method formed in accordance with the present invention for voltage level translation in a serial data interface between transmitting and receiving circuits includes establishing at least one signal path to the receiving circuit for an incoming signal transmitted by the transmitting circuit. The method further includes sensing an incoming voltage level of the incoming signal in the signal path, and providing at least one switched capacitive segment in such signal path. The method also includes selectively switching the switched capacitive segment between alternative operational configurations so as to translate the incoming signal in voltage to a preselected compatible level for the receiving circuit. 
     In each switched capacitive segment a first capacitive device is maintained in the signal path. In one operational configuration, a second capacitive device is selectively coupled in parallel to the first capacitive device, while in the other operational configuration the second capacitive device is coupled between a first reference voltage node at the incoming signal level and a second voltage reference node offset by a predetermined voltage shift therefrom. The second capacitive device in the other operational configuration is decoupled from said first capacitive device. The incoming signal is thereby translated in voltage by the predetermined voltage shift for transmission to the receiving circuit. 
     In certain exemplary embodiments of the present invention, a differential pair of the signal paths is defined. A pair of the switched capacitive segments is provided, with one switched capacitive segment being disposed in each of the differentially paired signal paths. In certain applications of such exemplary embodiments, the method provides for the second voltage reference node of each switched capacitive segment to be coupled to a correction signal generated by a differential offset correction circuit, such that the predetermined voltage shift in each said signal path is selectively set responsive thereto. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block-diagram of the transmission circuit illustrating a common mode voltage level translation interface in conjunction with an offset correction loop in accordance with one exemplary embodiment of the present invention; 
         FIG. 2  is a block diagram illustrating in greater detail certain portions of the embodiment illustrated in  FIG. 1 ; 
         FIGS. 3A-3B  are waveform diagrams illustrating the voltages established across respective capacitors of each switched capacitive segment in the embodiment of  FIG. 1  in one example of system operation; 
         FIGS. 4A-4B  are waveform diagrams illustrating non-overlapping clock signals of phase φ 1  and φ 2  for controlling switches in the embodiment of  FIG. 1 ; and, 
         FIG. 5  is a waveform diagram of a level translated common mode voltage obtained upon repeated cycles of system operation in the embodiment of  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In broad concept, the subject system and method provide a voltage level shifting circuit interface to preserve operational compatibility between transmitting and receiving integrated circuits. A system realized in accordance with one exemplary embodiment of the present invention employs a switched capacitor technique to translate the DC voltage level of an incoming signal in a controlled manner to a desired level. In the particular example of serial transmission of a signal through differentially paired lines, the system operates to level shift an incoming differential signal&#39;s common mode voltage to a desired level, and does so without appreciable attenuation of the signal “riding” on the incoming common mode voltage, or any reduction in the prevailing signal bandwidth. 
     In an exemplary embodiment, the system provides a serial interface for high speed differential data transfer between transmitting and receiving integrated circuits which may not otherwise be compatible in the common mode voltage levels they may safely support. The system provides the required shift of common mode voltage to a level safe for the receiving circuit. The shifted level is preferably defined by the outputs of a differential offset correction unit for the receiving circuit. 
     In accordance with one aspect of the present invention, the system ensures that the common mode voltage of the input is not high enough to harm any thin gate oxide transistor devices employed in the receiving circuit, yet is of sufficient level to reliably drive that circuit in the intended manner. The system employs switched capacitive means which effectively enable the passage of signal components having frequencies down to and even including DC. 
     Referring to  FIG. 1 , a system  10  formed in accordance with an exemplary embodiment of the present invention is disposed as an interface between a transmitting circuit  12  and a receiving circuit  14  to facilitate high speed serial differential data transfer therebetween. Preferably, the system  10  is formed on-chip, as part of a circuit unit or subsystem that is typically provided already with the given receiving circuit  14 . 
     For example, in circuits receiving signals through differentially paired lines, the difference of the lines&#39; output currents generally depend on the difference of their input voltages. If there is no difference in the input voltages, a null difference in the output currents should ideally occur, as a result. Due to non-ideal conditions, this is usually not the case in actual practice. Factors such as defects in component matching defects and the like leave the lines sufficiently imbalanced that the paired lines are invariably characterized by some non-zero DC offset voltage, the input voltage necessary for a null output to be generated. When this DC offset voltage is present on the signal lines, it is potentially amplified by direct coupled amplifiers, thereby distorting the signal being processed or even saturating the amplifiers&#39; output at their supply limits and rendering them inoperable. Consequently, a feedback differential offset correction loop or other such measures are often used to apply a suitable compensation voltage between the paired lines at the input. 
     System  10  is preferably incorporated in the illustrated embodiment as part of the differential offset correction unit  16  suitably employed with the given receiving circuit  14 . System  10  may be implemented on-chip with this differential offset correction unit  16 , so that the number of external off-chip components may be minimized. 
     As schematically shown in  FIG. 1 , system  10  includes a common mode voltage level translation interface unit  18  operably coupled to the differential offset correction unit  16 . Interface unit  18  includes a pair of switched capacitive segments  30   p ,  30   m  respectively disposed in the paired lines  36 ,  38 . Each switched capacitive segment  30   p ,  30   m , detailed in following paragraphs, operates responsive to an output of a digital-to-analog converter  20  employed within the differential offset correction unit  16  to impose a variable drop in voltage thereacross. The voltage levels of the signals on the paired lines  36 ,  38  will undergo a sufficient shift to present a translated common mode voltage VcmT at the input of the receiving circuit  14 . The switched capacitive segments  30   p ,  30   m  operate in this regard much as a battery varied in voltage responsive to the DAC  20 , as shown. 
     Level translation interface unit  18  further includes a common mode voltage sensing unit  24  to sense the common mode voltage of the incoming signal on lines  36 ,  38 . Unit  24  is formed in the embodiment shown as a voltage divider having balanced resistors  26 ,  28  of equal resistance R. Vcm In, the average voltage on the paired lines  36 ,  38 , is then tapped from the point between the resistors  26 ,  28 . 
     The switching in each of the switched capactive segments  30   p ,  30   m  is controlled by a controller  46 . Preferably, controller  46  generates phase-offset versions of a reference clock generated at block  48  to synchronize the switching of capacitive elements within the segments  30   p ,  30   m  in suitable manner. 
     Referring to more detailed schematic of  FIG. 2 , system  10  is configured in the embodiment shown for high speed differential data transfer between a transmitting circuit  12  and receiving circuit  14 . The transmitting circuit  12  may provide a differential digital bit stream having an input common mode voltage Vcm In at a level which exceeds the maximum voltage level that the receiving circuit  14  formed with low voltage high speed transistor devices may safely accommodate. 
     The switched capacitive segments  30   p ,  30   m  of interface unit  18  are disposed to established alternatively switched first and second configurations between the input lines  38 ,  36  and input terminals  88 ,  90 . Segment  30   p  includes a first capacitor  34  connected between input line  38  and an input terminal  88  of the receiving circuit  14 . Segment  30   p  also includes a second capacitor  42  which, in a first configuration selected responsive to a first clock signal Φ 1 , is coupled at nodes  60 ,  76  in parallel interconnection to the first capacitor  34  by switches  51 , S 4 . In a second configuration selected responsive to a second clock signal Φ 2 , the second capacitor  42  is decoupled from its parallel interconnection with the first capacitor  34  and alternatively coupled across nodes  58 ,  78  which respectively receive the sensed voltage Vcm In and a differential offset correction voltage DAC_P provided by DAC  20 . 
     Segment  30   m  likewise includes a first capacitor  32  connected between input line  36  and an input terminal  90  of the receiving circuit  14 . Segment  30   m  also includes a second capacitor  40  which, in a first configuration selected responsive to the first clock signal Φ 1 , is coupled at nodes  66 ,  70  in parallel interconnection to the first capacitor  32  by switches S 2 , S 3 . In a second configuration selected responsive to the second clock signal Φ 2 , the second capacitor  40  is decoupled from its parallel interconnection with the first capacitor  32  and alternatively coupled across nodes  64 ,  72  which respectively receive the sensed voltage Vcm In and a differential offset correction voltage DAC_M provided by DAC  20 . 
     The first capacitors  32 ,  34  of the switched capacitive segments  30   m ,  30   p  are of equal capacitance C 1  and C 2 , while the segments&#39; second capacitors  40 ,  42  are of equal capacitance C 2  and C 4 . The first capacitors  32 ,  34  are disposed in respective signal paths  36  and  38  of the transmission channel, while the second capacitors  40 ,  42  are disposed off the signal paths  36 ,  38 . Preferably, the first capacitance C 1 , C 2  in each of the segments  30   m ,  30   p  is considerably greater in value than the second capacitance C 3 , C 4 . 
     As mentioned, switches S 1 , S 2 , S 3 , and S 4 , best shown in  FIG. 2 , are controlled by unit  46  which includes a clock generator  48  which generates non-overlapping clock signals  50 ,  52  of phases Φ 1 , Φ 2 . Switch S 1  is preferably a high voltage switching device coupled to terminal  54  of the second capacitor  42  to be switchable responsive to clock signals  50 ,  52  between the high voltage node  58  and terminal  60  of the first capacitor  34  in segment  30   p . Switch S 2  is also a high voltage switching device, and is coupled to terminal  62  of the second capacitor  40  to be switchable responsive to clock signals  50 ,  52  between the high voltage node  64  and terminal  66  of the first capacitor  32  in segment  30   m.    
     Switch S 3  is a low voltage switching device coupled to terminal  68  of the second capacitor  40  to be switchable responsive to clock signals  50 ,  52  between terminal  70  of the first capacitor  32  and the desired referenced voltage node  72  in segment  30   m . The switch S 4  is also a low voltage switching device, and is coupled to terminal  74  of the second capacitor  42  to be switchable responsive to clock signals  50 ,  52  between terminal  76  of the first capacitor  34  and the desired reference voltage node  78  in segment  30   p.    
     The differential digital-to-analog converter (DAC)  20  of differential offset correction unit  16  generates at its paired outputs  82 ,  84  voltage signals DAC_P, DAC_M which not only serve offset compensation purposes for the receiving circuit  14 , but also define the common mode voltage level VcmT to which the incoming Vcm In is to be translated. Reference voltages DAC_P, DAC_M thus represent the sufficiently low voltage levels desired for safe delivery of signals to the differential receiving circuit  14 . 
     In operation, when a high speed differential input data signal  86  generated by the transmitting circuit  12 , two components of the signal, in_p and in_m ( FIG. 1 ) are applied on the differential lines  38 ,  36 . The signal components ride on the high incoming common mode voltage level Vcm In. Depending on the application, this may be at be at typical ranges of 1.8V-3.3V or even higher, which cannot be tolerated by the low voltage high speed thin oxide transistor devices  44  in the differential receiving circuit  14 . Level translating interface unit  18  operates as follows to the shift the high common mode voltage level Vcm In to a lower common mode voltage level VcmT and provide the needed compatibility. 
     Vcm In is sensed at the voltage divider sensor  24 . The sensed Vcm In is applied at the high voltage nodes  58 ,  64  of the switched capacitive segments  30   p ,  30   m , while the desired input voltage levels DAC_P, DAC_M provided by the differential DAC  20  are applied at the nodes  78 ,  72  of the segments  30   p ,  30   m . In this regard, if the outputs of the differential DAC  20  are equal, e.g., DAC_P=DAC_M, the equal voltage becomes the translated common mode voltage VcmT applied at the receiving circuit  14 . On the other hand, if the DAC output voltages DAC_P, DAC_M are not equal, the difference therebetween becomes the offset voltage and is added to the receiving circuit input terminals  88 ,  90  as part of the offset correction routine, their average voltage defining Vcm T. 
     With reference to  FIGS. 3A-3B  and  4 A- 4 B, the non-overlapping clock signals  50 ,  52  control the switches S 1 -S 4  to selectively set the switched capacitive segments  30   p ,  30   m  into their alternative first and second operational configurations. The first configuration establishes a sampling mode of operation, whereby the clock signal  52  of phase φ 2 , such as illustrated in  FIG. 4B , drives switches  51 , S 4 , and S 2 , S 3  to connect second capacitor  42  of segment  30   p  to the power nodes  58  and  78 , and second capacitor  40  of segment  30   m  to the power nodes  64  and  72 . Over repeated cycles of this sampling mode of operation, the voltage difference between Vcm In and DAC_P is ‘sampled’ by the second capacitor  42 , and the voltage difference between Vcm In and DAC_M is ‘sampled’ by the second capacitor  40 . That is, the second capacitor  42  is incrementally charged to the sampled voltage difference between Vcm In and DAC_P over repeated cycles of operation in the sampling mode, while the second capacitor  40  is incrementally charged in much the same manner to the sampled voltage difference between Vcm In and DAC_M. 
     The voltage across each of these second capacitors  40 ,  42  during this sampling mode of operation is represented by the waveform illustrated in  FIG. 3A . Because this process involves the redistribution of charge from one capacitive element to another through low impedance switching elements, rapid voltage transitions occur. The time scale for the illustrated waveform is such that the rapid voltage transitions appear as if to occur instantaneously, but one skilled in the art will readily recognize this to be due to compressed scaling, and that the transitions necessarily occur across the capacitive elements necessarily over a non-zero time period. 
     The second configuration establishes an accumulation mode of operation, whereby the clock signal  50  of phase φ 1 , such as illustrated in  FIG. 4A , drives switches S 1 -S 4  to connect the second capacitor  42  in parallel to the first capacitor  34  in capacitive segment  30   p , and the second capacitor  40  in parallel to the first capacitor  32  in capacitive segment  30   m . Preferably, the first capacitors  34 ,  32  are much greater in capacitance than the second capacitors  42 ,  40 , such that over successive cycles of accumulation mode operation, the charged second capacitors  42 ,  40  serve much like batteries. The first capacitors  34 ,  32  are thus charged until the voltages across them match the sampled voltages across their corresponding second capacitors  42 ,  40 . 
     The voltage across each of the first capacitors  34 ,  32  during this sampling mode of operation is represented by the waveform illustrated in  FIG. 3B . Again, the voltage transitions are sufficiently rapid that they appear seemingly instantaneous, though they actually occur over a non-zero time period. 
     The successive cycles of intermittent sampling and accumulation mode operations occur at a predetermined clock frequency, as illustrated in  FIGS. 3A-3B , in conjunction with  FIGS. 4A-4B . Once the voltage across each of the first capacitors  32 ,  34  settles to the sampled voltage value for the given signal line ( FIG. 3B ), the input common mode voltage is translated in each signal line by the line&#39;s sampled voltage amount. The receiving circuit  14  is presented at its input with signal lines at the desired common mode voltage VcmT defined by DAC_P and DAC_M. The receiving circuit  14  is then ready to safely receive data through the paired signal lines  36 ,  38 . In the example illustrated in  FIG. 5 , the incoming Vcm In of approximately 2V is translated to a desired common mode voltage level (of, for example, 0.6V in the example illustrated) which is tolerable to the low voltage high speed transistor devices in receiver circuit  14 . 
     Preferably, offset correction for common mode voltage VcmT may be effected by varying DAC_P and DAC_M levels, one relative the other. In the illustrated example of operation, the voltages across corresponding first capacitors  32 ,  34  of the segments  30   p  and  30   m , and across their corresponding second capacitors  40 ,  42 , are substantially identical because no offset correction is applied. The DAC_P and DAC_M levels in this case are substantially equal. Where these levels are varied to apply a measure of offset correction, the respective voltages of corresponding first and second capacitors would be accordingly offset. 
     When the first capacitors  34 ,  32  of the capacitive segments  30   p ,  30   m  become fully charged, they remain at their fully charged voltages and serve effectively as voltage level shifters. So long as the load (receiver circuit  14 ) is of sufficiently high impedance, even DC voltage levels may be shifted and passed to the load (with no current draw). That is, the common mode level may be reliably shifted and passed through to the receiver even for differential DC voltages levels. Serial data links which heretofore proved problematic for having to pass extensive strings of 1&#39;s and 0&#39;s may be suitably accommodated in accordance with the present invention. Data down to DC—or an infinite string of consecutive 1&#39;s or 0&#39;s—may be reliably passed. Although this invention has been described in connection with specific forms and embodiments thereof, it will be appreciated that various modifications other than those discussed above may be resorted to without departing from the spirit or scope of the invention as defined in the appended claims. For example, equivalent elements may be substituted for those specifically shown and described, certain features may be used independently of other features, and in certain cases, particular applications of elements may be reversed or interposed, all without departing from the spirit or scope of the invention as defined in the appended claims.