Abstract:
A method of testing a proximity communication system for voltage margin by impressing a voltage upon the data link between the transmitter on one chip and the receiver on the other chip coupled to the transmitter through a capacitively coupling circuit formed by juxtaposed capacitor pads on the respective two chips. The impressed voltage is varied and the output of the receiver is monitored to determine an operational voltage margin. The floating inputs on the receiver may be continuously biased by connecting them to variable biasing supply voltages through high impedances. When the floating inputs are periodically refreshed to a refresh voltage during a quiescent data period, the refresh voltage is varied between successive refresh cycles. The variable test voltage may be applied to transmitter output when it is in a high-impedance state, and the output of the receiver is measured.

Description:
RELATED APPLICATION  
       [0001]    This application hereby claims priority under 35 U.S.C. §119 to U.S. Provisional Application No. 61/024,832, entitled “Voltage Margin Testing for Proximity Communication,” by Robert J. Drost, Ronald Ho, and Justin Schauer, filed 30 Jan. 2008 (Atty. Docket No.: SUN07-0098PSP). 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    The present invention relates to biasing techniques in integrated circuit (IC) chips. In particular, the invention relates to testing the error margin as a function of biasing in a system that supports proximity communication. 
         [0004]    2. Related Art 
         [0005]    Advances in semiconductor technology presently make it possible to integrate large-scale systems, including hundreds of millions of transistors, into a single semiconductor chip. Integrating such large-scale systems onto a single semiconductor chip increases the speed at which such systems can operate because signals between system components do not have to cross chip boundaries, and are not subject to lengthy chip-to-chip propagation delays. Moreover, integrating large-scale systems onto a single semiconductor chip significantly reduces production costs because fewer semiconductor chips are required to perform a given computational task. 
         [0006]    However, these semiconductor chips still need to communicate with each other, and unfortunately these advances in semiconductor technology have not been matched by corresponding advances in inter-chip communication technology. Semiconductor chips are typically integrated onto a printed circuit board that contains multiple layers of signal lines for inter-chip communication. However, signal lines are typically 100 to 1000 times denser on a semiconductor chip than on a printed circuit board. Consequently, only a tiny fraction of the signal lines on a semiconductor chip can be routed across the printed circuit board to other chips. This problem is creating a bottleneck that is expected to worsen as semiconductor integration densities continue to increase. 
         [0007]    One solution to the above problem is to replace the direct conductive coupling with direct chip-to-chip capacitive coupling, referred to as “proximity communication” (PxC). Proximity communication is an I/O technology that allows two chips in face-to-face alignment to communicate without wires as has been explained by Drost et al. in “Proximity Communication,”  IEEE Journal of Solid - State Circuits,  vol. 39, no. 9, September 2004, pp. 1529-1535. In the most widely used implementation, corresponding arrays of electrode plates or pads are formed in the opposing surfaces of the two chips, which are then fixed together with a dielectric layer in between to form a large number of capacitively coupled communication links between the chips. One embodiment of a capacitively coupled communication system is illustrated in the circuit diagram of  FIG. 1 . A first integrated circuit chip, here called a transmit chip  10 , includes a transmitter  12  as well as other integrated circuitry typical of a modern IC. A second integrated circuit chip, here called a receive chip  14 , includes a receiver  16  as well as other integrated circuitry which needs to be coupled to the circuitry of the transmit chip  10 . In this embodiment, the transmitter  12  and receiver  16  are differential, each having either two outputs or two inputs for usually complementary versions of the same signals. Conductive transmit pads  18 A,  18 B are formed in the surface of the transmit chip  10  and are connected to the differential outputs of the transmitter  12  receiving an input signal V IN  across its differential inputs. Similarly, conductive receive pads  20 A,  20 B are formed in the surface of the receive chip  14  in positions to be aligned with the transmit pads  18 A,  18 B of the transmit chip  10 . The receive pads  20 A,  20 B are connected to the differential inputs of the receiver  16  outputting an output signal V OUT , which should correspond to V IN . 
         [0008]    Typically the pads  18 A,  18 B,  20 A,  20 B are covered with thin dielectric layers as part of their formation process. To achieve a proximity communication system, the transmit and receive chips  10 ,  14  are permanently or semi-permanently juxtaposed with the transmit pads  18 A,  18 B aligned with respective ones of the receive pads  20 A,  20 B with a dielectric layer between them, thereby forming two capacitive coupling circuits  22 A,  22 B between the two chips  10 ,  14  for the differential signal to be coupled between them. 
         [0009]    However, the capacitive coupling circuits  22 A,  22 B provide DC isolation between the two chips  10 ,  14  and the high input impedance of the differential amplifier of the receiver  16  provides very little conductive discharge to ground. As a result, receiver nodes  26 A,  26 B receiving the capacitively coupled signals are floating relative to the transmit chip  10  and are not DC tied to the outputs of the transmitter  12 . As a result, the receiver nodes  26 A,  26 B may suffer DC wander due to leakage and the input offset voltage of the receiver  16  subtract from any signal coupled to it across the capacitive coupling circuits  22 A,  22 B. 
         [0010]    The signals coupled across a DC-isolated capacitively coupled channel can be very small, on the order of tens of millivolts. The input offset voltages in the differential amplifier associated with the receiver  16  used to receive the signals can approach the same order of signal level. Mitigating the effect of the receiver offset advantageously increases their sensitivity and performance. Some prior offset cancellation schemes add capacitance to sensitive nodes in the proximity channel to bias the nodes  26 A,  26 B toward desired voltage levels. Some prior art schemes dynamically refresh nodes  26 A,  26 B such that the nodes  26 A,  26 B are intermittently charged to desired voltage levels during a quiescent period of data transmission. 
       SUMMARY 
       [0011]    One embodiment provides a system that enables testing of voltage margin and other circuit characteristics of a proximity communication channel having capacitive coupling between transmit and receive chips and a receiver on the receive chip having high-impedance inputs such that the receiver inputs are effectively floating. 
         [0012]    In some embodiments, variable voltages are impressed on one or more nodes between the transmitter on the transmit chip and the receiver on the receive chip, and the output of the receiver is monitored as a function of the impressed voltage. 
         [0013]    In some embodiments, the output of the receiver is analyzed against a sequence of data signals applied to the transmitter to determine a bit error rate (BER) of the transmission as a function of the impressed voltages. 
         [0014]    In some embodiments, the impressed voltage or voltages are swept over a range of voltages around the nominal operating voltages. 
         [0015]    In some embodiments involving continuously biased floating receiver inputs, the receiver inputs are coupled to bias voltages through coupling elements such as large resistance and the bias voltages are swept. 
         [0016]    In some embodiments involving periodic refreshing of the floating inputs during cessation of data transmission, the refresh voltages during successive refresh cycles are swept. 
         [0017]    In some embodiments, during a quiescent data period, the transmitter outputs are put into high impedance states, for example, with tri-state inventers, and known voltages are applied to one or more output nodes at the transmitter and the receiver output is monitored. The voltages may be swept over a range bracketing the nominal output voltages of the inverters during data operation. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0018]      FIG. 1  is a schematic circuit diagram of a proximity communication channel. 
           [0019]      FIG. 2  is a schematic circuit diagram of a proximity communication channel in accordance with an embodiment of the invention utilizing continuous biasing of the floating receiver inputs. 
           [0020]      FIG. 3  is a schematic diagram of testing apparatus in accordance with an embodiment of the invention usable for testing the proximity communication channel of  FIG. 2 . 
           [0021]      FIG. 4  is a chart exhibiting a voltage margin measured for the proximity communication channel of  FIG. 2 . 
           [0022]      FIG. 5  is a schematic circuit diagram of a proximity communication channel in accordance with another embodiment of the invention utilizing refreshed biasing of the floating receiver inputs. 
           [0023]      FIG. 6  is a schematic diagram of testing apparatus in accordance with an embodiment of the invention usable for testing the proximity communication channel of  FIG. 5  in accordance with an embodiment. 
           [0024]      FIG. 7  is a schematic diagram of a tri-state inverter usable with the proximity communication channel of  FIG. 5 . 
       
    
    
       [0025]    In the figures, like reference numerals refer to the same figure elements. 
       DETAILED DESCRIPTION 
       [0026]    The following description is presented to enable any person skilled in the art to make and use the embodiments, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present disclosure. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
         [0027]    The degree of biasing of floating input nodes is largely built into the system. That is, the hardware elements associated with the biasing is part of the fabricated chips and the voltage supply levels used in biasing the input nodes are either predetermined or difficult to vary between chips of different designs. Furthermore, in fielded systems, the voltages of power supplies can vary during operation. However, the degree of biasing the input nodes partially determines the signals entering the circuitry through the receiver  16 . During chip development, it would be desirable to determine the margin for the biasing voltage outside of which the received signal becomes unacceptably degraded. The characterization of the operating voltage margin of communicated signals for proximity communication is critical during system bring-up to compare simulation and analytic models with measurements to aid the debugging. It would also be useful to monitor the margins during normal operation to anticipate and correct potential problems before they occur. If voltage margins cannot be accurately measured, it will be difficult to productize proximity communication. 
         [0028]    Schauer, Hopkins, and Drost have described one biasing method in U.S. patent application Ser. No. ______, filed ______, Docket SUN07-0119 and entitled “Offset Cancellation for DC Isolated Nodes.” As illustrated in the circuit diagram of  FIG. 2 , the transmit chip  10  includes two inverters  30 A,  30 B receiving complementary signals datap, datan. The inverters  30 A,  30 B may represent the output amplifiers of the differential transmitter  12  of  FIG. 1 . Their inverted outputs are coupled across the capacitive coupling circuits  22 A,  22 B respectively to the negative and positive inputs of a differential receiver  32  on the receive chip  14 . The differential receiver  32  in this representation has a single output Rxdata, which is the data signal used on the receive chip  14 . The receiver  32  can easily be generalized to a bipolar receiver having complementary outputs. The floating input nodes  26 A,  26 B are biased by being connected through large resistances  32 A,  32 B to respective bias supply voltages biasn, biasp. The high-impedance resistors  32 A,  32 B are preferably greater than 1 gigaohm (10 9  ohm) and less than 1000 gigaohm and may be formed by resistors, weak pass transistors, or leakage through parasitic leakage paths of off transistors. In operational chips, the power supply nodes supplying the bias supply voltages biasn, biasp are connected to two internal power buses which are considered to have fixed voltages. However, in practicing some aspects of the invention especially during system development, the nodes are externally accessible to provide variably controlled and known values of the bias supply voltages biasn, biasp. Alternatively, internal digital-to-analog converters may generate a range of bias supply voltages biasn, biasp based on digital commands issued to them. 
         [0029]    Voltage margins may be determined by connecting the PxC circuit of  FIG. 2  to a communication system analyzer  40  of  FIG. 3 . The analyzer produces a digital data stream of 1 and 0 data signals, usually pseudo-random but having a specified run limit or unbalance limit of 1s and 0s to prevent excessive signal charging of floating nodes. The analyzer  40  supplies directly or indirectly the transmit data signals datap, datan on the transmit chip  10 . The receiver  32  on the receive chip  14  receives through its floating input nodes  26 A,  26 B the signals corresponding to datap, datan and outputs the corresponding signal Rxdata. The Rxdata signal is coupled back to the analyzer  40 , which determines if the received signal Rxdata is the same as the data datap, datan it transmitted. The data are usually compared bit by bit. Any bit received as the wrong polarity is counted as an error. Over a long data stream, the analyzer determines a bit error rate BER. Communication systems are typically designed to accommodate a maximum bit error rate. If the BER is too high, the system is not working acceptably and must be redesigned or adjusted. 
         [0030]    The communication system analyzer or auxiliary equipment of this embodiment of the invention also provides the capability of controlling the biasing voltages produced by two DC power supplies  42 A,  42 B producing the bias supply voltages biasp, biasn of  FIG. 2 . According to an embodiment of the invention, the bit error rate is measured for different values of the bias supply voltages biasp, biasn to determine the voltage margin beyond which the BER exceeds a predetermined limit. During voltage margin testing, a differential voltage may be introduced by shifting biasp and biasn in opposite directions from their design values or by a mixed common-mode and differential voltage biasing in which only one of biasp or biasn is shifted. Shifting only one bias voltage is simpler to implement and can be acceptable if the bipolar receiver provides a high common-mode rejection ratio. 
         [0031]    It is understood that the invention can be practiced with equipment and methods other than the testing equipment of  FIG. 3 . The signal generation may be performed with an analyzer outputting a value of the BER while separate equipment is varying the bias voltages biasp, biasn and tracking the measured BER with changes in these voltages. 
         [0032]    Such a system was tested by offsetting one of the bias voltages from its nominal value in both directions in a PxC system operating at 1.60 Gb/s. The results from the test illustrated in the graph of  FIG. 4  demonstrate that, if a BER of 10 −14  is considered acceptable, the voltage margin from its nominal zero value is over 200 mV in both the positive and negative directions. If such a voltage margin is not acceptable, the system needs to be redesigned or adjusted. 
         [0033]    In an alternative method of correcting offset in receivers in proximity communication systems, the capacitively coupled input nodes float during data transmission but are periodically refreshed to a desired voltage while the data transmission is halted. In an embodiment of the invention, the refresh voltages are swept during testing to establish the voltage margin. As illustrated in the circuit diagram of  FIG. 5 , the input nodes  26 A,  26 B of a refreshed receiver  32  float during data transmission but, as previously described, are subject to voltage offset because they are capacitively coupled through the capacitive coupling circuits  22 A,  22 B to the signals driving them. However, periodically during a refresh cycle while data is not being transmitted, a receive refresh signal RxRefresh causes switches  50 A,  50 B to close, thereby connecting the input nodes  26 A,  26 B through a low resistance path to receiver refresh voltages RxRefn and RxRefp. The input nodes  26 A,  26 B quickly assume these voltages, and the receiver refresh signal RxRefresh is reversed to cause the switches  50 A,  50 B to open. Thereby the input nodes  26 A,  26 B are left floating initially at the refreshed voltages RxRefn and RxRefp. Data transmission then resumes and the input nodes  26 A,  26 B and the inputs of the receiver  32  are driven by the transmitter on the transmit chip  10 . The refreshing is repeated every refresh period, which is chosen to be often enough to prevent excessive offset from developing on the input nodes  26 A,  26 B but infrequently enough to prevent excessive reduction in effective bandwidth of the transmitted signal. In normal operation of data transmission, the receiver biasing voltages Rbiasn and Rbiasp are set to optimize the power and delay of the receiver  32  or alternatively to cancel the inherent offset voltages of the receiver. 
         [0034]    In one embodiment of the invention, the receiver refresh voltages are swept during testing to establish the voltage margin. However, in view of the periodic nature of the normal refreshing, a discrete voltage of desired shift is imposed on the refresh voltages RxRefn and RxRefp during each refresh cycle during the testing. The testing apparatus, illustrated in the block diagram of  FIG. 6 , is similar to that of  FIG. 3 , but the communication system analyzer  40  includes an additional output for the refresh signal RxRfresh. During the phase of testing to measure the bit error rate BER, the refresh signal RxRefresh is not enabled. However, during a refresh cycle, the refresh signal RxRefresh is enabled, and the communication system analyzer  40  controls the two DC voltage supplies  42 A,  42 B to produce the desired refresh voltages RxRefn and RxRefp for the next cycle of BER testing. If desired to accumulate sufficient data for dependable bit error rates, the same refresh voltages RxRefn and RxRefp may be imposed over multiple refresh periods. 
         [0035]    The refresh voltages RxRefn and RxRefp may be varied together in a differential or a common-mode variation or they may be varied individually for a single-ended test, analogously to the options with the continuously biased PxC system of  FIG. 2 . The sweep measures parameters, such as the offset voltage of the receiver and the amplitude of the input signal. 
         [0036]    If only receiver refreshing is being tested, the transmit chip  10  driving the receive chip  14  may be the transmit chip  10  illustrated in  FIG. 2 . However, in another aspect of the invention, the voltage margin of the transmitter may be tested. In another embodiment of the invention, as also illustrated in  FIG. 5 , the transmitter on the transmit chip  10  includes two tri-state inverters  52 A,  52 B having enable inputs en driven by a transmit refresh signal TxRefresh to select between the data states and the high-impedance state on the inverter outputs. The tri-state inverters have three output states, high and low voltage levels corresponding to the two data levels and a high-impedance state. When the output is in its high-impedance state and a voltage supply is externally connected to the inverter&#39;s output, the voltage on the output is determined by the voltage supply and is not significantly affected by whatever voltage is output by the tri-state inverter. Output nodes  54 A,  54 B positioned between the tri-state inverters  52 A,  52 B and the capacitive coupling circuits  22 A,  22 B are selectively connected to transmission refresh voltages TxRefn and TxRefp through switches  56 A,  56 B. The transmission refresh signal TxRefresh opens the switches  56 A,  56 B when it enables the tri-state inverters  52 A,  52 B to enable data transmission and closes the switches  56 A,  56 B when it disables the tri-state inverters  52 A,  52 B during the voltage testing. 
         [0037]    During the voltage testing, the transmission refresh voltages TxRefn and TxRefp are imposed on the transmit side of the capacitive coupling circuits  22 A,  22 B instead of the data signals datap and datan, and corresponding coupled signals are received by the differential receiver  32 . The receiver  32 , in turn, during testing outputs the output signal Rxdata based upon the difference between its input signals derived from the transmission refresh voltages TxRefn and TxRefp. It is understood that varying the refresh voltages TxRefn and TxRefp on the transmitter outputs includes varying the voltage to more levels than the two nominal data levels associated with the two data states, for example, to at least three levels. 
         [0038]    During normal operation outside of testing, the average voltages developed by the data stream on the output nodes  54 A,  54 B would be the average of the high and low signal voltages during data transmission, assuming balanced coding for the data. During testing, one or both of the transmit refresh voltages TxRefn and TxRefp are swept over a range of output voltages expected in normal operation of the tri-state inverters to analyze the voltage margin versus the amplitude of the transmit signals output by the tri-state inverters  52 A,  52 B. Similarly to the other voltage sweeps, the transmitter refresh voltages TxRefn and TxRefp may be swept together, either differentially or in common-mode variation, or may be separately swept. 
         [0039]    The switches  50 A,  50 B,  56 A,  56 B may be implemented using NMOS, PMOS or CMOS pass gates. The tri-state inverters  52 A,  52 B may be implemented, as illustrated in the circuit diagram of  FIG. 7 , with first and second PMOS transistors  60 ,  62  and first and second NMOS transistors  64 ,  66  serially arranged across their sources and drains between a positive power supply voltage  68  and ground. The input signal INto the tri-state inverter is applied to the gates of the first PMOS transistor  60  and the second NMOS transistor  66 . The enable signal Enable is applied to the gate of the second PMOS transistor  62  and through an inverter  68  to the gate of the first NMOS transistor  64 . The output signal Out of the tri-state inverter is taken from between the second PMOS transistor  62  and the first NMOS transistor  64 . As a result, in the presence of a low Enable signal, the second PMOS and NMOS transistors  62 ,  66  are turned on and the Out signal is the inverse of the In signal. However, in the presence of a high Enable signal, the second PMOS and NMOS transistors  62 ,  66  are turned off and the node associated with the Out signal assumes a high-impedance state. 
         [0040]    The voltage margin testing of the invention relies upon the pre-existing receiver on the receive chip  14  used in operational data transmission. As a result, the experimentally determined voltage margin is more accurate than if additional circuitry bypassing the receiver is used for testing. 
         [0041]    The foregoing descriptions of various embodiments have been presented only for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention.