Abstract:
A high speed signal transmission system employs differential receivers for receiving data signals transmitted over circuit transmission lines. One input each receiver is coupled to the output of a transmission line and to a termination network. The termination network generates a termination voltage and a source impedance that is matched to the characteristic impedance of the transmission line. The other input of the receiver is coupled to a reference voltage. The termination voltage may be adjusted by programming signals while keeping the source impedance constant and matched to the transmission line. A test mode may be employed where known data signals are transmitted and received and the termination voltage is adjusted while monitoring the states of the received signals on the output of the receivers. In this manner, the system may be optimized or tested for noise margin in an actual operation environment without resorting to probing methods. The clock signal used to time the transmission of the data signals is likewise transmitted along with its complement on two additional transmission lines. The clock signals are received in termination networks like the data signals. Additionally, the two clock signals are coupled to the reference signal with resistor/capacitor filter networks generating a low frequency tracking voltage superimposed on the reference voltage further improving noise margins.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates in general to circuits for transmitting digital signals between integrated circuits on circuit lines treated as transmission lines.  
         BACKGROUND INFORMATION  
         [0002]    Computer systems typically have the integrated circuits (ICs) interconnected on a motherboard. These ICs send signals between various circuit functions using drivers and receivers. On-chip circuits use drivers and receivers configured for the on-chip transmission lines which are typically very short but highly resistive and lossy. Off-chip drivers are used for signals transmitted between ICs. As off-chip communication speeds have increased, the lines interconnecting the ICs should be treated as transmission lines when their lengths are long relative to the fundamental wavelengths of the signals they transmit. Off-chip transmission lines are typically low-loss with characteristic impedances typically between 50 and 70 ohms.  
           [0003]    Off-chip drivers that have signal swings extending to the power supply rails require high currents relative to the currents for on-chip driver circuits. As off-chip driver speeds have increased so has the number of bits for the data buses used in off-chip communication. Most logic in the various ICs making up a computer system are synchronous, wherein a system clock is used to time when data is transmitted or applied to one end of a transmission line and when it is sampled by the receiver at the other end of the transmission line.  
           [0004]    Various types of noise may result from off-chip driving including simultaneous switching noise, electric and magnetic field coupling between signal transmission lines, etc. Likewise, the power supply noise resulting from large current spikes during simultaneous switching in an IC transmitting signals may be different from the power supply noise present at an IC receiving the signals. As system clocking speeds have increased, the power supply voltages have been decreased to manage power. The higher speed operation along with lower power supply voltages may lead to decreased noise margins. Some of the noise experienced in off-chip communication may be common mode where the noise appears simultaneously on both the voltage and ground lines (planes) of the motherboard. To improve the noise rejection when receiving off-chip signals, differential receivers are often used where the receivers are biased at a reference level (e.g., one half the power supply voltage).  
           [0005]    It is also advantageous to terminate the transmission lines interconnecting ICs in the characteristic impedance of the transmission line. While the transmission line may be series or source terminated, far end termination often provides the best overall performance at the expense of power dissipation especially when the transmission line network has multi-drop nets (e.g., a particular bit is coupled to more than one IC). All the above factors create a signal transmission environment which is difficult to optimize so as to ensure the best possible noise margins.  
           [0006]    There is, therefore, a need for signal transmission circuitry and a method for optimizing the noise margins in high speed digital signal transmission and reception.  
         SUMMARY OF THE INVENTION  
         [0007]    Embodiments of the present invention disclose a signal transmission system and method for communication between integrated circuits (ICs) where the clock rates are at a frequency where the signal lines must be considered as transmission lines. The signal transmission system uses data line drivers that are each far-end terminated in a termination network which is coupled to one input of a differential receiver; the second input of the differential receiver is coupled to a reference network. The termination network generates a termination voltage by dividing the receiver power supply voltage in a programmable voltage divider network that allows the reference voltage to be varied up and down while maintaining a constant termination impedance. The data is clocked at the driver by a clock signal whose output levels are a function of the driver power supply voltage. Likewise, the data signals have output levels that are functions of the driver power supply voltage. At the same clock time that the data signals are transmitted, a clock and an inverted clock are transmitted using line drivers like those for the data signals. The clock signals are each terminated in a termination network like the data lines. In addition, each clock signal is coupled through a resistor/capacitor filter network to an output node generating the reference voltage. Since one of the clock signals is always at a logic one, representative of the driver power supply, the output node of the clock termination network has a variation corresponding to the driver power supply voltage at the time of signal transmission. The data termination networks have programmable resistor dividers that allow the termination voltage to be varied under system control to optimize noise margins or to test the signal transmission network. The clock termination voltage has filtered variations of the driver and receiver power supply voltages coupled to it to allow the threshold level of the receivers to track changes in the driver power supply voltages to further optimize noise margins.  
           [0008]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    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:  
         [0010]    [0010]FIG. 1 is a block diagram of a signal transmission system according to embodiments of the present invention;  
         [0011]    [0011]FIG. 2 is a detailed circuit diagram of an embodiment of the present invention showing data and clock lines terminated at receivers;  
         [0012]    [0012]FIG. 3 is a detailed circuit diagram of a programmable terminator according to embodiments of the present invention;  
         [0013]    [0013]FIG. 4 is a table illustrating programming of the terminator of FIG. 3; and  
         [0014]    [0014]FIG. 5 is a block diagram of a data processing system suitable to practice embodiments of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0015]    In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. In other instances, well-known circuits have been shown in block diagram form in order not to obscure the present invention in unnecessary detail. For the most part, details concerning timing considerations and the like have been omitted in as much as such details are not necessary to obtain a complete understanding of the present invention and are within the skills of persons of ordinary skill in the relevant art.  
         [0016]    Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. In the following detailed descriptions, a logic zero is a low or zero voltage and a logic one is a high or a plus supply voltage to simplify explanation of embodiments of the present invention.  
         [0017]    [0017]FIG. 1 is a block diagram of a signal transmission system according to embodiments of the present invention. A data signal  101  is coupled to driver (DR)  103  which is “clocked” by clock (CLK) signal  116 . DR  103  is powered by power supply (PS)  102  and generates a data output (DO) signal  104  in response to clock edges of CLK signal  116 . DO signal  104  is transmitted by transmission line (TL)  105  to receiver (REC)  112  which is powered by PS  107  which has a return ground  127 . PS  102  and PS 107  may be the same potential but are differentiated because they may be remote from each other and may have different noise conditions. All the ground returns of PS  102  and PS  107  may not be shown or labeled in FIG. 1 to simplify the drawing. REC  112  generates receiver output  111  in response to the difference in the voltages received on it inputs  109  and  110 . In a synchronous system, REC  112  is “sampled” by edges of a clock synchronous with CLK  116 . Termination network (TN)  108  is used to match the impedance of the receiver to the characteristic impedance of TL  105 . TN  108  generates a termination voltage (VT)  109  and has a termination source impedance (ZT). TN  108  is programmed by program signals  106  to vary VT  109  while keeping ZT at a fixed value. ZT is not shown in FIG. 1 but is internal to the termination network  108 . Clock driver (CDR)  119  receives CLK signal  116  and generates a clock output (CLK_Out) signal  117  and a complement CLK_Out signal  118 . CLK_Out  117  is transmitted on TL  114  and received at output  120  which is coupled to reference network (RN)  113 . CLK_Out  118  is transmitted on TL  115  and received at output  121  which is also coupled to reference network (RN)  113 . RN  113  comprises TN  123  for TL  114  and TN  124  for TL  115 . The outputs of TN  123  and  124  are coupled to a filter network (FN)  125  which generates reference output  110 . RN  113  is programmed with program signals  122 . RN  113  generates an output  110  in response to the received CLK_Out  117  and  118  . Output  110  is coupled to REC  112  and generates a modified threshold voltage for REC  112 .  
         [0018]    A repetitive data pattern may be transmitted on a TL  105 . When a repetitive data pattern is transmitted and received, successive transitions between a logic one and a logic zero, after reception, may not happen at precisely the same time even though they may have been transmitted at the same time. This is due to the uncertainties in the transmission and detection during reception of the transmitted data in exemplary REC  112 . If one views a transition of a repetitive data pattern on an oscilloscope, an “eye” pattern would be apparent. This occurs as a particular observed transition in the repetitive data pattern varies around the ideal timing due to uncertainties. The eye pattern results as a larger number of transitions occur on either side of an ideal transition timing leaving an open area in the display.  
         [0019]    Using a repetitive data pattern, termination network  108  may be programmed with program signals  106  to vary VT  109  while keeping the termination impedance ZT constant. Per bit error registers maybe used in a system that employs signal transmission according to embodiments of the present invention to log failures that occur on received signals from output  111 . Stress tests may be exercised to determine failure margins by varying exemplary voltage VT  109  while monitoring the error rate of the output of exemplary REC  112 . Only one data line (e.g., DO  104 ) is shown in FIG. 1 to simplify the drawing. It is understood that embodiments of the present invention use multiple data lines with corresponding drivers, transmission lines, termination networks, program signals and receivers.  
         [0020]    [0020]FIG. 2 is a circuit diagram of multiple data signals (D  206 -D  208 ) with corresponding transmission lines (TL- 211 -TL  213 ) and receivers (REC  223 -REC  225 ), respectively. Nodes  230 - 232  are terminated in voltage divider termination networks TN  216 -TN  218 . A termination network (e.g., TN  216 ) is designed to have terminator voltage (V  230 ) determined by PS  107  and the divider ratio of resistors  228  and  229 . While the resistors in each of the dividers  216 - 220  are shown fixed resistors, dividers  216 - 220  may receive program inputs which allow characteristics of the dividers to be varied. The programming of dividers  216 - 220  is not shown in FIG. 2 for simplicity, but a divider may be like a programmable divider  108  as shown in FIG. 3.  
         [0021]    A differential receiver (e.g., REC  223 ) receives a reference voltage VREF  226  and generates an output in response to the difference in the voltages received on its inputs. Common mode noise (noise appearing of both the power supply  107  and ground  127  is reduced (rejected) by the differential characteristics of a differential receiver (e.g., REC  223 ). Clock driver DR  204  transmits CLK_Out  209  on TL  214  to TL output  233  which is terminated in TN  219 . Likewise, the complement clock CLK_NOut  210  is transmitted on TL  215  to TL output  234  which is terminated in TN  220 . TL output  233  is coupled to capacitor (C)  227  with resistor (R)  221  and TL output  234  is coupled to C  227  with R  222 . C  227  operates to low pass filter the signals on TL output  233  and TL output  234  to modify VREF  226 . C  227  maybe configured as two capacitors of one-half the value of C  227 , one from VREF  226  to ground  127  and one from VREF  226  to the power supply voltage  107 . VREF  226  will be a composite of the direct current (DC) levels on node  233  and  234  as well as the filtered alternating current (AC) signals caused by the dynamics of CLK_Out  209  and CLK_NOut  210 . Either CLK_Out  209  or CLK_NOut  210  is always at a logic one except for the times the two signals have concurrent signal transitions. R  221  and R  222  may be sized along with C  227  so that VREF  226  has a level that tracks the low frequency variations of PS  102  and PS  107 . These low frequency variations may be caused by distribution losses, simultaneous switching noise, or other noise sources. If these variations appear on both the clock signals and the data signals, then the receivers (e.g., REC  223 -REC  225 ) reduce the effects of the variations by their common mode rejection characteristics. Additionally, if TN  216 -TN  220  are programmable, then the terminating voltages on nodes  230 - 232  may be modified to optimize signal detection and improve noise margins.  
         [0022]    [0022]FIG. 3 is a circuit diagram of exemplary termination network TN  108  which is programmable with program signals  106  according to embodiments of the present invention. In one embodiment of the present invention, resistors R  325 -R  344  all have the same value. R  325 -R  329  and R  335 -R  339  are grouped in one circuit corresponding to their common program signal (PRS)  301 . If PRS  301  is a logic zero, then R  325 -R  329  are coupled in parallel to node  109  and PS  107 . Likewise if PRS  301  is a logic one, then R  335 -R  339  are coupled in parallel to node  109  and ground  127 . The resistors R  325 -R  334  and R  335 -R 344  are selected in a group of five, one, and two groups of two. This allows the selection of all groups of resistors in pairs (5,5), (6,4), (4,6), (7,3), (3,7), (8,2), (2,8), (9,1), (1,9) (10,0) and (0,10). The fact that the resistors are the same value allows the parallel combination of the resistors as “seen” at node  109  to always be a constant resistance. For example, if each resistor R 325 -R 344  is 500 ohms, then the circuit in FIG. 3 allows the voltage at node  109  to be varied from PS  107  to ground  127  in ten incremental steps. Likewise, the resistance as seen at node  109  is a constant 50 ohms, 500 ohms divided by 10. Ten is the number of resistors in each possible parallel grouping.  
         [0023]    It may be desirable to make the terminating networks out of groups of like resistors to improve matching. For the above example ten parallel 500 ohm resistors are used for the upper and lower branches for TN  108 . Other values of resistors may be used for other impedances by scaling the resistor values. If it is desired to use groups of like resistors to generate a number of N increments of the terminating voltage according to embodiments of the present invention, there is only one minimum number of M of program inputs  106  corresponding to each number N. For N=10 there are a minimum of four parallel networks with equivalent branch resistances of, 500 ohms, 250 ohms, 250 ohms and 100 ohms. If twelve increments of the terminating voltage are desired, while maintaining a terminating resistance of 50 ohms, then twelve 600 ohm resistors grouped in four parallel networks would be necessary with equivalent branch resistances of, 600 ohms, 300 ohms, 200 ohms and 100 ohms. Other increments may be used and still be withing the scope of the present invention.  
         [0024]    In FIG. 4, table  401  shows the number of parallel resistors in columns  402  (for ten increments) in the “UP” circuit (circuit between node  109  and PS  107 ) and the “DOWN” circuit (circuit between node  109  and ground  127 ). The columns  403  in table  401  list the states of the program signals P 1   301 , P 2   302 , P 3   303  and P 4   304  used to generate the variable termination conditions on node  109 . Columns  405 , in table  402 , list the resulting parallel resistances of the UP circuit and the DOWN circuit. The resulting terminator source resistance is shown in column  404 .  
         [0025]    [0025]FIG. 5 is a high level functional block diagram of a representative data processing system  500  suitable for practicing the principles of the present invention. Data processing system  500 , includes a central processing system (CPU)  510  operating in conjunction with a system bus  512 . System bus  512  operates in accordance with a standard bus protocol, such that as the ISA protocol, compatible with CPU  510 . CPU  510  operates in conjunction with electronically erasable programmable read-only memory (ROM)  516  and random access memory (RAM)  514 . Among other things, ROM  516  supports storage the Basic Input Output System (BIOS) data. RAM  514  includes, DRAM (Dynamic Random Access Memory) system memory and SRAM (Static Random Access Memory) external cache. I/O Adapter  518  allows for an interconnection between the devices on system bus  512  and external peripherals, such as mass storage devices (e.g., a hard drive, floppy drive or CD/ROM drive), or a printer  540 . A peripheral device  520  is, for example, coupled to a peripheral control interface (PCI) bus, and I/O adapter  518  therefore may be a PCI bus bridge. User interface adapter  522  couples various user input devices, such as a keyboard  524 , mouse  526 , touch pad  532  or speaker  528  to the processing devices onbus  512 . Display  538  which maybe, for example, a cathode ray tube (CRT), liquid crystal display (LCD) or similar conventional display units. Display adapter  536  may include, among other things, a conventional display controller and frame buffer memory. Data processing system  500  may be selectively coupled to a computer or telecommunications network  541  through communications adapter  534 . Communications adapter  534  may include, for example, a modem for connection to a telecom network and/or hardware and software for connecting to a computer network such as a local area network (LAN) or a wide area network (WAN). CPU  510  may comprise one or more processor ICs with one or more line drivers (e.g., DR  103 ) transmitting data (e.g., DATA  101 ) and clock signals (e.g., CLK_Out  117  and CLK_ONout  118 ) on motherboard transmission lines (e.g., TL  105 , TL  114  and TL  115 ) to supporting ICs that have one or more corresponding differential receivers (e.g., REC  112 ), termination network (e.g., TN  108 ) and a reference network (e.g., RN 113 ) while using embodiments of the present invention to optimize or test noise margins. Likewise, CPU  510  and other components of data processing system  500  may transmit data and clock signals on motherboard transmission lines (e.g. TL  105 , TL  114  and TL  115 ) to subsystems (e.g. RAM  514 ) which have one or more receivers (e.g., REC  112 ) coupled to termination networks(e.g, TN  108 ) according to embodiments of the present invention. CPU  510  may contain circuits for modifying a terminating voltage level (e.g., VT  109 ) of a termination network (e.g., TN  108 ) while maintaining a constant source impedance to set or test noise margin according to embodiments of the present invention.  
         [0026]    The present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.