Apparatus and method for multilevel signaling

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a multilevel signaling system for transmitting signals across a multiconductor transmission line.

2. Background

Various techniques and systems are available for transmitting data between a source and a destination. When data is electrically transmitted across a conductor, a particular signaling technology or protocol is utilized. A set of symbols may be associated with specific signaling technologies. The symbols are used to encode the data into various electrical patterns on the transmission line conductors such that each symbol may be distinguished from other signals by analyzing the electrical pattern on the conductors. The conductors used to transmit data include wires, cables, traces on printed circuit boards, conductors embedded within a substrate, and various other conductive materials.

In certain data transmission systems, the conductors are treated as transmission lines and analyzed by considering various electrical and electromagnetic wave properties and characteristics. In these systems, the signaling technology may include the coupling of signal drivers, signal receivers, conductors, and termination devices.

A particular type of data transmission system transmits signals differentially. For example, FIG. 1 illustrates a known differential transmission system using a pair of conductors. A differential driver 10 receives data on input 12 and transmits differential signals across conductors 14 and 16. Conductor 14 is coupled to the non-inverting output of driver 10 and conductor 16 is coupled to the inverting output of driver 10. A differential receiver 18 (also referred to as a differential comparator) receives the differential signals from conductors 14 and 16, and generates an output on line 20. Conductor 14 is coupled to the non-inverting input of receiver 18 and conductor 16 is coupled to the inverting input of receiver 18. A pair of terminating resistors 22, 24 are coupled between conductors 14 and 16, and a terminating voltage V.sub.term.

In the system of FIG. 1, the pair of conductors 14, 16 are capable of transmitting two symbols representing a binary zero or binary one. The data provided to driver 10 represents one of two possible symbols; e.g., a binary zero or one. Driver 10 transmits a particular signal pattern on conductors 14, 16 based on the input data provided to the driver. For example, when a binary zero is the input data, driver 10 generates a logic low signal on its non-inverting output which is coupled to conductor 14. Driver 10 also generates a logic high signal on its inverting output which is coupled to conductor 16. Conversely, when a binary one is the input data, driver 10 generates a logic high signal on its non-inverting output and generates a logic low signal on its inverting output. Thus, the polarity of the outputs from differential driver 10 are always opposite one another. The output polarity is controlled by the input signal applied to driver 10.

Differential driver 10 may be a current mode driver which produces output currents (i.sub.0 and i.sub.1) in response to the input provided. The value of v.sub.0 is defined as v.sub.0 =V.sub.term -i.sub.0 R.sub.t. Similarly, the value of v.sub.1 is defined as v.sub.1 =V.sub.term -i.sub.1 R.sub.t. Receiver 18 compares the voltage levels on its two inputs and generates the data output signal corresponding to the input provided to driver 10.

The differential signaling system illustrated in FIG. 1 requires two conductors 14, 16 to transmit a single bit of data. Therefore, this method results in an inefficient use of data interconnect resources (number of conductors =2.times.number of bits transmitted). Certain applications may require a more efficient use of interconnect resources in a differential transmission system. Thus, it is desirable to provide a system having the advantages provided by differential signaling, but without the inefficient ratio of the number of conductors to the number of bits transmitted.

SUMMARY OF THE INVENTION

The present invention provides a multilevel signaling system using multiple conductors for transmitting data from a source to a destination.

An embodiment of the present invention includes at least three conductors coupled between the transmission source and the transmission destination. Multiple drivers are coupled to the conductors at the transmission source. Multiple comparators are coupled to the conductors at the transmission destination. Each comparator is coupled to a pair of conductors.

Another feature of the invention provides that the drivers maintain a constant current on the multiple conductors. The constant current is maintained for all signal patterns transmitted along the conductors.

Each signal pattern generates a linear combination of eigenvectors. A particular embodiment of the invention utilizes linear combinations of equal speed eigenvectors.

Another aspect of the invention includes a first translator coupled to the drivers. The first translator generates control signals for controlling the drivers.

Additionally, a second translator may be coupled to the comparators. The second translator generates an output signal in response to the signals generated by the comparators.

A specific feature of the invention couples multiple comparator inputs such that an "n choose two" combinatorial matrix is generated.

A specific embodiment of the invention provides a substantially symmetrical arrangement of the multiple conductors.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example in the following drawings in which like references indicate similar elements. The following drawings disclose various embodiments of the present invention for purposes of illustration only and are not intended to limit the scope of the invention.

FIG. 1 illustrates a known differential transmission system using a pair of conductors.

FIG. 2 illustrates an embodiment of a system capable of performing multilevel signaling according to the present invention.

FIG. 3A illustrates an embodiment of a driver for use with a three-conductor transmission line.

FIG. 3B illustrates an embodiment of a detector for use with a three-conductor transmission line.

FIG. 4 illustrates another embodiment of the invention utilizing a pair of three-conductor transmission lines.

FIG. 5 illustrates an embodiment of a driver for use with a four-conductor transmission line.

FIG. 6 illustrates an embodiment of a detector for use with a four-conductor transmission line.

FIGS. 7A-7G illustrate various examples of terminations that may be used with a transmission system.

FIG. 8 is a flow diagram illustrating an embodiment of a procedure for defining a set of symbols transmitted by a particular transmission system.

FIG. 9 is a flow diagram illustrating an embodiment of a procedure for transmitting information from a source to a destination.

FIG. 10 illustrates an embodiment of the invention using a driver and a receiver to transmit signals across a three-conductor transmission line.

FIG. 11A is a side cross-sectional view of a printed circuit board having multiple conductors.

FIGS. 11B and 11C illustrate the capacitances between the multiple conductors shown in FIG. 11A.

FIG. 12 illustrates a side cross-sectional view of a specific arrangement of conductors in a printed circuit board.

FIG. 13 illustrates a capacitance model for the three-conductor transmission system shown in FIG. 12.

FIG. 14 illustrates an alternate symmetric arrangement of multiple conductors.

FIGS. 15A and 15B illustrate two embodiments of a symmetrical arrangement of four conductors.

DETAILED DESCRIPTION

The following detailed description sets forth numerous specific details to provide a thorough understanding of the invention. However, those skilled in the art will appreciate that the invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to obscure the invention.

The present invention is related to a multilevel signaling system that utilizes multiple transmission lines to transmit information from a source to a destination. Information is transmitted using symbols (or codes) formed from multiple signal levels. A signal level is the physical state of a conductor that can be determined by a detector coupled to the conductor. The symbols are defined such that the sum of the currents flowing on any group of conductors is constant for all symbols.

Two or more possible signal levels are carried by each conductor. These signal levels may, for example, be generated by different currents flowing through the conductors. Different symbols are transmitted across the multiple conductors by using a permutation of the signal levels. For example, three different current values i.sub.0, i.sub.1, and i.sub.2 may be available for any particular conductor. Based on the combination of current values provided on each conductor, a specific symbol is represented. For example, Table 1 below illustrates a symbol set for use with a three-conductor transmission system using three different current values. The transmission system maintains a constant current on the conductors by providing each of the current values on one of the conductors; i.e., current i.sub.0 is provided on one conductor, current i.sub.1 is provided on another conductor, and current i.sub.2 is provided on the remaining conductor. Therefore, the sum of the currents flowing on the conductors for any symbol is i.sub.0 +i.sub.1 +i.sub.2. The use of constant current on the signal conductors reduces ground bounce. Ground bounce is the shift in the ground reference voltage due to output switching. If a terminating voltage is used in a termination network, noise on the voltage in the termination network is reduced, thereby providing a better signal-to-noise ratio in the system.

TABLE 1 ______________________________________ Symbol b.sub.1 b.sub.0 Current ______________________________________ A 00 i.sub.0, i.sub.1, i.sub.2 B 01 i.sub.0, i.sub.2, i.sub.1 C 10 i.sub.1, i.sub.0, i.sub.2 D 11 i.sub.2, i.sub.0, i.sub.1 E i.sub.1, i.sub.2, i.sub.0 F i.sub.2, i.sub.1, i.sub.0 ______________________________________

As shown in Table 1, three conductors are capable of transmitting six different symbols. In contrast, the known differential transmission system shown in FIG. 1 uses two conductors to transmit two symbols. By adding one additional conductor, the present invention triples the number of symbols that may be transmitted. Thus, the present invention is more efficient with respect to the utilization of interconnection resources.

As shown in Table 1, six different symbols may be used with a three conductor differential transmission system. Although six different symbols are available, a particular embodiment of the three conductor transmission system may utilize only four of the symbols. The four different symbols may be used to transmit two bits of data (b.sub.1 and b.sub.0). In this embodiment, the remaining two symbols (E and F) are unused.

FIG. 2 illustrates an embodiment of a system capable of performing multilevel signaling according to the present invention. This system transmits information from a source to a destination across multiple conductors. A translator 100 is constructed using various logic devices to convert an input signal 108 into a driver control signal for controlling a driver 102. Input signal 108 may represent a symbol or other information to be transmitted from a source to a destination. Driver 102 is controlled by the driver control signal to generate m-ary (e.g., binary, trinary, and the like) signals on a transmission line 112 coupled to driver 102. For example, driver 102 may generate a trinary signal (m=3) on transmission line 112. The variable m identifies the number of possible signal levels on transmission line 112. Thus, a trinary signal has three possible signal levels. Additional details regarding the driver control signal and the generation of m-ary signals are discussed below.

A detector 104 (also referred to as a receiver) is coupled to transmission line 112 and receives the trinary signal generated by driver 102. Detector 104 detects which signal level is on each conductor in transmission line 112 and provides that signal level information to a translator 106. Translator 106 converts the signal level information into a destination code. Translator 106 generates an output 116 indicating the symbol transmitted on transmission line 112. Output 116 matches the input 108, thereby completing the transmission of the information from input 108 to output 116. Although not shown in FIG. 2, transmission line 112 may include a ground conductor (also referred to as a reference conductor).

Although the invention may be used with a transmission line having any number of conductors, specific embodiments of the invention will be discussed by way of example. Specifically, transmission systems using three signal conductors and four signal conductors are illustrated and described. Those of ordinary skill in the art will appreciate that an N+1conductor transmission line may include N signal conductors and one reference conductor. Thus, a four-conductor transmission line may include three signal conductors and one reference conductor.

FIG. 3A illustrates an embodiment of a driver for use with a transmission line having three signal conductors 112a, 112b, and 112c. Switches 118, 120, and 122 are coupled to current sources 124, 126, and 128, respectively. Each switch 118-122 is coupled to two of the three conductors. Thus, each switch 118-122 is capable of "steering" the current generated by the associated current source 124-128 to one of two conductors. For example, switch 118 steers the current generated by current source 124 to conductor 112a or conductor 112b. The position of each switch 118-122 is determined by a control signal (ControlA, ControlB, or ControlC) generated by translator 100 (shown in FIG. 2). The control signals are generated such that the condition ControlA=ControlB=ControlC never occurs, thereby avoiding the situation where all conductors receive a current of 1i. In the driver of FIG. 3A, one conductor receives current from two current sources (2i), another conductor receives current from one current source (1i), and the remaining conductor receives no current (0i). The various combinations of signal levels are shown in Table 3.

In an embodiment of the invention, each current source 124-128 generates a current i. Thus, depending on the position of switches 118-122, each conductor 112a-112c may carry 0i, 1i, or 2i. When switches 118-122 are in the positions shown in FIG. 3A, each conductor 112a-112c carries 1i. However, if the position of switch 118 is changed, then conductor 112a carries 2i, conductor 112b carries 0i (no current), and conductor 112c carries 1i. Thus, various combinations of currents may be generated on the conductors based on the position of switches 118-122. Note that the condition shown in FIG. 3A (each conductor carrying current 1i) is not actually used because the control signals are generated to avoid this condition.

In the example shown in FIG. 3A, three different signal levels (0i, 1i, and 2i) may be transmitted on each conductor. A set of symbols is created by selecting all permutations of signal levels such that each signal level is used at least once. The set of symbols is created such that the order of duplicate signal levels is not considered as a separate symbol. In this example, there are six permutations of the three signal levels, using each signal level once. The six permutations are illustrated below in Table 2.

TABLE 2 ______________________________________ Symbol Signal Levels Sum of Currents ______________________________________ A 2i, 1i, 0i 3i B 1i, 0i, 2i 3i C 2i, 0i, 1i 3i D 0i, 2i, 1i 3i E 1i, 2i, 0i 3i F 0i, 1i, 2i 3i ______________________________________

As shown in Table 2, the sum of all signal level currents for each symbol is constant (3i). Since each signal level is used at least once, the transmitted signal levels can be decoded by comparing voltages between all possible pairs of conductors. This comparison is performed by detectors 104 shown in FIG. 2, and discussed below with respect to FIG. 3B.

Table 2 above illustrates the conductor signal levels associated with each symbol A-F. Table 3 below illustrates the control signals generated by translator 100 to control drivers 102. Additionally, Table 3 illustrates the signals generated by detectors 104 in response to the conductor signal levels.

TABLE 3 ______________________________________ Code Control Signal Detector Code (Source) Signals Levels Symbol Output (Dest.) ______________________________________ 000 001 2i, 1i, 0i A 001 000 001 010 1i, 0i, 2i B 010 001 010 011 2i, 0i, 1i C 011 010 011 100 0i, 2i, 1i D 100 011 100 101 1i, 2i, 0i E 101 100 101 110 0i, 1i, 2i F 110 101 ______________________________________

The columns of Table 3 represent exemplary signals generated at different stages of a transmission system having three signal conductors. Symbols A-F correspond to a particular binary code, as illustrated in column 1. The code in column 1 is generated at the source and provided to the input of translator 100. Translator 100 then generates control signals for controlling the position of switches 118-122 shown in FIG. 3A. The switch positions are controlled such that the signal levels shown in column 3 are provided on the conductors 112a-112c. These signal levels are received by detector 104 and converted into signals corresponding to the control signals shown in column 2. The detector output is then converted by translator 106 into a destination code corresponding to the input code shown in column 1. Thus, the transmission system reproduces the source information at the destination.

FIG. 3B illustrates an embodiment of a detector for use with a transmission line having three signal conductors. Differential comparators 129, 130, and 131 are positioned between each possible pair of conductors 112a-112c. Each comparator 129-131 compares the signal levels on the conductors coupled to the comparator. Since comparators 129-131 determine the difference between two signal levels, a threshold voltage reference is not required. Instead, comparators 129-131 determine the difference between the two signal levels, thereby eliminating the need to determine the actual value or magnitude of the signal level on each conductor. By comparing the two signals, common-mode noise does not interfere with signal recovery because substantially the same noise signal is present on each conductor.

Based on the comparison of signal levels, each comparator 129-131 generates an output signal (labeled A, B, and C, respectively), used by translator 106 (FIG. 2) to generate the proper code or symbol corresponding to the conductor signal levels. The output signal is shown, for example, in column 5 of Table 3.

As illustrated in FIG. 3B, comparators 129-131 are coupled to conductors 112a-112c such that an "n choose two" combinatorial matrix is created; i.e., each possible combination of two conductors is coupled to one of the comparators. Thus, comparators 129-131 are coupled to the "n choose two" combinatorial matrix and perform "pairwise differential comparisons."

FIG. 4 illustrates another embodiment of the invention utilizing a pair of transmission lines, each having three conductors, for transmitting data between a source and a destination. Since two different transmission lines are used, each capable of transmitting six symbols, a total of 36 (6.times.6) symbols may be transmitted. The system of FIG. 4 includes a translator 132 coupled to receive five bits of data (in.sub.4 -in.sub.0). The five bits of data represent 32 different states, thereby using 32 of the 36 possible symbols. Translator 132 generates six different control signals, three of which are provided to a first driver 133 and the remaining three are provided to a second driver 134. Drivers 133 and 134 may be similar to those described above with reference to FIGS. 2, 3A, and 5. Drivers 133 and 134 generate output signals on multiple conductors that are coupled to receivers 135 and 136. As with the drivers, receivers 135 and 136 are similar to those discussed above in FIGS. 2, 3B, and 6. The outputs of receivers 135 and 136 are coupled to a translator 137.

In operation, receivers 135 and 136 generate output signals that are provided to translator 137. Translator 137 generates a five-bit output signal (out.sub.4 -out.sub.0) in response to the signals received from receivers 135 and 136. The five-bit output signal corresponds to the five bit input signal (in.sub.4 -in.sub.0) received by translator 132.

FIGS. 5 and 6 illustrate an embodiment of a driver and a detector for use in a transmission system having four signal conductors and three different signal levels. Table 4 below illustrates the twelve symbols available for use with a transmission system of the type shown in FIGS. 5 and 6. Each conductor may carry 0i, 1i or 2i, thereby creating twelve permutations of current values as shown in the third column of Table 4. The number of permutations is determined by the equation: ##EQU1## In the above equation, n is the number of conductors and p is the number of like-kind repeating symbols. In this example, n=4 (four conductors). Since three signal levels are used, one signal level must be repeated (thus, p=2 because there are two like-kind repeating symbols).

A particular implementation of a transmission system having four signal conductors may use the first eight symbols to transmit three bits of data (b.sub.2, b.sub.1, b.sub.0), leaving the remaining four symbols (I, J, K, and L) to transmit other information.

TABLE 4 ______________________________________ Symbol b.sub.2 b.sub.1 b.sub.0 Current ______________________________________ A 000 0i, 1i, 1i, 2i B 001 0i, 1i, 2i, 1i C 010 0i, 2i, 1i, 1i D 011 1i, 0i, 1i, 2i E 100 1i, 0i, 2i, 1i F 101 1i, 1i, 0i, 2i G 110 1i, 1i, 2i, 0i H 111 1i, 2i, 0i, 1i I 1i, 2i, 1i, 0i J 2i, 0i, 1i, 1i K 2i, 1i, 0i, 1i L 2i, 1i, 1i, 0i ______________________________________

Another embodiment of a transmission system having four signal conductors may select among four different current values (0i, 1i, 2i, and 3i) instead of three current values as discussed above. By adding a fourth current value, the number of available symbols which may be transmitted over four conductors is doubled to 24. Using the above equation: ##EQU2##

The additional symbols are provided because no signal levels are repeated. In this embodiment, each switch is capable of steering current to one of three different outputs. Thus, each current switch is coupled to three of the four conductors. Table 5 below illustrates the symbols and corresponding current values used in this embodiment.

TABLE 5 ______________________________________ Symbol Current Symbol Current ______________________________________ A 0i, 1i, 2i, 3i M 2i, 0i, 1i, 3i B 0i, 1i, 3i, 2i N 2i, 0i, 3i, 1i C 0i, 2i, 1i, 3i O 2i, 1i, 0i, 3i D 0i, 2i, 3i, 1i P 2i, 1i, 3i, 0i E 0i, 3i, 1i, 2i Q 2i, 3i, 0i, 1i F 0i, 3i, 2i, 1i R 2i, 3i, 1i, 0i G 1i, 0i, 2i, 3i S 3i, 0i, 1i, 2i H 1i, 0i, 3i, 2i T 3i, 0i, 2i, 1i I 1i, 2i, 0i, 3i U 3i, 1i, 0i, 2i J 1i, 2i, 3i, 0i V 3i, 1i, 2i, 0i K 1i, 3i, 0i, 2i W 3i, 2i, 0i, 1i L 1i, 3i, 2i, 0i X 3i, 2i, 1i, 0i ______________________________________

FIG. 5 illustrates an embodiment of driver 102 in a system using a transmission line 112 having four signal conductors 112a, 112b, 112c, and 112d. As discussed above, three different signal levels (0i, 1i, and 2i) are provided on the four conductors 112a-112d. Each signal level must be used at least once. Since three signal levels are used on four conductors, one signal level must be used on two conductors. In the embodiment of FIG. 5, four switches 138, 139, 140, and 142 are coupled between various pairs of conductors as shown. Each switch 138-142 is coupled to a current source 144, 146, 148, or 150, and "steers" current generated by the current source toward one of the two conductors coupled to the switch. Each switch 138-142 has a control input (labeled A-D) generated by translator 100 (FIG. 2) that controls the position of the switch.

The transmission systems described above use switches to "steer" current from current sources to the multiple conductors. In alternate embodiments, switches may be used to "steer" voltages onto the conductors. In this embodiment, a voltage driver switches one of three possible voltage values onto its output. As discussed above, a termination voltage (V.sub.term) is used in conjunction with the current mode drivers. However, this alternate embodiment does not require a termination voltage. Instead, the conductors may be terminated by coupling terminating resistors between each pair of conductors.

FIG. 6 illustrates an embodiment of detector 104 in a system using a transmission line 112 having four signal conductors 112a-112d. The detector shown in FIG. 6 may be used with driver 102 shown in FIG. 5. Detector 104 includes six comparators 152, 154, 156, 158, 160, and 162. Each comparator is coupled between a unique pair of conductors 112a-112d and generates a signal (OUT1-OUT6) based on a comparison of the signal levels on the pair of conductors. The operation of comparators 152-162 is similar to the operation of comparators 130-134 discussed above with respect to FIG. 3B.

Table 6 below illustrates the various codes, control signals, and signal levels at different stages of the transmission system.

TABLE 6 ______________________________________ Code Control Signal Detector Code (Source) Signals Levels Symbol Outputs (Dest.) ______________________________________ 0000 0100 0i, 1i, 1i, 2i A 010x01 0000 0001 1011 0i, 1i, 2i, 1i B 0x0011 0001 0010 1110 0i, 2i, 1i, 1i C 0001x1 0010 0011 0200 1i, 0i, 1i, 2i D x11001 0011 0100 0210 1i, 0i, 2i, 1i E 01101x 0100 0101 2100 1i, 1i, 0i, 2i F 11x101 0101 0110 2111 1i, 1i, 2i, 0i G 00x010 0110 0111 1120 1i, 2i, 0i, 1i H 10010x 0111 1000 1121 1i, 2i, 1i, 0i I x00110 1000 1001 2210 2i, 0i, 1i, 1i J 1100x0 1001 1010 2120 2i, 1i, 0i, 1i K 1x1100 1010 1011 2121 2i, 1i, 1i, 0i L 101x10 1011 ______________________________________

The columns of Table 6 represent example signals generated at different stages of a transmission system having four signal conductors. Symbols A-L correspond to a particular binary code illustrated in column 1. The code in column 1 is generated at the source and received by translator 100. Translator 100 then generates control signals for controlling the position of switches 138-142 shown in FIG. 5. The switch positions are controlled such that the signal levels shown in column 3 are provided on conductors 112a-112d. These signal levels are received by detector 104 and converted into signals as shown in column 5. The control signals are then converted by translator 106 into a destination code corresponding to the input code shown in column 1. Thus, the transmission system correctly reproduces the source information at the destination.

FIGS. 7A-7G illustrate examples of terminations that may be used with the transmission systems de