# Patent

## Title
Apparatus and method for detecting a fault condition in a common-mode signal

## Abstract

The invention is directed to an apparatus and a method for generating a fault detection signal when a differential signal is in a fault condition. The fault condition arises when the data transmission path in a differential signaling device is either open, shorted, or terminated by an abnormal means, and is such that the inputs are within the valid common-mode range and a valid differential signal cannot be obtained. The invention is buffered from the differential signal source, and an intermediate signal is produced in response to the differential signal. Portions of the intermediate signal are compared to a reference signal, and based on the comparisons, fault condition control signals are produced. A fault detection signal is produced when two fault condition control signals indicate the presence of a fault. The fault detection signal is made available for invocation of a failsafe state.

## Background
FIELD OF THE INVENTION
The present invention relates to an apparatus and a method for detecting a fault condition in a common-mode signal, and generating a fault detection signal. The fault detection signal can be used by another device to activate a failsafe condition.
BACKGROUND OF THE INVENTION
Differential signaling has been in existence for many years. For example, teletypes were some of the first equipment to use differential signaling to communicate. One common technique of differential signaling utilizes a current loop to send and receive information between a receiver and transmitter.
According to this differential signaling technique, current is sent in one direction around the loop, or the other direction around the loop, to represent information. A pulse of current in the loop in one direction may correspond to a logic value of “1”, while a pulse in the opposite direction may correspond to a logic value of “0.”
Current loops have several advantages over other signaling techniques. For example, data sent utilizing a current loop can travel further than data sent through a common RS-232 interface. Current loop differential signaling techniques also provide protection against electrical interference. Additionally, current loop differential signaling techniques can reliably make connections when other communication techniques cannot.
One type of a current loop differential signaling technique is Low Voltage Differential Signaling (LVDS). LVDS is a differential signaling technique commonly used in data transmission systems. LVDS uses relatively low supply voltages; VDD is generally in the range of 2.5 volts and VSS is generally zero. The valid common-mode range for a LVDS receiver is generally between VSS+50 mV and VDD−50 mV. A low voltage differential signal produced by a line driver typically has peak-to-peak amplitudes in the range from 250 mV to 450 mV. The low voltage swing minimizes power dissipation, while maintaining high transmission speeds. Typical transmission speeds are over 100 Mbps (Mega-bits per second).
SUMMARY OF THE INVENTION
In accordance with the invention, the above and other problems are solved by an apparatus and method for detecting a fault condition in a differential signal and generating a fault detection signal. A fault condition prevents a valid differential signal from being obtained. A fault condition can arise when the data transmission path in a differential signaling device is either open, shorted, or terminated by an abnormal means. The present invention is directed to providing an apparatus and a method for generating a fault detection signal for those fault conditions where the differential signal remains within the proper common-mode range and a valid LVDS signal cannot be obtained.
According to one example of the invention, the invention is buffered from the differential signal source, and an intermediate signal is produced in response to the differential signal. Portions of the intermediate signal are compared to a reference signal. Fault condition signals are produced when amplitude differences between portions of the buffered signal and the reference signal meet predetermined criteria. A fault detection signal is produced when fault condition signals are produced from comparisons from two portions of the buffered signal. The fault detection signal is available to a device for activation of a failsafe state.
According to another example of the invention, the fault detection occurs without loading, altering, or disturbing the differential signal source.
According to another example of the invention, AC components of the intermediate signal are integrated to eliminate rapid signal swings and provide the comparators with a relatively stable input. By integrating the intermediate signal components, the comparators have sufficient time to perform their function without their DC voltages falling prematurely.
According to a further example of the invention, the presence of a fault condition is signaled by a high control signal.
A more complete appreciation of the present invention and its improvements can be obtained by reference to the accompanying drawings, which are briefly summarized below, to the following detailed description of presently preferred embodiments of the invention, and to the appended claims.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a LVDS driver and receiver;
FIG. 2 is a graph generally illustrating LVDS signal swing;
FIG. 3 is a block diagram generally illustrating an exemplary operating environment;
FIG. 4 is a block diagram generally illustrating an exemplary apparatus overview;
FIG. 5 is a schematic diagram of an exemplary input buffer;
FIG. 6 is a schematic diagram of an exemplary level shifter and integrator;
FIG. 7 is a schematic diagram of an exemplary comparator;
FIG. 8 is a schematic diagram of an exemplary reference signal circuit;
FIG. 9 is a schematic diagram of an exemplary output circuit; and
FIG. 10 is a schematic diagram of an exemplary embodiment of the invention.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanying drawings, which form a part hereof, and which illustrate specific exemplary embodiments by which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims.
DEFINITIONS
Throughout the specification and claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise. The term “connected” means a direct electrical connection between the things that are connected, without any intermediary devices. The term “coupled” means either a direct electrical connection between the things that are connected, or an indirect connection through one or more passive or active intermediary devices. The term “circuit” means one or more passive and/or active components that are arranged to cooperate with one another to provide a desired function. The term “signal” means at least one current signal, voltage signal or data signal. The meaning of “a”, “an”, and “the” include plural references. The meaning of “in” includes “in” and “on.”
INTRODUCTION
Under certain conditions, a LVDS device may experience a fault condition where its signal path is open, shorted, or terminated by abnormal means. Although the differential signal may be within the valid common-mode range, any one of these conditions alone may prevent a valid signal from being obtained. A fault condition is produced because the differential signal's peak-to-peak signal swing may be very low or near zero (i.e., a short circuit AC condition.). The output signal is unknown when the device does not receive a valid input signal. An unknown output signal can create bad data or signals.
The apparatus and method of the present invention is directed toward detecting a fault condition in common-mode differential signals and generating a fault detection signal for certain types of faults. The present invention is directed to providing an apparatus and a method for generating a fault detection signal for those fault conditions where the differential signal remains within the proper common-mode range and a valid LVDS signal cannot be obtained. As used herein, the terms “fault” and “fault condition” include the situation when a data transmission path in a differential signaling device is open, shorted, or terminated by an abnormal means while the differential signal remains within the proper common-mode range. This invention does not address other forms of differential signaling faults, for example an open condition where the differential signal exceeds the proper common mode range. The instant invention only detects faults occurring within the common-mode range. Therefore, the term “normal condition” means those situations not included within “fault condition,” which includes those instances when a valid differential signal can be obtained and when a fault occurs outside the common-mode range. This definition is a broad definition of normal and is intended to include both conditions not detected by the invention and conditions where valid signals are obtained.
The type of transistors is generally not designated in the drawings, specifications, and claims herein. For the purposes of this invention, p-type and/or n-type transistors may be used unless expressly indicated otherwise. The transistors may be bipolar devices, MOS devices, GaAsFET devices, JFET devices, as well as one or more components that are arranged to provide the function of transistors.
Referring to the drawings, like numbers indicate like parts throughout the views. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or inconsistent with the disclosure herein.
Briefly described, a LVDS fault condition detector generates a control signal when a LVDS device experiences a fault condition. The control signal, termed a fault detection signal herein, can be used by another apparatus to invoke a known, valid state. In one embodiment, the LVDS fault condition detector employs an input buffer that isolates a LVDS signal from any upstream device, and creates single-ended signals representing the LVDS inputs. Each single-ended signal is shifted to a level appropriate for comparators. During a normal LVDS condition, AC components of the single-ended signal are integrated to provide relatively stable input to the comparators. The comparators separately compare each single-ended signal to a reference signal, and generate a control signal for each single-ended signal. The magnitude of a control signal depends on whether the single-ended signal is above or below the reference signal. A high comparator signal represents a normal condition and a low comparator signal represents a potential fault condition. In a normal condition, at least one single-ended signal is above the reference signal, and results in one high comparator signal being generated. In a fault condition, both single-ended signals are below the reference signal, and both comparator signals are low. Lastly, the output stage of the LVDS fault condition detector produces a low output stage control signal in a normal condition, and a high output stage control signal in a fault condition. The high output stage control signal constitutes a fault detection signal and is used by a downstream device to invoke the failsafe condition and retain a valid LVDS signal in a fault condition.
The output stage may be arranged to provide a high signal in a normal condition and a low signal when a fault condition exists.
ILLUSTRATIVE ENVIRONMENT
FIG. 1 is an exemplary schematic diagram illustrating a LVDS driver and receiver system being monitored by the fault condition detector. The system includes driver 101, receiver 130, differential communication line pair 120, resistive load R135, and fault condition detector 400.
Driver 101 includes a current source and two pairs of transistors, M103b and M105a driving current in one direction, and M103a and M105b driving current in the opposite direction. A typical current produced by driver 101 could be in the range of 4.0 mA. Differential line pair 120 couples driver 101 to receiver 130. Receiver 130 has a high input impedance and can be any device configured to accept a LVDS input. Resistive load R135 is a termination resistive load that is connected across the LVDS inputs of receiver 130, and can be in the range of 100 ohms. Receiver 130 detects a voltage signal that is driven across resistive load R135. When driver 101 switches directions, the current flow changes direction across R135, and the signal across R135 changes polarity. Receiver 130 detects the change in polarity as a “high” or “low” logic state (i.e., logic “1” or logic “0.”)
Fault condition detector 400 monitors the differential signal (Sod) output by receiver 130 without significantly loading it.
FIG. 2 illustrates a typical LVDS signal swing that may appear across resistive load R135 at the input terminals of receiver 130 shown in FIG. 1. For example, driving 4.0 mA through R135 will produce a 400 mV differential signal across the input terminals of receiver 130. A typical LVDS system may have a common-mode voltage (VCM) between +0.2V and +2.2V. As shown in FIG. 2, a typical LVDS signal may have a common-mode voltage (VCM) centered at 1.5 V, a high output signal level (Sh) at 1.7V, and low signal output level (Sl) at 1.3V, yielding a differential signal (Sd) with a 400 mV peak-to-peak.
In the present invention, a fault condition is determined to occur when the voltage difference between Sh and Sl is less than 100 mV. This may occur when there is an open or a shorted conduction path in differential communication line pair 120, or when the conduction path or the input to receiver 130 is terminated other than by resistive load R135. Receiver 130 does not receive a valid logic state when a fault condition exists for reasons previously discussed. For the present invention, a fault condition is determined to occur when the voltage difference between Sh and Sl is less than approximately 100 mV. This may occur when there is an open or a shorted conduction path in differential communication line pair 120, or when the conduction path or the input to receiver 130 is terminated other than by resistive load R135. Receiver 130 does not receive a valid logic state when a fault condition exists. The present invention detects fault conditions that occur within the allowable common-mode range of the LVDS system.
FIG. 3 is a block diagram generally illustrating an exemplary operating environment for a LVDS fault condition detector. A typical operating environment can include LVDS receiver stage 330, fault condition detector 400, current gain stage 350, and appropriate couplers (not shown). LVDS receiver stage 330, fault condition detector 400, and current gain stage 350 can each be a separate device, or some or all of the devices may be combined into one or more single devices.
LVDS receiver 330 has a differential input that is arranged to receive differential input signal (+Sid, −Sid). The LVDS receiver (330) produces a differential output signal (+Sod, −Sod) at nodes N301 and N302 respectively. Fault condition detector 400 has a differential input that is arranged to receive a differential signal (+Sod, −Sod) from nodes N301, and N302 respectively. Fault condition detector 400 provides a pair of control signals (SCH, SCL) in response to its differential input. Current gain stage 350 receives control signals (SCH, SCL) and the differential output signal (+Sod, −Sod). Current gain stage 350 produces a differential output signal (+Sod2, −Sod2) in response to its inputs.
LVDS receiver stage 330 can be any apparatus or portion of an apparatus capable of receiving a LVDS signal, represented as Sid. LVDS receiver stage 330 preferably has a high input impedance. In operation, the input differential signal Sid is produced across resistive load R135 (see FIG. 1). LVDS receiver stage 330 receives differential signal Sid, rejects any common-mode noise in Sid, and produces an output differential signal (Sod) that is characterized by a relatively constant common-mode voltage. Fault condition detector 400 receives differential signal Sod as inputs Sih and Sil. Fault condition detector 400 outputs control signals SCH and SCL to current gain stage 350. The downstream device, illustrated by current gain stage 350 receives differential signal Sod and fault condition control signals (SCH, SCL) as inputs, and outputs signal Sod2.
The LVDS fault condition detector 400 generates either a normal condition control signal or a fault condition control signal (fault detection signal). The control signals indicate normal or fault condition to the downstream device. A normal condition control signal is generated when LVDS receiver 330 outputs a valid LVDS signal (Sod). In the event of a fault, fault condition detector 400 provides control signal SCH greater than SCL as a fault detector signal. For example, in a normal condition, fault condition control signal SCL may be greater than SCH by roughly 600 mV, and in a fault condition, fault condition control signal SCH may be greater than SCL by roughly 600 mV, constituting a fault detection signal.
In a fault condition, current gain stage 350 receives fault condition control signal SCH greater than SCL, constituting a fault detection signal, and invokes the failsafe condition by outputting a logic high signal as Sod2. A “failsafe condition” means a specific output by an apparatus of a predetermined value upon the occurrence of a fault condition. An example of a failsafe condition is blinking a traffic light red in all directions when a fault condition occurs. When the fault condition ends, fault condition control signal SCH becomes less than SCL, and current gain stage 350 outputs signal Sod2 as a function of signal Sod.
FIG. 3 reflects a preferred embodiment with fault condition detector 400 coupled signal downstream of LVDS receiver stage 330. This allows LVDS receiver 330 to function normally without being altered or disturbed by fault condition detector 400. In an alternative embodiment, fault condition detector 400 is located upstream of LVDS receiver stage 330 and receives signal Sid as its input signal.
FIG. 4 shows a block diagram for LVDS fault condition detector 400, generally illustrating an embodiment of the invention. LVDS fault condition detector 400 includes input buffer 410, two level shifters 420a and 420b, two integrators 430a and 430b, two comparators 440a and 440b, reference signal supply 450, and output stage 460.
Input buffer 410 has one signal input coupled to one part of the LVDS differential signal (Sod) and one signal input coupled to another part of LVDS differential signal (Sod). Input buffer 410 provides an output that is coupled to an input of level shifter 420a, and another output that is coupled to an input of level shifter 420b. Level shifter 420a has an output coupled to node N403a. Integrator 430a is coupled to node N403a. Comparator 440a has an output that is coupled to node N403a, and another input that is coupled to node N404. Level shifter 420b has an output coupled to node N403b. Integrator 430b is coupled to node N403b. Comparator 440b has an input that is coupled to node N403b, another input that is coupled to node N404. Reference signal supply 450 has one output coupled to node N404, and another output coupled to output 460. Output stage (460) has inputs coupled to comparators 440a and 440b, and the Sref2 output of Reference Signal Circuit 450, and outputs differential signal (SOH, SOL).
In operation, input buffer 410 receives differential input signal Sod (S2h), S2l). Input buffer 410 is configured to have a relatively high input impedance to prevent LVDS fault condition detector 400 from loading, altering, or disturbing the source of input signal Sod. Input buffer 410 also provides isolation from any external LVDS common-mode changes that might be interpreted by the LVDS fault condition detector 400 as a fault condition. Input buffer 410 may be configured to provide additional signal gain. Input buffer 410 outputs in-phase, single-end signals S2h and S2l to level shifters 420a and 420b, respectively.
In operation, level shifters 420a and 420b are arranged to receive in-phase, single-end signals S2h and S2l, respectively, and to shift these signals to a level appropriate for comparators 440a and 440b. Level shifters 420a and 420b output signals S3h and S3l to integrators 430a and 430b, and to comparators 440a and 440b, respectively. Integrators 430a and 430b are included because many devices used for comparators begin to operate as integrators when the LVDS switching speeds increase dramatically. The function of integrators 430a and 430b becomes significant when LVDS switching speeds approach AC frequencies in the range of approximately 400 MHz. In a preferred embodiment of the invention, integrators 430a and 430b are configured to average the AC component of signals S3h and S3l present at nodes N403a and N403b, respectively.
The integrators function as a “speed trap,” generating an average of the signal's peak value, and eliminating rapid signal swings. When transient signals approach a range of approximately 400 MHz, the gate voltages of comparators 440a and 440b start falling. Averaging effectively slows down the AC components of signals S3h and S3l by holding signals S3h and S3l at average peak values. This provides a relatively stable input signal to comparators 440a and 440b, and allows them sufficient time to perform their function. The transistors used in comparators 440a and 440b may be very fast, and without averaging or a “speed trap,” S3h or S3l signal swings in AC domain could cause a comparator to generate an erroneous fault detection signal or glitches.
In further operation, reference signal supply 450 provides reference signals Sref1 and Sref2. Reference Sref1 is provided to comparators 440a and 440b at node N404. Reference Sref2 is provided to output stage 460.
In further operation, comparators 440a and 440b receive signals S3h, S3l, and Sref1, and are arranged to compare reference signal Sref1 to signals S3h and S3l. Each comparator (440a, 440b) generates a control signal (SA, SB) based on the signals received by it. The magnitude of the comparator control signal depends on whether the input signal is above or below reference signal Sref1. In normal LVDS operating mode, at least one of signals S3h and S3l is above reference signal Sref1. This results in at least one high control signal being generated by a comparator. In a fault condition, both control signals will be low. For example, control signal SA will be high when S3h is above reference signal Sref1 (normal condition). Control signal SA will be low when S3h is below reference signal Sref1 (possible fault condition). Comparator 440b functions in a manner similar to comparator 440b. 
In further operation, output stage 460 receives comparator signals SA and SB from comparators 440a and 440b, respectively, and reference signal Sref2 from reference signal supply 450. Output stage 460 generates control signals SCH and SCL. Output stage 460 is configured to generate a signal such as SCL>SCH to signal to a downstream device that a normal condition (non-fault condition) exists. Output stage 460 is configured to generate a fault condition control signal such as SCH>SCL, constituting a fault detection signal, to inform a downstream device that a fault condition exists in Sod. The fault condition may originate either in LVDS receiver stage 330 or upstream. The fault detection signal SCH>SCL can be used by a device to invoke a failsafe condition.
For the purposes of illustrating an embodiment of the invention, a LVDS differential signal (Sod) is in a fault condition when Sod has a magnitude that is less than 100 mV peak-to-peak. For the purposes of further illustrating an embodiment of the invention, the difference between fault condition control signals SCL and SCH is approximately 600 mV. In a normal condition (non-fault condition) when the LVDS signal differential magnitude is 100 mV or more (see FIG. 2), LVDS fault condition detector (400) provides an fault condition control signal such that SCL is greater than SCH by approximately 600 mV. In a fault condition (when the LVDS signal differential magnitude is below 100 mV), LVDS fault condition detector (400) is configured to provide a fault condition control signal such that SCH is greater than SCL by approximately 600 mV. This latter fault condition control signal constitutes a fault detection signal, and indicates to a downstream device that a fault condition exists.
In an alternative embodiment, integrators 430a and 430b, and level shifters 420a and 420b are omitted when LVDS switching speeds are sufficiently slow such that the comparators do not produce a false fault condition.
FIGS. 5–10 illustrate preferred embodiments of the invention. FIGS. 6–9 illustrate a fault condition detector processing the S2h side of a LVDS differential signal. The components processing the S2l side of a LVDS differential signal are substantially similar, and are illustrated in FIG. 10.
FIG. 5 is a schematic diagram of an exemplary input buffer 500 according to one embodiment of the invention. As shown in FIG. 5, input buffer 500 includes resistive loads R502, R504, R506, and R508; transistors M1, M2, M5, and M6; and current sources I1 and I3.
Resistive loads R502, R504, R506, and R508 are coupled between local power supply VDD and nodes N510, N512, N516, and N518, respectively. Transistor M1 has a gate coupled to node N301 (Sih), a drain coupled to node N510, and a source coupled to node N514. Transistor M2 has a gate coupled to node N302 (Sil), a drain coupled to node N512, and a source coupled to node N514. Current source I1 is coupled between node N514 and local power supply VSS. Transistor M5 has a gate coupled to node N510, a drain coupled to node N516, and a source coupled to node N520. Transistor M6 has a gate coupled to node N512, a drain coupled to node N518, and a source coupled to node N520. Current source I2 is coupled between node N520 and local power supply VSS. Transistors M1 and M2 form a first differential pair and transistors M5 and M6 form a second differential pair.
In operation, the LVDS differential input signal (Sih, Sil) is applied to the gates of the first differential pair of transistors M1 and M2. When Sih rises above Sil, transistor M1 has a larger gate-to-source voltage than that for transistor M2, causing M1 to drive more current than transistor M2. The higher current through M1 and resistive load R502 pulls the voltage at node N510 lower, and the lower current through M2 and resistive load R504 pulls the voltage at node N512 higher. When the voltage at node N510 is lower than the voltage at node N512, the gate-to-source voltage applied to second transistor M5 decreases below the gate-to-source voltage applied to transistor M6. Less current is steered through M5 and R506, and more current is steered through transistor M6 and R508 such that signal S2h rises toward VDD and signal S2l falls toward VSS. Signals S2h and S2l, are in-phase with respect to signals Sih and Sil. When Sil, rises above Sih, more current is steered through transistor M2 than through transistor M1, and the process is reversed.
In a preferred embodiment, the first and second differential transistor pairs are relatively small compared to the other transistors in the LVDS fault condition detector 400, and M1 and M2 are smaller than M5 and M6. Drain resistive loads R502 and R504 typically are approximately equal, and are matched to provide approximately equal current through M1 and M2 when the input signals (Sih, Sil) are equal. Current sources I1 and I3 can be any type of current source capable of supplying a relatively constant current to the other components of input buffer 500. Transistors M1 and M2 have a relatively high input impedances such that they do not load down the source of signals Sih and Sil, and provide isolation from any external LVDS common-mode changes that might be interpreted by a LVDS fault condition detector 400 as a fault condition.
FIG. 6 is a schematic diagram of an exemplary level shifter 600 and integrator 650 according to one embodiment of the invention. A preferred embodiment of a LVDS fault condition detector employs two, separate level shifters 420a and 420b, and two, separate integrators 430a and 430b. Level shifter 420a and integrator 430a form part of the path analyzing Sih, while level shifter 420b and integrator 430b form part of the path analyzing Sil (see FIG. 4). FIG. 6 illustrates a preferred embodiment of a level shifter and an integrator.
In a preferred embodiment, level shifter 600 comprises transistor M12, and current source I3. Transistor M12 has a gate coupled to node N516, a drain coupled to local power supply VDD, and a source coupled to node N622. Current source I3 is coupled between N622 and local power supply VSS. In operation, transistor M12 operates as a source follower (level shifter) in the DC situation with I3 providing a pull-down current source. Signal S2h is coupled to the gate of M12. In operation, the source of M12 tracks gate signal S2h, and produces a laterally shifted (stepped down) signal S3h at node N622. M12 is also configured to provide a very fast low-to-high transition time and a very slow high-to-low transition time in the transient domain.
In a preferred embodiment, integrator 650 comprises capacitive circuit C602, which is coupled between node N622 and signal ground. In operation, capacitive circuit C602 is configured to operate as an integrator when the LVDS switching speeds approach high frequencies (i.e., in the range of approximately 700 to 800 MHz). Capacitive circuit C602 is configured to provide an average signal S3h at node N622 (See also node N403a at FIG. 4). Capacitive circuit C602 functions as a “speed trap,” generating an average of the signal's value, and eliminating rapid signal swings. For example, without integrator 650, the signal at node N622 will rapidly fluctuate between VSS and (V2h−VT) as M12 turns on and off. The rapid fluctuation may cause comparator 440a to have an output that rapidly oscillates between high and low.
The comparators determine when the signal level of the differential input is in a normal condition. Since at least one comparator must provide an output in a normal condition, both comparators turning off at the same time may cause a momentary “glitch” in the output, giving a false fault detection signal. The transistors of comparators 440a and 440b are very fast, and without averaging or without a “speed trap,” signal S3h may cause a comparator to generate an erroneous fault condition signal.
When M5 is off (see FIG. 5), node N516 (S2h) will be pulled up to VDD, resulting in M12 being fully on, and the source voltage for M12 at node N622 will be pulled high. Integrator C602 rapidly charges to (VDD−VT) while M12 is fully conducting. When M5 is turned on (see FIG. 5), node N516 would be rapidly pulled lower and M12 is turned off. However, the charge stored in C602 is temporarily “trapped” because the only discharge path is through current source I3, and it has high input impedance. Integrator C602 holds the voltage at node N622 high after M12 turns off. The “speed trap” or signal averaging effectively slows fast transient components of signal S3h at node N622 (N403a) by holding signal S3h close to the peak value. This provides comparator 700 (see FIG. 7) with a relatively stable input signal, and allows sufficient time for the comparator to perform its function.
Other embodiments of level shifter 600 and integrator 650 can be employed. Level shifter 600 can be any circuit configured to latterly shift (step down) signal S2h where a high impedance discharge path is created when the input signal (S2h) decreases sufficiently low. Integrator 650 can be any device or circuit capable of integrating AC components of signals S3h to a degree necessary for comparator 700 to function without generating a false LVDS fault condition signals (SA,SB). In addition to the capacitor illustrated in FIG. 6, alternative embodiments of integrator 650 include an op amp circuit, an op amp arranged with a capacitor to provide an integrator function, and parasitic capacitances of a transistor.
FIG. 7 illustrates a schematic diagram of comparator 700 according to an embodiment of the invention. A preferred embodiment of the LVDS fault condition detector employs more than one comparator as shown in FIG. 4. Comparator 440a forms part of the path analyzing Sih, and comparator 440b forms part of the path analyzing Sil. FIG. 7 illustrates a preferred embodiment of a comparator according to the invention.
Comparator 700a comprises resistive loads R710 and R712, differential pair transistors M3 and M4, and current source I6. Transistor M3 has a gate coupled to node N404 (Sref1), a drain coupled to node N732 (SA), and a source coupled to node N730. Transistor M4 has a gate coupled to node N622 (S3b), a drain coupled to node N734, and a source coupled to node N730. Resistive load R710 is coupled between supply voltage VDD and node N734. Resistive load R712 is coupled between supply voltage VDD and node N732. Current source I6 is coupled between node N730 and supply voltage VSS, and maintains approximately a constant current for the operation of comparator 700.
In operation, M3 and M4 form a differential pair. A fixed reference signal, Sref1, is applied to a gate of M3. Single-ended signal S3h is applied to a gate of M4. Restive loads R710 and R712 are sized to allow approximately equal current flow through the differential pair when S3h is substantially equal to Sref1. When single-ended signal S3h rises above Sref1 transistor M4 has a larger gate-to-source voltage than transistor M3 causing more current to be steered through transistor M4 and less current to be steered through transistor M3. As a result, signal SA at node N732 is pulled up toward VDD, resulting in a high comparator control signal SA when signal S3h is above Sref1. 
When single-ended signal S3h falls below Sref1, transistor M3 has a larger gate-to-source voltage than transistor M4, causing less current to be steered through transistor M4, and more current to be steered through transistor M3. As a result, signal SA at node N732 is pulled down toward VSS, resulting in a low comparator control signal SA when signal S3h is below Sref1.
FIG. 8 is a schematic diagram of an exemplary reference signal circuit 800 according to one embodiment of the invention and is an exemplary embodiment of reference signal circuit 450 shown in FIG. 4.
Reference signal circuit 800 includes resistors R820, R822, and R824; transistors M14, M15, and M16; and current sources I7, I8, and I9.
Resistors R820 and R822 are coupled between local power supply VDD and nodes N840 and N842 respectively. Resistor R824 is coupled between nodes N842 and N844. Transistor M14 has a gate coupled to node N840, a drain coupled to local power supply VDD, and a source coupled to node N404. Transistor M15 has a gate coupled to local power supply VDD, a drain coupled to node N840, and a source coupled to node N846. Transistor M16 has a gate coupled to local power supply VDD, a drain coupled to node N844, and a source coupled to node N848. Current sources I7–I9 are coupled between local power supply VSS and nodes N404, N846, and N848, respectively. Transistors M15–M16 can be long channel devices that are arranged to act as resistors. M14 and I7 are arranged to operate as a follower.
In operation, transistors M14 and M15, resistor R820, and current sources I7 and I8, provide signal Sref1 used by comparator 700. Transistor M15 functions as a voltage-controlled resistor, and in conjunction with resistor R820 and current source I8, form a controlled voltage drop from local power supply VDD at node N840. Transistor M14 functions as a source follower, has a gate biased at node N840, and in conjunction with current source I7, provides signal Sref1, at node N840. Transistors M14 and M15 are always enabled.
In operation, transistor M16, resistors R822 and R844, and current source I9, provide signal Sref2 used by output circuit 900 in FIG. 9. Transistor M16 has a gate biased at VDD. Transistor M16 functions as a voltage-controlled resistor, and in conjunction with resistors R820 and R824, and with current source I9, forms another controlled voltage drop from local power supply VDD, providing signal Sref2 at node N842. Transistor M16 is always enabled.
In alternative embodiments, signals Sref1 and Sref2 can be provided by any voltage source configured to provide the required signal levels.
FIG. 9 illustrates a schematic diagram of output circuit 900 according to one embodiment of the invention, and is an embodiment of output circuit 460 as shown in FIG. 4.
Output circuit 900 includes transistors M9, M10, and M11; resistive loads R952 and R954; and current source I10. Transistor M9 has a gate coupled to node N732 (SA), a drain coupled to node N952 and a source coupled to node N956. Transistor M10 has a gate coupled to node N950 (SB), a drain coupled to node N952 (SCH), and a source coupled to node N956. Transistor M11 has a gate coupled to node N842 (Sref2), a drain coupled to node N954 (SCL) and a source coupled to node N956. Current source I10 is coupled between node N956 and local power supply VSS. Resistive loads R952 and R954 are coupled between local power supply VDD and nodes N952 and N954 respectively.
In operation, output circuit 900 is similar to a wired NOR gate, or a three output comparator. A differential pair is formed by transistor M 11 and the parallel pair of transistors M9 and M10. Resistive loads R952 and R954 are approximately equal and function as pull-up devices for nodes N952 and N954, respectively. Current source I10 provides a constant current source at node N956, which is coupled to the sources of transistors M9, M10, and M11. Signal Sref2 is applied to a gate of transistor M11, and provides a uniform M11 gate-to-source bias voltage to establish the level of signal SCL at node N954. Transistor M11 is always enabled.
In normal LVDS condition, either transistor M9 or M10 is on and steering current away from transistor M11. As a result, the current through resistive load R954 decreases, and fault condition control signal SCL is pulled high toward VDD. The current through resistive load R952 increases, and fault condition control signal SCH is pulled low toward VSS. The components are sized so that this results in SCH being lower than SCL, indicating a normal LVDS condition. In a LVDS fault condition, both transistors M9 and M10 are off and all of current I10 flows through transistor M11. As a result, the current through resistive load R954 increases, and fault condition control signal SCL is pulled low toward VSS. The current through resistive load R952 decreases, and fault condition control signal SCH are pulled high toward VDD. Components M9, M10, M11. R952, R954, and I10 are sized so that fault detection signal SCH is greater than SCL when Sa and Sb are low. In an alternative embodiment, the transistors are scaled with respect to each other such that transistors M11 and M12 are twice the capacity of M9 (i.e., ((W/L)11=(W/L)12=2×(W/L)9). In an alternative embodiment, the component parameters of output circuit 900 are sized so that the difference between signal SCH and signal SCL is approximately 600 mV.
Many types of wired NOR gates, or three input comparators, can be used to generate fault condition control signals SCH and SCL.
FIG. 10 illustrates a schematic diagram of LVDS fault condition detector 1000 according to a preferred embodiment of the invention, and combines the preferred embodiments illustrated in FIGS. 5–9 into one circuit. FIG. 10 also illustrates a preferred embodiment of LVDS fault condition detector 400 illustrated in the block diagram of FIG. 4. FIG. 10 also schematically illustrates a fault condition detector processing both the Sih and Sil, sides of a LVDS differential signal. Where fault condition detector 1000 employs two devices, one for the Sih side and another for the Sil side of a differential signal, the two similar devices are shown by “a” and “b” suffixes. For example, fault condition detector 1000 employs two comparators of the type illustrated in FIG. 700. The comparator for the Sih side is designated 700a and the comparator for the Sil side is designated 700b. Similar components are similarly labeled.
LVDS fault condition detector 1000 comprises input buffer 500, two level shifters and integrators 600a and 600b, two comparators 700a and 700b, reference signal supply 800, and output stage 900. The above elements are enclosed in dashed boxes on FIG. 10. Current sources for the elements are not included within the dashed boxes for clarity. The components of LVDS fault condition detector are coupled and function as described in FIGS. 4–9.
The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.

## Claims

1. An apparatus for detecting a fault condition in a differential signal having a valid common-mode voltage, comprising:
an input buffer circuit that is arranged to produce a first buffered signal and second buffered signal in response to the differential signal;
a first comparator circuit that is arranged to produce a first signal in response to the first buffered signal and a first reference signal;
an integrator that is arranged to provide a first integrated signal in response to the first buffered signal, and the first comparator circuit is arranged to produce the first signal in response to the first integrated signal and the first reference signal, whereby the integrator operates as a speed trap, and wherein the integrator includes a level shifter circuit and a capacitance circuit, the level shifter circuit providing isolation between the input buffer circuit and the capacitance circuit, and the capacitance circuit is arranged to average the first buffered signal;
a second comparator circuit that is arranged to produce a second signal in response to a second buffered signal and second reference signal; and
an output circuit that is arranged to produce a fault detection signal in response to the first signal and the second signal, whereby the fault detection signal indicates the fault condition that is due to at least one of a short condition, an open condition, and a termination by abnormal means.

2. An apparatus as in claim 1, wherein the first reference signal and the second reference signal are the same.

3. An apparatus as in claim 1, wherein detecting the fault condition occurs without loading, altering, or disturbing the signal source.

4. A method for detecting a fault condition in a differential signal having a valid common-mode voltage, the method comprising:
producing a first buffered signal and a second buffered signal in response to the differential signal;
generating a first signal when the amplitude of the first buffered signal meets a first predetermined criteria;
employing an integrator to generate a first integrated signal in response to the first buffered signal, and the generation of the first signal occurs in response to the first integrated signal and the first reference signal, and wherein a level shifter provides for isolation between the first and second buffered signals and a capacitance circuit that is arranged to average the first buffered signal;
generating a second signal when the amplitude of the second buffered signal meets a second predetermined criteria; and
producing a fault detection signal in response to the first signal and the second signal, wherein the fault detection signal indicates the fault condition when the amplitude of the differential signal is below a predetermined amplitude level.

5. A method as in claim 4, further comprising detecting the fault condition occurs without loading, altering, or disturbing the signal source.

6. A method as in claim 4, wherein the first predetermined criteria is the same as the second predetermined criteria.

7. A method as in claim 4, wherein the first predetermined criteria corresponds to the first buffered signal being below a predetermined signal level.

8. A method as in claim 4, wherein the first predetermined criteria corresponds to the first buffered signal being above a predetermined signal level.

9. A method as in claim 4, further comprising averaging at least one of the first buffered signal and the second buffered signal with a corresponding integrator.

10. An apparatus for detecting a fault condition in a differential signal having a valid common-mode voltage, comprising:
a buffering means that is arranged to provide a first and second buffered signal in response to the differential signal;
a first comparison means that is arranged to produce a first signal in response to a comparison between the first buffered signal and a first reference signal;
an integration means that is arranged to provide a first integrated signal in response to the first buffered signal, and the first comparison means is arranged to produce the first signal in response to the first integrated signal and the first reference signal, whereby the integration means operates as a speed trap, and wherein the integration means includes a level shifter means and a capacitance means, the level shifter means providing isolation between the buffering means and the capacitance means, and the capacitance means being arranged to average the first buffered signal;
a second comparison means that is arranged to produce a second signal in response to a comparison between the second buffered signal and second reference signal; and
an output means that is arranged to produce a fault detection signal in response to the first signal and the second signal, such that the fault detection signal indicates the fault condition that is due to at least one of a short condition, an open condition, and a termination by abnormal means.

11. An apparatus as in claim 10, wherein detecting the fault condition occurs without loading, altering, or disturbing the signal source.