Patent Publication Number: US-8538362-B2

Title: Squelch detection circuit and method

Description:
FIELD OF DISCLOSURE 
     The present disclosure is related to communications and more specifically, to a squelch detection circuit having an inexpensive current source to generate offset and having programmable threshold and calibration settings and associated method. 
     BACKGROUND 
     Squelch detectors are commonly used in communication links, such as high speed serial links, to provide an indication of when data signals are present on a link as distinguished from noise signals and the like. When the data signal is not present, the receiver can be momentarily turned off, thus limiting power consumption. Over time, such off-intervals can result in increased battery life for hand held receivers and other benefits. 
     In order to operate effectively, squelch detectors are typically configured with a large gain value and a large bandwidth so as to ensure maximum detection probability. The bandwidth of the squelch detector is generally wider than the input bandwidth of the receiver. Such requirements can reduce the efficiency of the overall receiver since power gains achieved by receiver shut-off during intervals where no signal is detected are lost by the need for additional power to support wider bandwidth can gain requirements of the detection stage. 
     A typical signal configuration in a modern high speed data link involves differential signal pairs that are of opposite polarity and transition to different and opposite levels during data transitions. Such operation is known in the art as differential signaling and is advantageous, inter alia, because of the inherent noise rejecting properties, tolerance to DC offset and the like. A receiver using differential signaling can experience enhanced common mode noise rejection, although such rejection can be due to the balanced nature of the typical transmission line. The signals on each line are compared only to each other and thus external influences of factors such as DC offset can be minimized. 
     In order to create a detection envelope, squelch detectors introduce offset as will be described in greater detail hereinafter in connection with, for example, the conventional circuit shown in  FIG. 1 . By using, for example, a replica circuit, a current mismatch can be introduced on the two branches of a differential amplifier. Signals that fall outside the detection envelope established by the offset are considered data signals while signal energy within the offset is considered noise. 
     Disadvantages can arise in conventional squelch detection circuits in that that linearity constraint can be exceeded and circuit sensitivities to current and replica matching can arise. Further, the common mode of the replica circuit is fixed, while that of the actual amplifier varies, for example in accordance with universal serial bus (USB) standard version 2.0. Accordingly, a systematic mismatch and hence additional and often non-linear offset can be introduced in a scheme that uses a replica circuit. Consequently, errors can arise due to mismatch and also due to any offset inherent in the replica circuit. Such errors are difficult to compensate for since they may vary across different signal levels and also across different fabrication processes and scales. 
     SUMMARY 
     Exemplary embodiments are directed to circuits and methods for calibration and squelch detection threshold setting in a squelch detection circuit. Accordingly, a first comparator having a first polarity output can be coupled to a complimentary input signal pair. A second comparator having a second polarity output can be coupled to the complimentary input signal pair. An offset associated with, for example, the difference between the input signal pair, and with the first polarity and the second polarity output can be used to establish a positive squelch threshold and a negative squelch threshold. A calibration unit can be coupled to the first comparator and the second comparator. The calibration circuit generates a digital output including threshold settings and calibration settings to the first comparator and to the second comparator. The digital output associated with establishing the offset and the positive squelch threshold and the negative squelch threshold. The squelch detection circuit can further include a logic circuit coupled to the first comparator and the second comparator. The logic circuit can have inputs for the first polarity output and the second polarity output and an output for a squelch signal. 
     In an embodiment, a first comparator means, which means can be constituted as the first comparator circuit, can provide a first polarity output when an offset associated with a complimentary input signal pair exceeds a first squelch threshold value. A second comparator means can be for providing a second polarity output when the offset associated with the complimentary input signal pair exceeds a second threshold value. A calibration means can be coupled to the first comparator means and the second comparator means and can be for generating a digital output including threshold settings and calibration settings to the first comparator and to the second comparator, the digital output for establishing the offset, the positive squelch threshold and the negative squelch threshold. 
     In accordance with an embodiment, a calibration circuit for a squelch detector can include a programmable resistor network coupled to a comparator circuit in the squelch detector. The programmable resistor network includes a plurality of resistors. A first portion of the plurality of resistors is for establishing a squelch detection threshold setting and a second portion of the plurality of resistors is for establishing a calibration setting for calibrating a mismatch between a first input and a second input of the comparator circuit. A switching network can be coupled to the programmable resistor network, and can include a plurality of switching elements coupled to the plurality of resistors. First ones of the plurality of switching elements can be coupled to the first portion and second ones of the plurality of switching elements coupled to the second portion of the resistor network. Programming signals can be coupled to the switching network for selecting ones of the plurality of resistors through corresponding ones of the switching elements to establish the squelch detection threshold setting and the calibration setting for the comparator circuit. 
     In accordance with an embodiment, a method for calibrating a squelch detector circuit can be provided. The method can include exemplary procedures such as setting an internal offset in a comparator stage of the squelch detector circuit, introducing an external offset to the comparator stage, resetting a plurality of programming bits input to the comparator stage; and incrementing the plurality of programming bits until a transition in an output of the comparator stage occurs. The transition can represent a squelch detection threshold level associated with a difference between the internal offset and the external offset. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are presented to aid in the description of embodiments and are provided solely for illustration of the embodiments and not limitation thereof. 
         FIG. 1  is a schematic circuit diagram illustrating a prior art offset generating circuit. 
         FIG. 2A  is a block diagram illustrating an exemplary squelch detector. 
         FIG. 2B  is a signal diagram illustrating exemplary threshold crossing. 
         FIG. 3  is a schematic circuit diagram illustrating an exemplary squelch offset generating circuit. 
         FIG. 4A  is a schematic circuit diagram illustrating features of an exemplary squelch comparator circuit. 
         FIG. 4B  is a schematic circuit diagram illustrating features of another exemplary squelch comparator circuit. 
         FIG. 4C  is a schematic circuit diagram illustrating exemplary details of one of the squelch comparator circuits of  FIG. 4A  and  FIG. 4B . 
         FIG. 5A  is a diagram illustrating an exemplary threshold programming circuit. 
         FIG. 5B  is a block diagram illustrating an exemplary digital calibration circuit. 
         FIG. 6  is a diagram illustrating an exemplary timing circuit at a given calibration threshold setting. 
         FIG. 7  is a timing diagram illustrating different signals in an exemplary timing circuit at a given calibration threshold setting. 
         FIG. 8  is a flowchart illustrating an exemplary calibration procedure. 
     
    
    
     DETAILED DESCRIPTION 
     Aspects of the embodiments are disclosed in the following description of and related drawings directed to specific embodiments. Alternate embodiments may be devised without departing from the scope of the invention. Additionally, well-known elements of the embodiments will not be described in detail or will be omitted so as not to obscure the relevant details of the embodiments. 
     The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. Likewise, the term “embodiments” does not require that all embodiments include the discussed feature, advantage or mode of operation. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising,”, “includes” and/or “including”, when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Further, many embodiments are described in terms of sequences of actions to be performed by, for example, elements of a computing device. It will be recognized that various actions described herein can be performed by specific circuits (e.g., application specific integrated circuits (ASICs)), by program instructions being executed by one or more processors, or by a combination of both. Additionally, these sequence of actions described herein can be considered to be embodied entirely within any form of computer readable storage medium having stored therein a corresponding set of computer instructions that upon execution would cause an associated processor to perform the functionality described herein. Thus, the various aspects of the embodiments may be embodied in a number of different forms, all of which have been contemplated to be within the scope of the claimed subject matter. In addition, for each of the embodiments described herein, the corresponding form of any such embodiments may be described herein as, for example, “logic configured to” perform the described action. 
     With reference to  FIG. 1 , a conventional squelch detection circuit  100  is show for performing squelch detection in, for example, a receiver circuit. A positive version inp  101  of, for example, a complimentary differential input signal pair and a negative version inn  102  of the input signal pair are inputted into elements  121  and  122  of input circuit  120 . In the prior art, a replica circuit  130  can be placed into the circuit to introduce a mismatch between the circuit branches. It will be noted that elements  131  and  132  are coupled to a 0 reference and a threshold reference such that the common mode of the circuit becomes fixed, while the common mode of the input circuit  120  varies based on the input signal excursions. Element  133  is coupled to vdda 18   110 . The input circuit  120  is situated between vdda 18   110  and vssa  111  and has additional elements  123 ,  124  and  125  forming a folded cascode differential amplifier configuration. Circuit portion  140  provides gain and includes elements  141 ,  142 ,  143  and resistors  145 . The loads of the differential pair are two current sources formed by transistors  141  and  142 . The resistors  145  are used to sense the output common mode level and apply it to the gate of current sources  141  and  142 . It should be noted that the resistors  145  are not related to establishing offset in connection with a squelch circuit as discussed and described herein. Also, it will be understood that the loads presented here are for illustrative purposes and can represent any general load. 
     A high speed communication circuit, such as a USB 2.0 circuit  200 , is shown in  FIG. 2A  and includes a comparator CMP 1   220 , comparator CMP  2   230  and logic circuit  240 . A complimentary differential signal including a positive input Inp  201  and a negative input Inn  202  can be input to CMP  1   220  and CMP  2   230  respectively in opposite relation, where, for CMP  1   220 , Inp  201  is coupled to the non-inverting terminal and Inn  202  is coupled to the inverting terminal thereof. For CMP  2   230 , Inn  202  is coupled to the non-inverting terminal and Inp  201  is coupled to the inverting terminal. Input signals can be relatively small such as between +/−10-20 mV. CMP  1   220  and CMP  2   230  can be coupled between a voltage vdda  210  and reference voltage vssa  211  and can generate output signals  221  and  231  respectively. The outputs  221  and  231  are coupled to a logic element  241  of logic circuit  240 , shown as an OR element. The output of the OR element is coupled to a one-shot circuit  242 , which generates a squelch signal  203 . It will be appreciated that the squelch signal can indicate the presence or absence of a squelch condition, by way of an active high or active low level on the output thereof. 
     As illustrated in  FIG. 2B , it will be appreciated that in accordance with the positive and negative supply voltage levels  210  and  211  that are applied to the comparators, and, for example, in accordance with signal input levels, the outputs of CMP  1   220  and CMP  2   230  can each swing, for example, from the positive output voltage limit or “rail” to the negative rail. It should first be noted that for signals with slow rise/fall times the input levels to the comparators CMP  1   220  and CMP  2  can be small, in the order of 10-20 mV, and thus require amplification.  FIG. 2B  shows the differential input to the comparator CMP 1  and CMP 2 . It will be appreciated that the comparator CMP 1  is configured to have an internal offset of 120 mV while comparator CMP 2  is configured to have an internal offset of −120 mV. It should be noted that in accordance with USB2.0, the inter-symbol interference (ISI) eye-opening of the signal, in the worst case, can be just 0.3 unit interval (UI) where unit interval means one bit width. In accordance with the worst case signal, which is shown in  FIG. 2B , the signal crosses the squelch detection thresholds for the comparator CMP 1  and CMP 2  +/−120 mV by a margin of only +/−10-20 mV respectively. In various exemplary embodiments, the squelch circuit should detect the above noted condition as a valid signal. It should also be noted that, due to process and other variations, the internal offsets of CMP 1  and CMP 2  can deviate from their ideal value of +/−120 mV. Thus, the two comparators need to be configured with a high gain in order to amplify a small 10-20 mV signal to full swing and a high bandwidth in order to detect a worst case signal crossing within a time margin of just 0.3UI. 
     The input offset described above can be advantageously established, in contrast to prior art approaches, using a simple current resistance (IR) value incorporated into a source follower input stage that is provided in connection with each of CMP  1  and CMP  2 . The positive and negative supply voltages  210  and  212 , which in the present example are vdda and ground, can be applied to a pair of source follower circuits, one for each input. In the present example, for the positive input Inp  201 , element  321  and  322  can be coupled to input element  325  through offset element  350 . It will be appreciated that in the present example offset element  350  is a resistive element. The offset element  350  can be coupled to, for example, a control line  351 , which can be a digital bus or the like, for controlling the offset element  350 . An output outp  301  can be taken from the A side of the offset element  350 . For the negative input Inp  202 , element  323  and  324  can be coupled to input element  326 . An output outn  302  can be taken from the ungrounded terminal of element  326 . 
     A version of the above described circuit can be used at the input of each of CMP  1   220  and CMP  2   230  as will be appreciated with reference to  FIG. 4A  and  FIG. 4B . In  FIG. 4A , an offset can be established using the offset element  350   a  in the positive signal input path to comparator  401 , which can correspond to CMP  1   220 , such that the positive input Inp  201  is coupled to the non-inverting terminal thereof and the offset is established with reference to the negative input signal Inn  202  which is coupled to the reference or inverting terminal. Outputs  301   a  and  302   a  can be input to comparator  401 , which generates output  458   a  that can slew from rail to rail in accordance with the input signals. Similarly, with reference to  FIG. 4B , an offset can be established using offset element  350   b  in the negative signal input path to comparator  402 , which can correspond to CMP  2   230 , such that the negative input Inn  202  is coupled to the non-inverting terminal thereof and the offset is established with reference to the positive input Inp  201 , which is coupled to the reference or inverting terminal Outputs  301   b  and  302   b  can be input to comparator  402   m  which generates output  458   b  that can slew from rail to rail in opposite relation to comparator  401 . 
     A more detailed view of an example of the comparator stages such as comparator  401  and  402  is illustrated in  FIG. 4C . The outputs  301  and  302 , generated from the source follower circuits can be input to initial amplification stage  420 , in connection with the other sections, an analog output that can slew from, for example, vdda 18   410  to vssa  411 . Stage  420  can include elements  421 - 426  in a known configuration such as a differential amplifier configuration and generating outputs  427  and  428  which can be input to stage  430  which can be coupled between vdd 18   412  and dvss  411  and includes element  431 - 434  configured as a slewing stage. The output of slewing stage  420  is coupled to a level-down shifter stage  440  implemented in the present example as inverter. A latching stage can be provided at the output to introduce an amount of hysteresis sufficient to prevent frequent toggling of comparator output if the input signal swing is close comparator offset. 
     To better understand the operation of the offset unit  350 , a resistor network  500  is illustrated in  FIG. 5A . It will be appreciated that RA  501  and RB  502  can correspond, for illustrative purposes, to nodes A and B of resistive element or resistor R in  FIG. 3 . An initial resistance value R  503  can be established based on a typical or minimum IR to establish a minimum or sub-minimum offset value. Portions of the resistor network, such as R  511 -R  513  and R  521 -R  523  can be included to address, for example, threshold setting as in the case of R  511 -R  513  and calibration setting, as in the case of R  521 -R  523 . The portions of the resistor network can be controlled by way of switching elements Th 1   514 -Th 3   516  and Pbit 1   524 -Pbit 3   526 , which can be used to switchably couple selected ones of the resistors in parallel with R  503  to calibrate residual offset and establish threshold levels and the like. It will be appreciated that the foregoing approach is vastly superior to the replica circuits and other circuits of the prior art in that it requires far fewer elements and uses less current and is more scalable across voltage and process variations. 
     A block diagram illustrating how a calibration unit  560  can be used in an exemplary squelch circuit is shown in  FIG. 5B . In the present embodiment, CMP  1   540 , which can correspond to CMP  1  as described herein including any modifications described hereinafter, can be coupled to digital calibration unit  560  through pbits bus  561 , which can be a serial or parallel data bus, or the like, and CMP  2   550 , which can correspond to CMP  2  as described herein, including any modifications described hereinafter, can be coupled to digital calibration unit  560  through nbits bus  562 . Digital calibration unit  560  can be a circuit, logic, or the like, or a combination of circuitry, as described in greater detail hereinafter. Alternatively, digital calibration unit  560  can be coupled to or can further include, or be under the control of a processor, controller, or the like such as processor  565  including a memory  566 . Although shown for illustrative purposes as being external to digital calibration unit  560  and coupled thereto, it will be appreciated by one of ordinary skill that, in an embodiment, processor  565  and memory  566  can be incorporated into the circuitry of digital calibration unit  560 . 
     Digital calibration unit  560  can be configured to monitor the output of CMP  1   540  and CMP  2   550  through, for example, configurable calibration feedback switches pcal  563  for the positive side and ncal  564  for the negative side. Based on measured readings, digital calibration unit  560  can set the various switching elements Th 1   514 -Th 3   516  and Pbit 1   524 -Pbit 3   526  during a calibration or setup phase or in a monitoring phase as will be appreciated. In accordance with various embodiments, the calibration and threshold settings can be permanently set such as during a manufacturing set up, can be fixable, or can be permanently configurable or some combination thereof. For example, the calibration settings can be fixed, and the threshold settings can remain configurable or the like. 
     In one embodiment, an exemplary calibration circuit as shown in  FIG. 6 , can be used, for example, to perform the calibration setting shown and described herein above, for example, in connection with digital calibration unit  560  of  FIG. 5B . A circuit such as comparator calibration circuit  620 , shown for illustrative purposes, for calibrating the positive signal side, can be coupled to pbit calibration sections  630 ,  640  and  650 , which produce pbit outputs that are then fed back to the comparator calibration circuit  620 . The calibration is a one time process which can be performed at the start or power-up of the chip and the calibrating settings can be stored for future use. In order to perform calibration, input inp  201  and inn  202  are blocked comparator  628 , by switching element  623  and  624 . A desired offset voltage is generated by dumping current through current source  621  into resistor  625  and applying the offset voltage as an input signal to the comparator CMP 1   628 . The comparator CMP 1   628  then compares the external offset with the internal offset generated by the resistor network shown in  FIG. 5A . If the external offset is greater than the internal offset, then output node  658  goes high. If external offset is less than the internal offset, then the output node  658  goes low. 
     The circuit generating calibration clocks is shown in  FIG. 7  where four clock signals S 1 , S 2 , S 3  and S 4  are generated. The clocks S 1   b , S 2   b , S 3   b  and S 4   b  are inverted versions of the clocks S 1 , S 2 , S 3  and S 4 . At the beginning of calibration all the pbits are driven to zero by pull down NMOS switches  634 ,  644  and  654  due to the initial state of S 1   b , S 2   b  and S 3   b  as all high. The output node  658  is driven to a low level turning ON PMOS switches  637 ,  647  and  657  which pre-charges middle input of all AND gates  635 ,  645  and  655  to high. At the next clock edge, S 1  goes high while S 2  and S 3  are still zero as shown in  FIG. 7 . Pbit calibration section  630  is then activated and pbit 1  goes high, thus reducing the internal offset of comparator CMP 1  by turning ON parallel resistor branch as shown in  FIG. 5A . If the output node  658  is still zero at next clock edge, section  640  of pbit calibration is activated such that the internal offset of comparator is reduced further. The sequence continues until pos or node  658  goes high at which point calibration is complete and the calibrated settings are latched by the latches  633 ,  643  and  653 . 
     As discussed earlier,  FIG. 7  shows an exemplary clock circuit  700  including flip flop elements  722 - 725 . With a vdd signal coupled to the data input of the first element  722 , a transition of clk  721  can produce a high level for S 1   726  on the output of element  722 , which can be coupled to the data input of element  723 . The next transition of clk  721  will produce a high level for S 2   727  on the output of element  723  which can be coupled to the data input of element  724 . The next transition of clk  721  will produce a high level for S 3   728  on the output of element  724 , which can be coupled to the data input of element  725 . The next transition of the clk  721  can produce a high level for S 4   729  on the output of element  725  and so on, as shown by the timing relationships in the lower portion of  FIG. 7 . These signals can be input to  FIG. 6  as noted above, for calibration purposes. 
     To the extent that the programmability of the calibration and the threshold level settings can be performed in a variety of manners consistent with embodiments described herein above and with other embodiments, an exemplary procedure for calibration is shown and described in connection with  FIG. 8 . It will be appreciated that the following description while referring to one side of a squelch detector circuit can be applicable to both comparator sides of, for example, a circuit that accommodates a differential input signal with a complimentary signal pair. After start at  801 , which can represent the power up of the detector circuit or the like as would be understood by one of skill in the art, an external offset to the comparator stage at  802 . A plurality of programming bits that are input to the comparator stage can be reset at  803 . After the programming bits are reset, they are incremented until a transition in an output of the comparator stage occurs at  804 . The programming bits can be incremented, for example, by turning on each of the plurality of programming bits in sequence. The transition represents a squelch detection threshold level associated with a difference between the internal offset and the external offset. Once the transition occurs, the levels, states or the like associated with each of the programming bits can be latched at  805  so as to store the calibration setting. While the procedure is indicated as ending at  806 , it can be repeated whenever calibration is required and when different internal offsets are desired or required. While the procedure shown in  FIG. 8  is described with various actions procedures or sub-procedures, embodiments are not limited solely to those described herein and particularly are not limited to the specific terms. It will be appreciated that the exemplary procedure can be embodied as a series of steps and associated functions as set forth in the claims appended hereto that can be performed using any suitable structures and procedures, for example, as described herein. 
     It should be noted that in an embodiment, the calibration of the squelch detector circuit can be performed during a calibration procedure during which signal inputs to the squelch detector circuit are prevented. The setting the internal offset includes switching a resistor in a resistor network coupled to the comparator stage. The incrementing the plurality of programming bits includes switching resistors in the resistor network with respective ones of the resistors corresponding to ones of the programming bits. More specifically, the setting the internal offset can include switching a first resistor in a first portion of the resistor network and incrementing the plurality of programming bits includes switching resistors in a second portion of the resistor network, with ones of the resistors corresponding to ones of the programming bits. Still further, in an embodiment, incrementing the plurality of programming bits can include switching a plurality switching elements coupled to a plurality of resistors in the resistor network. Ones of the plurality of switching elements are capable of being switched by corresponding ones of the plurality of programming bits. 
     Those of skill in the art will appreciate that information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof. 
     Further, those of skill in the art will appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the invention. 
     The methods, sequences and/or algorithms described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor itself, can be a controller, a digital signal processor (DSP), a field-programmable gate array (FPGA), or the like. 
     It will be further appreciated that the squelch detector circuit as described herein may be included within a mobile phone, portable computer, hand-held personal communication system (PCS) unit, portable data units such as personal data assistants (PDAs), GPS enabled devices, navigation devices, settop boxes, music players, video players, entertainment units, fixed location data units such as meter reading equipment, or any other device that stores or retrieves data or computer instructions, or any combination thereof. Accordingly, embodiments of the disclosure may be suitably employed in any device which includes active integrated circuitry including the squelch detector as disclosed herein such as by being integrated into at least one semiconductor die associated with circuits in such devices. 
     While the foregoing disclosure shows illustrative embodiments, it should be noted that various changes and modifications could be made herein without departing from the scope of the invention as defined by the appended claims. The functions, steps and/or actions of the method claims in accordance with the embodiments described herein need not be performed in any particular order. Furthermore, although elements of the embodiments may be described or claimed in the singular, the plural is contemplated unless limitation to the singular is explicitly stated.