Abstract:
A level shifter and method are provided for balancing a duty cycle of a signal. An input circuit receives a differential logic signal with two complimentary logic levels. A level transition balancing circuit balances the rise and fall times of a level shifted version of each complimentary logic level during a transition from a first to a second of the logic levels and a level shift. A logic element stores and provides outputs of the level shifted versions of the logic levels. The level transition balancing circuit can include a capacitor in parallel with a transfer element for each input. The capacitor destabilizes inputs to the logic element and balances the transition using a capacitance and a level previously stored in the logic element.

Description:
FIELD OF DISCLOSURE 
       [0001]    The present disclosure is related to high speed receivers and related circuits, and more specifically, to a latch-based level shifter with balanced rise and fall times 
         [0002]    Background Digital circuits commonly use a consistent logic level for internal signals. However, the logic level can vary from system to system and even within various subsystem circuits within the same system. A level shifter can be used to connect a digital circuit using one logic level to a digital circuit using another logic level. Multiple level shifters can be used, such as one in each system, where bidirectional shifting is required. A driver, for example, can convert from internal logic levels to levels that are compatible with standard interface line levels. A line receiver, for example, can converts from interface levels to internal voltage levels. Examples of standard interface levels include the transistor to transistor logic (TTL) or complimentary metal oxide semiconductor (CMOS) logic levels commonly found within integrated circuits. Within a digital system, such as a digital communication system, internal voltage levels can be referred to as logic levels, while external voltage levels can be referred to as line levels. 
         [0003]    Digital circuits generally operate based on the logic signals having logical transitions from one level to another level, for example, according to a specified time period for synchronous digital circuits. When a level shifter is required, it is often the case that the timing of the original signal transitions is distorted. For modern high-speed digital receivers and related phase locked loop (PLL) circuits stringent duty-cycle distortion requirements are present for level shifters since duty cycle or bit period distortion in a high speed digital receiver directly affects the jitter budget of CDR. 
         [0004]    While simple level shifters are available in the prior art that accurately and adequately translate levels associated with an input logic signal, such circuits have disadvantages in that they can distort the time domain characteristic of the signal, such as the duty-cycle, due to unmatched rise/fall transitions that do not correspond precisely enough with the timing characteristics of the original signal to be suitable for high speed digital communication applications. Such lack of matching or correspondence can lead disadvantageously to jitter, data errors, and the like. 
       SUMMARY 
       [0005]    Exemplary embodiments are directed to systems and method for balancing a transition in a level shifting circuit. An input circuit receives a differential logic signal from two input nodes associated with two complimentary logic levels. The two input nodes are coupled respectively to symmetric portions of the input circuit. The input circuit includes a level transition balancing circuit and a logic element coupled to the input circuit. The level transition balancing circuit balances the respective rise and fall times of level shifted versions of the two complimentary logic levels associated with the differential logic signal during a transition from a first to a second of the two complimentary logic levels and an associated level shift. The logic element stores and provides outputs of the level shifted versions of the two complimentary logic levels on two output nodes after the transition. The level transition balancing circuit includes a capacitor in parallel with a transfer element for each of the two input nodes. The capacitor balances the respective rise and fall times during the transition using a capacitance and a previous level shifted version of the complimentary logic level stored in the logic element. The transfer element includes a field effect transistor (FET). The logic element includes a latch that further holds the output levels of the level shifted versions of the two complimentary logic levels after the transition. The latch can be embodied as a pair of cross coupled inverters. 
         [0006]    In accordance with various aspects, an exemplary latch can include a first field effect transistor (FET) of a first polarity coupled to a first FET of a second polarity between a ground potential and a source potential. The first FETs can have gates thereof coupled to each other and to a first of the two output nodes, and to a second field effect transistor (FET) of the first polarity coupled to a second FET of the second polarity between the ground potential and the source potential. The second FETs can have gates thereof coupled to each other and to a second of the two output nodes. 
         [0007]    In accordance with an aspect, a circuit can be provided for balancing rise and fall times associated with a level transition in a level shifter. The circuit can include an input transfer element for each one of two input nodes associated respectively with one of two complementary logic levels of a differential logic signal, a latch coupled to the respective input transfer element, and a capacitor coupled in parallel with the input transfer element. The latch can hold level shifted versions of the two complimentary logic levels on two output nodes after the level transition from a first to a second of the two complimentary logic levels. The capacitor balances the respective rise and fall times of the level shifted versions of the two complimentary logic levels during the transition so as to eliminate a timing variance between the two complimentary logic levels and the level shifted versions on the two output nodes associated with the level transition. The capacitor further destabilizes the latch through capacitive coupling to differential inputs of the latch. 
         [0008]    In accordance with an aspect, a level shifter can include input means, such as various arrangements of the input circuits described in greater detail hereinafter, for receiving two complimentary logic levels of a differential logic signal from two input nodes coupled respectively to symmetric portions of the input means. A level transition balancing means can be included for balancing the respective rise and fall times of level shifted versions of the two complimentary logic levels during a transition from a first to a second of the two complimentary logic levels and an associated level shift. Logic means, such as various arrangements of the latching circuits described in greater detail hereinafter, can be coupled to the input means and may further be provided for holding the two complimentary logic levels and outputting the two complimentary logic levels on two output nodes after the transition from the first to the second of the two complimentary logic levels. The level transition balancing means can include a capacitive means, such as various arrangements of a capacitor and related circuits as described in greater detail hereinafter, in parallel with a transfer element for each of the two input nodes. The capacitive means can be provided for balancing the rise times of the level shifted versions of the two complimentary logic levels during the transition using a capacitance and a previous level shifted version of the complimentary logic level stored in the logic means. 
         [0009]    In accordance with an aspect, a method can be provided for level shifting. The method can include receiving a differential logic signal including two complimentary logic levels associated therewith, level shifting the differential logic signal to form level shifted versions of the two complimentary logic levels, and balancing the respective rise and fall times of the level shifted versions of the two complimentary logic levels during a transition from a first to a second of the two complimentary logic levels so as to eliminate a timing variance between the two complimentary logic levels and the level shifted versions thereof. The method can further include holding and outputting the level shifted versions of the two complimentary logic levels on two output nodes after a transition from a first to a second of the two complimentary logic levels. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    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. 
           [0011]      FIG. 1  is a circuit diagram illustrating an example of a common inverter-based level shifter. 
           [0012]      FIG. 2  is a circuit diagram illustrating another example of a common latched-based level shifter 
           [0013]      FIG. 3  is a block diagram illustrating an arrangement using of an exemplary level shifter block. 
           [0014]      FIG. 4  is a circuit diagram illustrating an exemplary level shifter circuit. 
           [0015]      FIG. 5A  is a graph illustrating performance characteristics consistent with the prior art level shifter of  FIG. 1 . 
           [0016]      FIG. 5B  is a graph illustrating performance characteristics consistent with the prior art level shifter  FIG. 2 . 
           [0017]      FIG. 5C  is a graph illustrating performance characteristics consistent with the exemplary level shifter of  FIG. 4 . 
           [0018]      FIG. 6  is a flow chart illustrating an exemplary procedure for level shifting. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Aspects are disclosed in the following description and related drawings directed to specific embodiments. Alternate embodiments may be devised without departing from the scope of the invention. Additionally, well-known elements will not be described in detail or will be omitted so as not to obscure the relevant details. 
         [0020]    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. 
         [0021]    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. 
         [0022]    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 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. 
         [0023]    Accordingly, with reference to the figures,  FIG. 1  and  FIG. 2  show conventional inverter circuit  100  and conventional latch circuit  200  respectively. It is known that an inverter such as the inverter circuit  100  can be used for level shifting. A signal input  101  can be input to the gates of elements  102  and  103 , which are transfer elements such as field effect transistors (FETs) having opposite polarities and can be coupled between a voltage source such as Vdd 12   110  and a reference voltage such as ground  111 . A level shifted and inverted output can be obtained from, for example, the junction between elements  102  and  103 , such as at output  104 . Further, conventional level shifting can be accomplished with a latch circuit, such as latch circuit  200  shown in  FIG. 2 . 
         [0024]    A differential input signal, that is a signal pair having complimentary, for example, opposite polarity, signals that represent complimentary versions of the same information, such as signals input positive (inp)  201  and input negative (inn)  202  can be input to elements  203  and  204  respectively of  FIG. 2 . The inputs can be a high voltage signal level and the shifter is for shifting the levels down, for example to the 1.2 volt logic domain. It should be noted that the latch circuit consists of negative metal oxide semiconductor (NMOS) elements and positive MOS (PMOS) element, each having different drive characteristics. Further, since the NMOS elements trigger during a high transition, there are different gate delays for high and low transitions and thus the rise and fall times are not symmetric or matched. When the input is positive, the corresponding one of elements  203  and  204  are turned on, coupling the corresponding one of output nodes  207  and  208  to ground  111  and biasing the corresponding one of opposite polarity elements  205  and  206  to an ON state, whereupon the other of the corresponding one of output nodes  207  and  208  is brought to the approximate level of source voltage  110 . The other of the opposite polarity elements  205  and  206  is biased OFF thus “latching” the result on the output nodes  207  and  208 . 
         [0025]    As will be appreciated, particularly with reference to the rise time and fall time graphs shown in  FIG. 5  and described in greater detail hereinafter, there are drawbacks to the use of conventional circuits shown in  FIG. 1  and  FIG. 2  for performing level shifting in high speed communications applications. For certain applications, such as high speed universal serial bus (USB) 2.0 receiver applications, a variations associated with rise fall mismatches, on the order of less than 40 ps is tolerated for high speed receiver operating, for example, at  450  MHz. In particular, due to the above described mismatches between PMOS and NMOS characteristics and the differences in drive characteristics and gate characteristics for turning on and turning off, timing variances can arise that, in high speed circuits, can cause distortion and anomalies such as jitter, inter-symbol interference (ISI), and the like, which, in turn, can lead to synchronization issues and data errors in clock data recovery (CDR). Such timing errors are generally due to the asymmetrical relation of the circuit elements to the input level transitions and the corresponding unbalanced propagation of the signal transitions through the prior art circuits. What is provided by various embodiments described herein is a relatively simple solution to eliminating the timing variances, balancing the signal transitions, and thus reducing the probability of synchronization and error issues. 
         [0026]    A block diagram of an exemplary system  300  can provide a better understanding of where balancing can be accomplished with reference to  FIG. 3 . A differential amplifier  310  can have inputs for positive and negative complimentary levels of a logic signal, shown as inp  311  and inn  312 . The amplifier  310  can further have outputs  321  and  322  that can be input to various circuits, such as high input impedance transconductance amplifiers and the like to maintain current requirements and other requirements. Ultimately, the input signals are input to a symmetric level shifter  330  for providing level shifting from the line level to the appropriate internal logic level and for passing the signal to a timing sensitive circuit such as a clock and data recovery (CDR) circuit or the like. 
         [0027]    An exemplary circuit  400 , various embodiments and portions thereof, providing symmetric level shifting, symmetric level down shifting, balancing, and the like for an input logic signal, such as a differential signal constituted as a pair of signals corresponding to complimentary logic levels is shown in  FIG. 4 . Symmetric portions A and B can correspond to, for example, input portions for receiving respective ones of the complimentary logic levels associates with Inp  401  and Inn  402 . It should be noted that in accordance with a level down shifting circuit, the input voltage levels can be greater than the internal circuit levels, which for illustrative purposes are shown, for example as 1.2 volts, or the like. The inputs  401  and  402  are also coupled respectively to gates of elements  410  in each of the portions A and B for controlling the provision of a level shifted output at output nodes  403  and  404 . With reference to portion A as an example, when input inp  401  falls to a voltage level less than Vdd 12 −V T , which is the voltage drop across element M 1   422 , M 1  is turned on. M 2  turns OFF when the level falls below vdd 12 +V T . It should be noted that the purpose of elements M 1  and M 2  is to maintain the DC logic levels in the circuit. For example, if Inp  401  is low then M 1  is ON and M 2  is OFF to ensure that outp  403  is low and outn  404  is high. Similarly, if Inp  401  is high then M 1  is OFF and M 2  is ON to ensure that outp  403  is high and outn  404  is low. During differential switching at  401  and  402 , the capacitor  421  by-passes the switch M 1  and forces outp  403  to follow Inp  401  and outn  404  to follow Inn  402 , which has the effect of destabilizing the latch and completing the transition. 
         [0028]    Since capacitors  421  have a high degree of matching and provide a balancing action for the latch, the rise/fall times at node  403  and  404  are correspondingly balanced or matched. The output provided on output node  403  through element M 1   422  of input portion  420  is configured, through the advantageous action of capacitor  421  and the previous latched level, to follow the input transition as nearly as possible. In other words, due to the simple placement of the capacitor  421  and the latching function of logic element  430 , outp  403  follows Inp  401 . 
         [0029]    After the input and output levels are stabilized at a low level, capacitor  421  is effectively open. The gate of M 1   422  is permanently coupled to a source voltage level Vdd 12   410  having the effect of permanently biasing M 1   422  such that any high level above vdd 12 −V T  on the input will cause M 1  to be placed into the OFF state. The negative voltage at output node  403  is coupled to the gates of elements  432  and  434  of logic element  430 , which are coupled between vdd 12   410  and ground  411 . It should be noted that logic element  430  is shown in  FIG. 4 , for example, embodied as a latch. An exemplary latching means can be supported with such a circuit structure as illustrated and described herein and other equivalent structures as described herein, such as processors, programmed logic and the like, as would be appreciated by one of ordinary skill. The output node  403  is further coupled to the junction node  435  between elements  431  and  433 . With reference now to portion B and the corresponding one of input portion  420 , the negative level on the gates of elements  432  and  434 , and the complimentary level of the input Inn  402  and action of element  422 , described above, holds the output node  404  at a high level. 
         [0030]    The advantages of various embodiments as described herein are apparent during a transition from one of the complimentary logic levels to another. Of particular interest in the present example is when, for example, the input Inp  401  transitions from a low level, for example a level greater than vdd 12 −V T , to a high level and, correspondingly, the high input level at Inn  402  transitions from a high level to a low level. With reference to portion A as an example, when the low level at Inp  401  begins to rise, a charge forms across capacitor  421 , acting as a transition balancing means or circuit. In effect, the charge across capacitor  421  acting together with the previously latched level, advantageously smoothes the transition of the level-shifted level on output node  403  from a low to a high level by allowing the level-shifted level to begin rising immediately in response to the change in the input signal at Inp  401 . As the input level rises, for example, above vdd 12 +V T , M 2  turns on and the level at output node  403  rises, the biasing of elements  432  and  434  is reversed, driving the level at output node  404  to a low level. Correspondingly, by a similar action but in response to a complimentary input of opposite polarity in the input circuit  420  coupled to the input Inn  402 , the biasing of the elements  431  and  433  is also reversed driving the output node  403  to a high level. 
         [0031]    In view of the above described transition balancing and by way of comparison, it can be seen from the graphs of  FIG. 5A ,  FIG. 5B  and  FIG. 5C  that acceptable transitions can be obtained from a simple combination of circuit elements. For example, with reference to  FIG. 5A  a transient response of the prior art inverter circuit of  FIG. 1  is shown. After the level transition begins from high to low, an overshoot appears at  511  based on the characteristics of the circuit elements and the effect of the reversal acting between the two elements. It can be seen that due to differences in the action of the characteristics based on the polarity of the input signal it can be seen that for the complimentary signal transition from low to high, an overshoot appears at  512 , but it is slightly delayed. While the transition smoothes as it approaches completion, it will be noted that the completion point for the high to low transition at  513  occurs earlier than the low to high transition completion point at  514 . Further the balance point  515  is delayed around 250 ps, and is negatively biased, which for an exemplary high tolerance receiver circuit such as a USB 2.0 circuit having a variation tolerance of only 40 ps, would be unacceptable. 
         [0032]    With reference to  FIG. 5B , a transient response of the prior art latch circuit of  FIG. 2  is shown. The latch circuit shows a slight improvement in the positive over shoot at  521 , however negative overshoot is still present at  522 . The completion points  524  and  523  are more closely balanced, however, the transition balance point is still significantly delayed for a tight tolerance application. 
         [0033]      FIG. 5C  shows the balanced transition associated with an exemplary transition balancing circuit as shown and described above in connection with  FIG. 4 . Both the high to low and low to high transition points  531  and  532  begin smoothly and at roughly the same instant. Since the rate of rise and fall are balanced, the transition balance point  535  occurs about mid voltage with little or no deviation from the specified rise and fall balance time of, for example, 360 ps. The transition completion points  533  and  534  are similarly smooth. With the exemplary balancing circuit as described herein in accordance with various embodiments, such as in a 45 nm process embodiment, a duty cycle variation can be reduced to around 0.5%. With 28 nm process embodiments further advantages are to be expected. 
         [0034]    It will be appreciated that an embodiment can include an exemplary procedure or method  600 , exemplary portions thereof which are illustrated in  FIG. 6 . After start at  601 , a differential logic signal including two complimentary logic levels can be received at  602 . It will be appreciated that in accordance with a level down shifting circuit, the logic signal levels may exceed the internal logic levels and are thus shifted down in level. Other configurations are possible where the levels are shifted up or the like. The two complimentary logic levels are shifted to level shifted levels, such as down shifted at  603 . During the transition of the complimentary logic levels and the level shifted versions thereof, such as from high to low at one input and from low to high on the complimentary input, the corresponding rise and fall times can be balanced at  604 , by operation of, for example, the transition balancing circuit, capacitor, and the like and making use of a previously stored level, as described herein above. After the transition, the level shifted versions of the complimentary logic levels are held, latched, or the like, and outputted at  605 . Although the exemplary procedure is indicated as ending at  606 , it will be appreciated that the procedure can be performed for every transition of the complimentary logic levels. While the procedure  600  is shown with various actions or sub-procedures, embodiments are not limited solely to those described herein. 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 using suitable structures and procedures, for example, as described herein. 
         [0035]    It will be further appreciated that the level shifter 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 level shifter as disclosed herein such as by being integrated into at least one semiconductor die associated with circuits in such devices. 
         [0036]    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. 
         [0037]    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 present invention. 
         [0038]    To the extent applicable, 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. 
         [0039]    Accordingly, an embodiment can include a computer readable media embodying a method for balancing a transition of a level shifted signal. Accordingly, the invention is not limited to illustrated examples and any means for performing the functionality described herein are included in embodiments. 
         [0040]    The foregoing disclosed devices and methods can be designed using computer aided design methods and systems and are configured into, for example, GDSII, GERBER, and similar computer files output from compute aided design systems, and stored on a computer readable media. These files are in turn provided to fabrication handlers who fabricate devices based on these files. The device designs can be replicated onto semiconductor wafers, for example, as stand alone circuits or as parts of other circuits, and can then be cut into semiconductor die and packaged into a semiconductor chip. The chips can then be employed in various devices. 
         [0041]    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 may be described or claimed in the singular, the plural is contemplated unless limitation to the singular is explicitly stated.