Abstract:
A system and method is disclosed for adaptively adjusting a duty cycle of a signal between a first and second chip in a 3D architecture/stack for adaptively calibrating a chip in a 3D architecture/stack. In one embodiment, the system includes a first chip and a second chip located within the 3D chip stack, wherein the first chip generates a calibration signal, the second chip receives the calibration signal and compares it to a reference signal to generate a comparison signal that further compared to a reference duty signal to generate a reference duty comparison signal, that is then provided to the first chip to generate a drive signal that adjusts a duty cycle of the calibration signal.

Description:
RELATED PATENT APPLICATIONS 
       [0001]    This application is a continuation of U.S. Utility application Ser. No. 14/489,508, filed on Sep. 18, 2014 and will issue as U.S. Pat. No. 9,231,585 on Jan. 5, 2016, which is a divisional and claims benefit of priority under 35 U.S.C. §119(e) to Utility application Ser. No. 13/355,024, now U.S. Pat. No. 8,866,508, entitled “System and Method for Calibrating Chips in a 3D Chip Stack Architecture”, filed on Jan. 20, 2012, each of which is incorporated by referenced herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    Chip architecture is moving beyond the traditional uniplanar arrangement of chips, i.e., where chips are positioned in a single plane. Three-dimensional (“3D”) architectures are becoming more common and offer many advantages over uniplanar architectures. 3D architectures or 3D chip stacks, as used herein, encompass architectures where chips are positioned on more than one plane and may be integrated both horizontally and vertically into a single circuit. Additionally, 3D architectures also encompass the situation where there exists more than one vertical stack of chips in the circuit. 3D architectures present a variety of challenges for calibrating the chips in the circuit. Connections between chips in a 3D architecture typically involve routing connections through a silicon interposer and at least one Through Silicon Via (“TSV”). Additionally, the chips in a 3D architecture may be of different varieties, such as, but not limited to, processors, memory (of various types and capacities), digital signal processors (“DSP”), radio frequency (“RF”) modules, etc., as would be familiar to those of skill in the art. 
         [0003]    One of the challenges of a 3D architecture is that certain chips may be more difficult to connect to for standard calibration or testing. Furthermore, the different varieties of chips may cause an unbalanced load between any two given chip sets in the 3D architecture. Thus, an optimal driving strength between a first set of two given chips may not be the same as the optimal driving strength between a second set of chips due to the difference in chip-to-chip loading. Additionally, there may be a range of operating speeds which need to be accounted for. 
         [0004]    Traditional calibration methods for uniplanar architectures use a direct current (“DC”) method along with optimization methodologies such as board routing to minimize load unbalances to meet specified maximum operating speeds. This results in a fixed driving strength for all chips for all operating regimes. However, in a 3D architecture, for example, routing and loading are dependent upon the permutation of chips in the 3D architecture. Thus, if the traditional calibration methods are used in a 3D architecture, serious penalties may accrue such as a power penalty during times when the operating speed is less than the specified maximum as well as degraded signal integrity and/or a simultaneous switching output (“SSO”) problem due to a minimal time for peak current. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a simplified schematic diagram of a chip-to-chip calibration arrangement according to an embodiment of the present subject matter. 
           [0006]      FIG. 2  is a pictorial view of a chip-to-chip calibration arrangement according to an embodiment of the present subject matter. 
           [0007]      FIG. 3  is a simplified schematic diagram of a single chip calibration arrangement according to an embodiment of the present subject matter. 
           [0008]      FIG. 4  is a pictorial view of a single chip calibration arrangement according to an embodiment of the present subject matter. 
           [0009]      FIG. 5  is a representation of a voltage level profile according to an embodiment of the present subject matter. 
           [0010]      FIG. 6  is a flow diagram of methods for a chip-to-chip calibration arrangement according to an embodiment of the present subject matter. 
           [0011]      FIG. 7  is a flow diagram of methods for a single chip calibration arrangement according to embodiments of the present subject matter. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    With reference to the figures where like elements have been given like numerical designations to facilitate an understanding of the present subject matter, the various embodiments of a system and method for calibrating chips in a 3D chip stack architecture are described. 
         [0013]    Embodiments of the present subject matter overcome the challenges associated with implementing calibration methods for 3D architectures and avoid the penalties that that must be paid when using traditional calibration methods that result in fixed driving strengths for all chip pairs in the 3D architecture at all times and operating conditions. In one embodiment, described in further detail below, the driving strength between a first and second chip in a 3D architecture is adaptively adjusted by sending a first signal from the first chip to the second chip, comparing the signal received by the second chip with a voltage input high (“VIH”) signal and a voltage input low (“VIL”) signal, comparing a version of each of the resultant signals with a version of a VIH duty signal and a VIL duty signal, respectively, sending the resultant compared signals back to the first chip which causes circuitry on the first chip to send a modified first signal in response to the received compared signals. In another embodiment, described in further detail below, the driving strength between a first and second chip in a 3D architecture, where the first chip may be a “near end” chip and the second chip may be a “far end” chip, is adaptively adjusted by sending a first signal from the first chip to each of four loops, two without output loads and two with output loads on the second chip. Thus, each of the loops carries a loop signal which is compared, on the first chip, with the appropriate VIH or VIL signal, as described below, and versions of the resultant signals are then compared with a version of the appropriate VIH or VIL duty signal. The resultant compared signals are then used to modify circuitry on the first chip to send a modified first signal. 
         [0014]    With attention now directed at  FIG. 1 , a simplified schematic diagram of a chip-to-chip calibration arrangement according to an embodiment of the present subject matter is presented. One of skill in the art will readily understand that the simplified schematic diagrams do not represent a detailed view of all circuitry and devices on a chip. Chip A,  110 , Chip B,  120 , and Chip C,  130 , are shown where Chip A includes transmission circuit  101  and Chip B includes receiver circuit  102 . Likewise, Chip C may also include a receiver circuit (not shown for sake of clarity) similar to receiver circuit  102  on Chip B. The transmission circuit on Chip A and the receiver circuit on Chip B are connected through a silicon interposer and/or one or more TSVs, shown as references  10 A and  10 B. In the case where Chip C also includes a receiver circuit, similar connections would be made to Chip C. Another connection, shown as reference  10 S, also connects the receiver circuit on Chip B with the transmission circuit on Chip A. The same connection  10 S would also connect to Chip C where Chip C included a receiver circuit. These connections, or pathways, will be discussed in further detail below. The transmission circuit  101  includes adjustable drivers  111  and  112 , and logic and driving code control  113 . The receiver circuit  102  includes receiver comparators  121  and  122 , duty generator counter  123 , duty-to-voltage converter low pass filters  141 ,  142 ,  151 , and  152 , and comparators  161  and  162 . 
         [0015]    In order to calibrate chips in a 3D architecture or chip stack, each driver  111  and  112  sends a signal, sometimes referred to herein as a VIH calibration signal or a VIL calibration signal, respectively, to Chip B through separate connections across the silicon interposer/TSV  10 A and  10 B, respectively. The VIH and VIL calibration signals are received by a respective receiver comparator  121  and  122 . Receiver comparator  121  also has an input for a VIH signal, compares the VIH signal with the VIH calibration signal received from driver  111 , and outputs a signal, sometimes referred to herein as a VIH comparison signal. Similarly, receiver comparator  122  also has an input for a VIL signal, compares the VIL signal with the VIL calibration signal received from driver  112 , and outputs a signal, sometimes referred to herein as a VIL comparison signal. As will be discussed in further detail below with respect to  FIG. 5 , the VIH and VIL comparison signals operate on the transition time between a low voltage value and a high voltage value; either increasing or decreasing the transition time and thus adaptively changing the duty value based on the current operating conditions. 
         [0016]    The VIH comparison signal is operated on by duty-to-voltage converter low pass filter  141  and the resultant output signal, which is representative of the VIH comparison signal, is input to comparator  161 . Likewise, The VIL comparison signal is operated on by duty-to-voltage converter low pass filter  142  and the resultant output signal, which is representative of the VIL comparison signal, is input to comparator  162 . 
         [0017]    Duty generator counter  123  sends a VIH duty signal to duty-to-voltage converter low pass filter  151  and the resultant output signal, which is representative of the VIH duty signal, is input to the comparator  161 . Duty generator counter  123  also sends a VIL duty signal to duty-to-voltage converter low pass filter  152  and the resultant output signal, which is representative of the VIL duty signal, is input to the comparator  162 . The VIH and VIL duty signals are representative of the current target duty cycle of Chip B. Comparators  161  and  162  compare their respective input signals and send a VIH duty comparison signal and a VIL duty comparison signal, respectively, to the logic and driving code control  113 . The VIH and VIL duty comparison signals travel from Chip B to Chip A through a shared channel, sometimes referred to herein as a feedback channel, through the silicon interposer and/or one or more TSVs  10 S. 
         [0018]    The logic and driving code control  113  receives the VIH and VIL duty comparison signals and, in response thereto, sends a signal, sometimes referred to herein as a driving signal, which modifies the state of operation of adjustable drivers  111  and  112 . Consequently, the drivers  111  and  112  send a modified VIH calibration signal and a modified VIL calibration signal, respectively, to Chip B. In certain conditions, the driving signal may be such that it does not modify the drivers  111  and  112  so that the VIH and VIL calibration signals do not change. Similarly, the driving signal may only modify one of the two drivers  111  and  112 . 
         [0019]    The above-described embodiment allows for on speed operation response of the chips to the then-current real load and transition times. Furthermore, the above-described system allows for adaptive adjustment of the driving strength between Chips A and B. Since the system includes an independent receiver comparator for VIH and VIL, there is no concern for propagation delay. Also, the transition times may be adjusted independently. The duty generator counter  123  outputs the VIH and VIL duty signals that may represent the high speed performance of Chip B and the VIH and VIL duty signals are compared in comparators  161  and  162 , respectively, with slower signals from the receiver comparators  121  and  122 . Thus, the shared feedback channel  10 S has the advantage of being able to operate at a speed lower than the design maximum while conserving valuable TSV resources in the event the number of chips in the 3D architecture increases. 
         [0020]    Directing attention now to  FIG. 2 , a pictorial view of a chip-to-chip calibration arrangement according to an embodiment of the present subject matter is depicted. Silicon interposer and one or more TSVs are represented by reference number  10 . Also depicted are Chip A  110 , Chip B  120 , Chip C  130 , Chip D  140 , Chip E  150 , Chip F  160 , Chip G  170 , and Chip H  180 . Those of skill in the art will understand that this particular arrangement is for convenience of explanation only and in no way is the present subject matter intended to be restricted to this particular arrangement of chips. Pathways through interposer/TSV  10  between specific pairs of chips are shown as  20 A,  20 B,  20 C, and  20 D. For the sake of clarity, in  FIG. 2  pathway  20 A represents both pathways  10 A and  10 B in  FIG. 1 , i.e., the outputs of the transmission circuit  101  to the inputs of the receiver circuit  102 . Likewise, pathways  20 B,  20 C, and  20 D represent similar multiple connections between the respective chips. In  FIG. 2 , pathway  20 A connects Chips A and E while pathway  20 D connects Chips A and H. Also shown in  FIG. 2  are connections between Chips A and B, Chips A and C, and Chips A and D, although no reference numbers are shown for the sake of clarity. These pathways also represent similar multiple connections between the respective chips. Additionally, shared feedback channel  10 S, which is described above, is shown connecting each of the chips, i.e, the output of each chip&#39;s receiver circuit to the input of each chip&#39;s transmission circuit. 
         [0021]    In  FIG. 2 , Chip A  110  is shown having transmission circuit  201  while Chip H  180  is shown having receiver circuit  202 . Transmission circuit  201  as shown differs from transmission circuit  101  in  FIG. 1  in that transmission circuit  201  includes a set of drivers for each connection to the other chips, i.e., one set of drivers for the connection to Chip B, one set of drivers for the connection to Chip C, etc. In another embodiment, transmission circuit  201  will be similar to transmission circuit  101  in  FIG. 1  with the exception that the single set of outputs of the transmission circuit  201  is switched by any known switching arrangement, so that the signals sent from the output of the transmission circuit  201  are directed towards only one selected other chip in the 3D architecture. In certain embodiments, receiver circuit  202  will be similar to the receiver circuit  102  except that the receiver circuit  202  will include multiple inputs to the receiver comparators in order to establish a connection with each of the other chips in the 3D architecture. While embodiments of the present subject matter contemplate that a plurality of Chips A-H may include a transmission circuit  201  and/or a receiver circuit  202 , only the transmission circuit  201  on Chip A  110  and the receiver circuit  202  on Chip H  180  are shown for clarity. Thus, in an embodiment where each chip has a transmission and receiver circuit, one of skill in the art will readily understand that chip-to-chip calibration between any pair of chips, as described above with reference to  FIG. 1 , can be undertaken. 
         [0022]    Looking now to  FIG. 3 , a simplified schematic diagram of a single chip calibration arrangement according to an embodiment of the present subject matter is shown. One of skill in the art will readily understand that the simplified schematic diagrams do not represent a detailed view of all circuitry and devices on a chip. Chip A,  110 , Chip B,  120 , and Chip C,  130 , are shown. Chip A includes calibration driver circuitry  301 , duty generator counter  323 , duty-to-voltage low pass filters  341 A,  341 B,  341 C,  342 A,  342 B,  342 C, average comparators  361  and  362 , and logic and driving code control  113 . Chip B includes circuit devices  321  and  322  which represent loading on the indicated circuits/connections. Likewise, Chip C may also include a similar circuitry to Chip B, although such circuitry is not shown for sake of clarity. Chip A and Chip B are connected through a silicon interposer and/or one or more TSVs, shown as references  10 E and  10 F. In the case where Chip C also includes load circuitry, similar connections would be made between Chip A and Chip C. 
         [0023]    The calibration driver circuitry on Chip A includes circuit devices  31 A,  31 B,  31 C, and  31 D, each of which receives a signal from an driver in a transmission circuit, for example transmission circuit  101  shown in  FIG. 1 , such as adjustable driver  111  (for circuit devices  31 A and  31 C) and adjustable driver  112  (for circuit devices  31 B and  31 D). Chip A further includes comparators  32 A,  32 B,  32 C, and  32 D. Comparators  32 A and  32 C each include a connection to a VIH signal while comparators  32 B and  32 D each include a connection to a VIL signal. 
         [0024]    Chip A also includes duty generator counter  323 , which provides a VIH duty signal and a VIL duty signal, in a manner similar to duty generator  123  described above with respect to  FIG. 1 . Also included on Chip A are duty-to-voltage low pass filters  341 A,  341 B,  341 C, and  342 A,  342 B,  342 C, which operate in a manner similar to duty-to-low pass filters  141 ,  142 ,  151 , and  152  described above with respect to  FIG. 1 . In a particular embodiment, the inputs to these low pass filters are paired with the outputs of the comparators  32 A,  32 B,  32 C, and  32 D and the outputs of the duty generator counter  323  as follows:  341 A with  32 A;  341 B with VIH duty;  341 C with  32 C;  342 A with  32 B,  342 B with VIL duty; and  342 C with  32 D. Those of skill in the art will readily recognize that certain other variations of these pairings are possible while being consistent with the teachings of the present subject matter. The outputs of duty-to-voltage low pass filters  341 A,  341 B,  341 C are input to average comparator  361  while the outputs of duty-to-voltage low pass filters  342 A,  342 B,  342 C are input to average comparator  362 . Average comparators  361  and  362  output signals, similar to the VIH duty comparison signal and the VIL duty comparison signal, respectively, as described in  FIG. 1  with reference to comparators  161  and  162 , respectively. The details of the operation of the average comparators  361  and  362  will be discussed in further detail with respect to  FIG. 5  below. The output of the average comparators  361  and  362  are sent to logic and driving code control  113  which operates as described above with respect to  FIG. 1 . Consequently the signal input to the circuit devices  31 A,  31 B,  31 C, and  31 D will be modified. 
         [0025]    Also shown in  FIG. 3  are loop  1 , loop  2 , loop  3 , and loop  4 . Loop  1 , as shown, includes circuit device  31 A and comparator  32 A. Loop  2 , as shown, includes circuit device  31 B and comparator  32 B. Loop  3 , as shown, includes circuit device  31 C, interposer/TSV  10 E, load circuit  321 , interposer/TSV  10 E, and comparator  32 C. Loop  4 , as shown, includes circuit device  31 D, interposer/TSV  10 E, load circuit  321 , interposer/TSV  10 E, load circuit  322 , and comparator  32 D. It should be noted that a signal traversing either loop  1  or loop  2  does not have any output loading while a signal traversing either loop  3  or loop  4  will have a higher path load. These path loads will be described in more detail with respect to  FIG. 5  below. 
         [0026]    The above-described embodiment allows for on speed operation response of the chips to the then-current real load and transition times. Furthermore, the above-described system allows for adaptive adjustment of the driving strength between Chips A and B. Since the system includes an independent receiver comparator for VIH and VIL, there is no concern for propagation delay. Also, the transition times may be adjusted independently. The above-described embodiment allows for a single-chip solution to the calibration challenges of a 3D architecture since the transmission circuit, receiver circuit, calibration driver, and other mentioned devices all reside on Chip A. Additionally, in an embodiment where Chip A is at the “near end” of the 3D architecture and the only connection is to Chip B which is at the “far end” of the 3D architecture, no other chips need to be connected in the calibration circuit. This is shown below in  FIG. 4 . 
         [0027]      FIG. 4  is a pictorial view of a single chip calibration arrangement in a 3D architecture according to the above-described embodiment of the present subject matter. Shown in  FIG. 4  are Chip A  110 , Chip B  120 , Chip C  130 , Chip D  140 , Chip E  150 , Chip F  160 , Chip G  170 , and Chip H  180 . Those of skill in the art will understand that this particular arrangement is for convenience of explanation only and in no way is the present subject matter intended to be restricted to this particular arrangement of chips. Chip A includes the operation and details of calibration driver  301 , as discussed above with respect to  FIG. 3 , with the addition of adjustable drivers as shown (reference numbers omitted for clarity). Chip A also includes logic and driving code control  113  the output of which is used to modify the adjustable drivers as discussed above with respect to  FIG. 1 . Additionally, Chip A includes receiver circuit  402  which incorporates the operations and details of the duty generator counter, duty-to-voltage converter low pass filters, and average comparators discussed above with respect to  FIG. 3 . 
         [0028]    Included in Chip A is transmission driver  401  which further include adjustable drivers as shown (reference numbers omitted for clarity). In this embodiment, calibration driver  301  is used only for calibration of the chips in the 3D architecture and is not used for information transfer between chips. Transmission driver  401  is used for information transfers between chips. It will be noted that the output of logic and driving code control  113  also modifies the adjustable drivers in the transmission driver so that they are modified the same way as the adjustable drivers in the calibration driver. 
         [0029]      FIG. 4  also includes silicon interposer/TSV  10 . Connections between Chip A, shown here to be at the “near end” of the 3D architecture, and Chip H, shown here to be at the “far end” of the 3D architecture, includes pathways  10 G,  10 H,  10 J, and  10 K through the silicon interposer/TSV. Chip H also includes circuit devices  421  and  422  which are representative of loads on their respective circuits/connections. Since Chips A and H are at opposite ends of the 3D architecture chip stack, calibration between these two devices is sufficient to calibrate the 3D architecture chip stack since the effects of all of the intervening chips will be necessarily taken into account. 
         [0030]    With reference now to  FIG. 5 , a representation of a voltage level profile according to an embodiment of the present subject matter is presented. Line  51  represents a conventional profile of a duty cycle as may be produced by a duty generator such as the duty generator counter  123  in  FIG. 1  or the duty generator counter  323  in  FIG. 3 . The profile  51  transitions between a low voltage value  5 L, which may be electrical ground, and a high voltage value  5 H, which may be a supply voltage level, as shown in  FIG. 5 . Additionally, the profile  51  is at or above a voltage input high (“VIH”) signal, shown as VIH in  FIG. 5 , and a voltage input low (“VIL”) signal, shown as VIL in  FIG. 5 , for a specific interval of time. The duty cycle for profile  51  may be changed by altering the transition times (rising time and falling time) between  5 L and  5 H thus changing the specific interval of time that the profile  51  is above VIH and VIL. 
         [0031]    For the embodiment shown in  FIG. 1 , if the duty cycle for profile  51  is to be increased one way to accomplish this is to shorten the transition times between low voltage value  5 L and high voltage value  5 H. For example, if the transition time in  FIG. 5  for the profile  51  is shortened, such as for the profile designated as  52 , the time spent above VIH and VIL is increased thus increasing the duty cycle so that the duty cycle of profile  52  is greater than the duty cycle of profile  51 . Similarly, if the transition time in  FIG. 5  for the profile  51  is lengthened, such as for the profile designated as  53 , the time spent above VIH and VIL is decreased thus decreasing the duty cycle so that the duty cycle of profile  53  is less than the duty cycle of profile  51 . 
         [0032]    For the embodiment shown in  FIG. 3 , there are four loops as discussed above. Representative duty cycles for each of the loops are shown. For each of loop  1  and loop  2 , the representative duty cycle is shown as profile  52 . For each of loop  3  and loop  4 , the representative duty cycle is show as profile  53 . In this embodiment, a representative profile of an estimated average of loop  1  and loop  3  is shown as the profile  54 . In an embodiment, profile  54  is also a representative profile of an estimated average of loop  2  and loop  4 . 
         [0033]    The rising time for each of loop  1 , loop  2 , loop  3 , and loop  4  for the embodiment shown in  FIG. 3  are proportional to the resistances and capacitances seen by the respective loop and can be calculated as follows: 
         [0000]      LOOP 1 RISING TIME α  R   PU   C   ChipA  
 
         [0034]    where R PU  is a pull-up resistance and C ChipA  is a capacitance value for Chip A. As stated above, loop  1  has no output loading off of Chip A. 
         [0000]      LOOP 2 FALLING TIME α  R   PD   C   ChipA  
 
         [0035]    where R PD  is a pull-down resistance and C ChipA  is a capacitance value for Chip A. As stated above, loop  2  has no output loading off of Chip A. 
         [0000]      LOOP 3 RISING TIME α  R   PU ( C   ChipA   +C   Sil/TSV   +C   ChipB   +C   Sil/TSV   +C   ChipA )
 
         [0036]    where R PU  is a pull-up resistance, C ChipA  is a capacitance value for Chip A, C Sil/TSV  is a capacitance value for the interposer/TSV, and C ChipB  is a capacitance value for Chip B. As shown in  FIG. 3  and stated above, loop  3  has off chip loading including traversing the silicon interposer/TSV path twice as well as a load with respect to Chip B. This may be seen as roughly a double path loading value. 
         [0000]      LOOP 4 FALLING TIME α  R   PD ( C   ChipA   +C   Sil/TSV   +C   ChipB   +C   Sil/TSV   +C   ChipA )
 
         [0037]    where R PD  is a pull-down resistance, C ChipA  is a capacitance value for Chip A, C Sil/TSV  is a capacitance value for the interposer/TSV, and C ChipB  is a capacitance value for Chip B. As shown in  FIG. 3  and stated above, loop  3  has off chip loading including traversing the silicon interposer/TSV path twice as well as a load with respect to Chip B. This may be seen as roughly a double path loading value. 
         [0038]    Using the above equations, the average of loop  1  and loop  3  is: 
         [0000]      AVG LOOPS 1 &amp; 3 RISING TIME α  R   PU ( C   ChipA   +C   Sil/TSV +( C   ChipA   +C   ChipB )/2)
 
         [0039]    Likewise, the average of loop  2  and loop  4  is: 
         [0000]      AVG LOOPS 2 &amp; 4 FALLING TIME α  R   PD ( C   ChipA   +C   Sil/TSV +( C   ChipA   +C   ChipB )/2)
 
         [0040]    For the embodiment represented in  FIG. 3 , the specification for the duty generator counter rising time for Chip A and Chip B is: 
         [0000]      DUTY GEN. SPEC RISING TIME FOR FAR END α  R   PU ( C   ChipA   +C   Sil/TSV   +C   ChipB )
 
         [0041]    Likewise, for the embodiment represented in  FIG. 3 , the specification for the duty generator counter falling time for Chip A and Chip B is: 
         [0000]      DUTY GEN. SPEC FALLING TIME FOR FAR END α  R   PD ( C   ChipA   +C   Sil/TSV   +C   ChipB )
 
         [0042]    As can be seen from the equations above, there is only a minor difference between the average rising and falling times and the specification rising and falling times, respectively. The difference, or inaccuracy, between the two can be represented by the following equations: 
         [0000]      RISING TIME INACCURACY= R   PU ( C   ChipA   −C   ChipB )/2) 
         [0000]      FALLING TIME INACCURACY= R   PD ( C   ChipA   −C   ChipB )/2) 
         [0043]    This inaccuracy is well within the capability of the single chip embodiment shown in  FIG. 3  to maintain and/or alter the duty cycle of the 3D architecture/chip stack. 
         [0044]    With attention now drawn to  FIG. 6 , a flow diagram  600  of methods for a chip-to-chip calibration arrangement in a 3D chip stack/architecture according to an embodiment of the present subject matter is shown. At block  602 , on a first chip, a VIH signal is compared with a first signal to produce a VIH comparison signal. The first signal typically is a VIH calibration signal from a second chip, as discussed above with reference to  FIG. 1 . At block  604 , a VIL signal is compared with a second signal to produce a VIL comparison signal. The second signal typically is a VIL calibration signal from the second chip, as discussed above with reference to  FIG. 1 . At block  606 , a first voltage signal representative of the VIH comparison signal is compared with a second voltage signal representative of a reference VIH duty signal to thereby produce a VIH duty comparison signal. At block  608 , a first voltage signal representative of the VIL comparison signal is compared with a second voltage signal representative of a reference VIL duty signal to thereby produce a VIL duty comparison signal. At block  610 , the VIH and VIL duty comparison signals are sent to the second chip. At block  612 , the VIH and VIL duty comparison signals are received at the second chip and in response to the VIH and VIL duty comparison signals, the second chip sends a modified first signal to the first chip. 
         [0045]    In a further embodiment, at block  614 , the modified first signal alters a duty cycle of the first chip by changing a transition time between a low level and a high voltage level. 
         [0046]    Now considering  FIG. 7 , a flow diagram  700  of methods for a single chip calibration arrangement in a 3D chip stack/architecture according to embodiments of the present subject matter is presented. In one embodiment at block  702 , on a first chip in a 3D architecture/chip stack, a calibration signal is generated using a calibration driver circuit. At block  704 , the calibration signal is sent to a first, a second, a third, and a fourth loop circuit. At block  706  the calibration signal sent to the first, second, third, and fourth loop circuits returns as a first, a second, a third, and a fourth loop signal, respectively. At block  708 , a driving signal is determined based at least on the first, second, third, and fourth loop signals. At block  720 , the calibration driver circuit is adjusted based on at least the driving signal to thereby produce a modified calibration signal. 
         [0047]    In another embodiment, blocks  702 ,  704 , and  706  are as described above. At block  710 , the first, second, third, and fourth loop signals are each individually compared with a predetermined one of two reference signals to thereby determine a respective first, second, third, and fourth comparison signal. As a non-limiting example, the first loop signal is compared with a VIH signal to produce the first comparison signal; the second loop signal is compared with a VIL signal to produce the second comparison signal, the third loop signal is compared with a VIH signal to produce the third comparison signal, and the fourth loop signal is compared with a VIL signal to produce the fourth loop signal. At block  712 , the first, second, third, and fourth comparison signals are converted into a first, a second, a third, and a fourth voltage signal, respectively. At block  714 , in one embodiment, the first and third voltage signals are compared with a first duty reference signal to thereby determine a first control signal. The first duty reference signal may be a voltage signal representative of a VIH duty signal generated by a duty generator counter such as is shown in  FIG. 3 . In an embodiment, the first and third voltage signals are averaged and the average is compared with the first duty reference signal. At block  716 , in one embodiment, the second and fourth voltage signals are compared with a second duty reference signal to thereby determine a second control signal. The second duty reference signal may be a voltage signal representative of a VIL duty signal generated by a duty generator counter such as is shown in  FIG. 3 . In an embodiment, the second and fourth voltage signals are averaged and the average is compared with the second duty reference signal. At block  718 , a driving signal is determined from at least the first and second control signals. At block  720 , as described above, the calibration driver circuit is adjusted based on at least the driving signal to thereby produce a modified calibration signal. 
         [0048]    As shown by the various configurations and embodiments illustrated in  FIGS. 1-7 , a system and method for location and network timing recovery in communications networks have been described. 
         [0049]    While preferred embodiments of the present subject matter have been described, it is to be understood that the embodiments described are illustrative only and that the scope of the invention is to be defined solely by the appended claims when accorded a full range of equivalence, many variations and modifications naturally occurring to those of skill in the art from a perusal hereof.