Abstract:
In a first aspect, a first method is provided for providing multiple termination values using a plurality of binary termination signals. The first method includes the steps of (1) determining a characteristic impedance of a first port by generating a plurality of binary termination signals; and (2) modifying a characteristic impedance of a second port by manipulating one or more of the plurality of binary termination signals. Numerous other aspects are provided.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention relates generally to integrated circuits, and more particularly to methods and apparatus for programmable and/or scalable terminations within integrated circuits. 
   2. Background 
   As the speed of transmission lines included in memory interfaces and buses increases, impedance “matching” become increasingly important. The characteristic impedance of a transmission line is the ratio of voltage to current of a signal moving along the transmission line. By terminating the transmission line with a load (e.g., an impedance) equal to the characteristic impedance of the transmission line, a signal pulse applied to the transmission line is transferred to the load without reflection. The benefits of impedance matching, such as reduced signal reflection and signal loss during signal transmission, are well known to one of skill in the art and is not be described further herein. 
     FIG. 1  is a diagram of a conventional programmable termination circuit  100 . The programmable termination circuit  100  includes an impedance evaluation control circuit  102  coupled to a resistive element  104 . The resistive element  104  is coupled to a port  106  included in a memory system (not shown). The port  106  may correspond to a transmission line included in a bus for the memory system, or a memory interface, for example. 
   The resistive element  104  includes an upper portion  108  of circuitry including a plurality of p-channel metal-oxide semiconductor field-effect transistors (PFETs) P 0 -P 7  connected in parallel between a high voltage level (e.g., V DDQ ) and the port  106 . The PFET P 0  is a default device that is always on and determines (along with NFET N 0 ) the maximum impedance that may be created by the resistive element  104 . The PFETs P 1 -P 7  are arranged in size order such that PFET P 1  is the narrowest transistor and PFET P 7  is the widest transistor. 
   The resistive element  104  includes a lower portion  110  of circuitry including a plurality of n-channel metal-oxide semiconductor field-effect transistors (NFETs) N 0 -N 7  connected in parallel between the port  106  and ground. The NFET N 0  is a default device that is always on and determines (along with PFET P 0 ) the maximum impedance that may be created by the resistive element  104 . The NFETs N 1 -N 7  are arranged in size order such that NFET N 1  is the narrowest transistor and NFET N 7  is the widest transistor. 
   The upper portion  108  of circuitry is connected in series with the lower portion  110  of circuitry to create a voltage divider that provides a termination for a signal output from circuitry that employs the programmable termination circuit  100 . The terminated impedance is created by the resistive element  104 , on the port  106 . Each PFET, NFET combination (e.g., P 0 -N 0 , P 1 -N 1 , P 2 -N 2 , etc.) is referred to herein as a stacked transistor pair. However, it should be understood that each of the transistors PFETs P 1 -P 7  and NFETs N 1 -N 7  may operate independently. 
   The impedance evaluation control logic  102  outputs a fixed set of control or binary termination signals (e.g., binary counts) p 1 -p 7  and n 1 -n 7  to the PFETs P 1 -P 7  and NFETs N 1 -N 7 , respectively, for selectively activating or de-activating the transistors (thereby creating a resistive element  104  of a fixed impedance (e.g., once programmed via the impedance evaluation control logic  102  described below with reference to FIG.  2 ), which is used for outputting a signal on the port  106 ). In one embodiment, the most significant bit of a binary count is provided to the widest transistor, and the least significant bit is provided to the narrowest transistor. As stated, because the default devices P 0 , N 0  are always on, the default devices P 0 , N 0  sets the maximum impedance value of the resistive element  104 . 
     FIG. 2  is a diagram of the conventional impedance evaluation control circuit  102  of FIG.  1 . The impedance evaluation control circuit  102  may include control logic  202  coupled to a plurality  204  of PFETs  204   a-h  connected in parallel between a high voltage level (e.g., V DDQ ) and a port  206  (e.g., a chip pad) included, for example, in a memory system (not shown). The PFETs  204   a-h  may be arranged in size order in a manner similar to the PFETs P 1 -P 7  included in the upper portion  108  of the resistive element  104  of FIG.  1 . 
   The control logic  202  may be coupled to the port  206  via a feedback line  208 . A resistor  210  (e.g., an external resistor connected to a system board) is coupled between the port  206  and ground. Consequently, the impedance evaluation control circuit  102  acts as a voltage divider. 
   The control logic  202  outputs bits of a binary count signal (e.g., signals p 1 -p 7 ) to the plurality  204  of PFETs  204   a-h , respectively, and in response thereto receives a value indicating the voltage at the port  206  via the feedback line  208 . In one embodiment, the most significant bit of the binary count signal is provided to the widest transistor, and the least significant bit is provided to the narrowest transistor. The control logic  202  compares the voltage at the port  206  with a reference voltage (e.g., a desired value such as V DDQ /2) included in the control logic  206  and outputs a different binary count signal until the voltage at port  206  matches the reference voltage (e.g., V DDQ /2). Once the voltage at port  206  matches the reference voltage, the impedance evaluation control circuit  102  fixes and outputs the binary count (e.g., control signals p 1 -p 7 ) used for creating the voltage at port  206  to the PFETs P 1 -P 7  of FIG.  1 . Although not shown in  FIG. 2 , the impedance evaluation control circuit  102  may create control signals n 1 -n 7  in a similar manner and provide the same to the NFETs N 1 -N 7  of FIG.  1 . In this manner, the impedance evaluation control circuit  102  generates control or binary termination signals p 1 -p 7 , n 1 -n 7  used for creating a resistive element  104  (e.g., terminator) of a fixed impedance (e.g., the characteristic impedance) based on the value of the external resistor  210 . Thus, the conventional impedance evaluation control circuit  102  determines a characteristic impedance of a port by generating a plurality of binary termination signals. 
   Different applications and different types of signals corresponding to an application, such as data, address, and/or clock signals, may require different termination values for optimal transmission. Although a different programmable termination circuit  100  may be used for creating the required termination value for each different port of an application (e.g., a memory system) such a solution requires the above circuitry for each port. 
   SUMMARY OF INVENTION 
   To overcome the disadvantages of the prior art, in one or more aspects of the present invention, methods and apparatus for scalable terminations within integrated circuits are provided. For example, in a first aspect of the invention, a first method is provided for providing multiple termination values using a plurality of binary termination signals. The first method includes the steps of (1) determining a characteristic impedance of a first port by generating a plurality of binary termination signals; and (2) modifying a characteristic impedance of a second port by manipulating one or more of the plurality of binary termination signals. 
   In a second aspect of the invention, a second method is provided for providing multiple termination values using a set of control signals. The second method includes the steps of (1) employing the set of control signals to provide a fixed output impedance on a first port; and (2) employing the set of control signals to provide a variable output impedance on a second port. Numerous other aspects are provided, as are systems and apparatus in accordance with these and other aspects of the invention. 
   Other features and aspects of the present invention will become more fully apparent from the following detailed description, the appended claims and the accompanying drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a diagram of a conventional programmable termination circuit. 
       FIG. 2  is a diagram of the conventional impedance evaluation control circuit of FIG.  1 . 
       FIG. 3  is a block diagram of a first exemplary scalable termination circuit provided in accordance with the present invention. 
       FIG. 4  is a block diagram of a second scalable termination circuit for providing a fixed termination value on a first port and a variable termination value on a second port using a single set of control signals in accordance with an embodiment of the present invention. 
       FIG. 5  is a block diagram of a third scalable termination circuit for providing a variable impedance on a port of a memory system in accordance with the present invention. 
       FIGS. 6A and 6B  are a block diagram of a fourth exemplary scalable termination circuit for providing a fixed termination value on a first port and a variable termination value on a second port using a single set of control signals in accordance with an embodiment of the present invention. 
       FIG. 7  is a block diagram of a fifth exemplary scalable termination circuit for providing a fixed termination value on a first port and a variable termination value on a second port using a single set of control signals in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 3  is a block diagram of a first exemplary scalable termination circuit  301  provided in accordance with the present invention. The scalable termination circuit  301  provides a variable termination on a transmission line coupled to a port  302 . The scalable termination circuit  301  includes scalable termination logic  300  having a resistive element  304  coupled to the port  302  of a memory system, transistor enable logic  306  and the conventional impedance evaluation control circuit  102 . The resistive element  304  and the transistor enable logic  306  may be coupled to and receive control signals p 1 -p 7 , n 1 -n 7  from the conventional impedance evaluation control circuit  102 . When a logic enable signal, ENABLE, is not asserted via an input  308  of the transistor enable logic  306 , the scalable termination logic  300  provides a first termination value on the port  302 . Alternatively, when the logic enable signal, ENABLE, is asserted, the scalable termination logic  300  provides or outputs a second termination value on the port  302 . 
   In one embodiment, the resistive element  304  may include a plurality of transistors similar to the transistors (e.g., P 1 , P 2 , N 1 , N 2 , etc.) of resistive element  104  shown in FIG.  1 . However, for each transistor (e.g., P 1 ) included in the resistive element  104 , the resistive element  304  includes a group of transistors (e.g., P 1 A-P 1 B-P 1 C-P 1 D), referred to as a fingered transistor set, connected in parallel. The width of each transistor included in the fingered transistor set is the width of the transistor (e.g., P 1 ) shown in  FIG. 1  to which the fingered transistor set (e.g., P 1 A-P 1 B-P 1 C-P 1 D) corresponds reduced by a factor based on the number of transistors in the fingered transistor set. For example, the group of PFETs P 1 A-P 1 B-P 1 C-P 1 D shown in  FIG. 3  corresponds to the transistor P 1  shown in FIG.  1 . Each of the PFETs P 1 A-P 1 D is ¼ the width of transistor P 1 . Likewise, the group of NFETs N 1 A-N 1 B-N 1 C-N 1 D shown in  FIG. 3  corresponds to the transistor N 1  shown in  FIG. 1 ; and each of the NFETs N 1 A-N 1 D is ¼ the width of transistor N 1 . 
   The group of PFETs P 1 A-P 1 D may be connected in series with the group of NFETs N 1 A-N 1 D as shown. Consequently, the programmable termination circuit  300  includes a stacked fingered transistor pair P 1 A-P 1 D/N 1 A-N 1 D, which include a plurality of stacked transistor pairs (e.g., P 1 A-N 1 A, P 1 B-N 1 B, P 1 C-N 1 C, P 1 D-N 1 D), corresponding to the stacked transistor pair P 1 -N 1  shown in FIG.  1 . 
   Although the resistive element  304  only illustrates one stacked fingered transistor pair P 1 A-P 1 D/N 1 A-N 1 D that corresponds to the stacked transistor pair P 1 -N 1  included in the resistive element  104  of  FIG. 1 , it should be understood that in practice the resistive element  304  includes a stacked fingered transistor pair that corresponds to each of the remaining stacked transistor pairs (e.g., P 2 -N 2 , P 3 -N 3 , P 4 -N 4 , P 5 -N 5 , P 6 -N 6 , and P 7 -N 7 ) shown in FIG.  1 . Although not illustrated, the resistive element  304  may include a stacked fingered transistor pair that corresponds to the default stacked transistor pair (e.g., P 0 -N 0 ) shown in FIG.  1 . 
   The signals (e.g., modified or manipulated control signals) output by the transistor enable logic  306  may be coupled to and selectively activate or de-activate one or more stacked transistor pairs (e.g., P 1 D-N 1 D) in each stacked fingered transistor pair (e.g., P 1 A-P 1 D/N 1 A-N 1 D). For example, if the logic enable signal, ENABLE, is not asserted, the scalable termination logic  300  outputs a first termination value (e.g., the characteristic impedance of the transmission line coupled to the port  302  with transistors P 1 D-N 1 D on). Alternatively, if the transistor enable logic, enable signal, ENABLE, is asserted, the scalable termination logic  300  outputs a second termination value that is a scaled value of the first termination value (as the stacked transistor pair P 1 D-N 1 D are off). 
   The numerator of the scaling factor provided by the scalable termination logic  300  is the number of stacked transistor pairs included in each stacked fingered transistor pair when the enable signal is of a first logic state. The denominator of the scaling factor is the number of stacked transistor pairs that are activated in each stacked fingered transistor pair, which includes stacked transistor pairs that are activated by the control signals provided by the impedance evaluation control circuit  102  when the enable signal is of a second logic state. Therefore, the exemplary scalable termination logic  300  shown in  FIG. 3  may scale or adjust the first termination value by {fraction (4/3)} when the logic  306  enable signal, ENABLE, is asserted. Through use of the scalable termination logic  300  shown in  FIG. 3 , a variable termination value may be provided on a port  302  using control signals provided by transistor enable logic  306  and the impedance evaluation control circuit  102 . 
     FIG. 4  is a block diagram of a second scalable termination circuit  401  for providing a fixed termination value on a first port and a variable termination value on a second port using a single set of control signals in accordance with an embodiment of the present methods and apparatus. The second scalable termination circuit  401  includes scalable termination logic  400  having a first resistive element  402  coupled to a first port  404  of a memory system (not shown) and the conventional impedance evaluation control circuit  102 . The scalable termination logic  400  includes the scalable termination logic  300  shown in  FIG. 3  coupled to a second port  406  of the memory system. The conventional impedance evaluation control circuit  102  may be coupled to and provide signals to the first resistive element  402  and the resistive element  304  of the scalable termination logic  300 , which serves as a second resistive element in the scalable termination logic  400 . 
   The structure of the first resistive element  402  may be similar to the structure of the resistive element  104  shown in FIG.  1 . However, the first resistive element  402  includes a fingered transistor set (e.g., a group of transistors connected in parallel) for each transistor included in the resistive element  104  shown in FIG.  1 . Therefore, the scalable termination logic  400  includes a stacked fingered transistor pair P 1 A-P 1 D/N 1 A-N 1 D corresponding to the stacked transistor pair P 1 -N 1  of FIG.  1 . Although the first resistive element  402  shown in  FIG. 4  illustrates only one stacked fingered transistor pair P 1 A-P 1 D/N 1 A-N 1 D, it should be understood that the first resistive element  402  includes a stacked fingered transistor pair for each of the remaining stacked transistor pairs included in the resistive element  104  shown in FIG.  1 . Although not illustrated, the first resistive element  402  may include a stacked fingered transistor pair that corresponds to the default stacked transistor pair (e.g., P 0 -N 0 ) shown in FIG.  1 . Because the structure of the scalable termination logic  300  was described in detail above, it is not described again herein. 
   In operation, the scalable termination circuit  401  employs a set of control signals for providing a fixed output impedance on a first port. More specifically, the first resistive element  402  of the scalable termination logic  400  may receive control signals (e.g., binary counts) p 1 -p 7 , n 1 -n 7  from the conventional impedance evaluation control circuit  102  that serve to selectively activate or deactivate one or more stacked transistor pairs (e.g., P 1 D-N 1 D in each stacked fingered transistor pair (e.g., P 1 A-P 1 D/N 1 A-N 1 D) to create a resistive element of a fixed impedance. Because the resistive element  402  is coupled to the first port  404 , an output impedance (e.g., the characteristic impedance) is provided on the first port  404  based on the set of control signals. The output impedance terminates a transmission line coupled to the first port  404 . 
   A set of control signals may be employed to provide a variable output impedance on a second port. The conventional impedance evaluation circuit  102  may provide the same set of control signals p 1 -p 7 , n 1 -n 7  provided to the first resistive element  402  to the scalable termination logic  300 . As stated above while describing  FIG. 3 , the control signals p 1 -p 7 , n 1 -n 7  along with secondary control signals output by the transistor enable logic  306  may serve to selectively activate or de-activate one or more stacked transistor pairs included in one or more stacked fingered transistor pairs (e.g., P 1 A-P 1 D/N 1 A-N 1 D of the second resistive element  304  to create a resistive element  304  of a first impedance. However, it should be understood that these transistors operate (e.g., may be activated) independently. If a logic  306  enable signal, ENABLE, is asserted, the transistor enable logic  306  may modify or manipulate one or more portions of the control signals p 1 -p 7 , n 1 -n 7  and output modified secondary control signals to the resistive element  304 . The control signals p 1 -p 7 , n 1 -n 7  along with the modified secondary control signals serve to selectively activate or de-activate one or more stacked transistor pairs (e.g., P 1 D-N 1 D) included in one or more stacked fingered transistor pairs of the second resistive element  304 . In this manner, the resistive element  304  may be modified to create another (e.g., a second) impedance. Because the resistive element  304  may be modified and is coupled to the second port  406 , an impedance that may be adjusted (e.g., a variable impedance) is provided or output on the second port  406  based in part on the control signals p 1 -p 7 , n 1 -n 7 . 
   The scalable termination circuit  401  uses resistive elements that include stacked fingered transistor pairs to provide a fixed termination value on a first port and a variable termination value on a second port using a single set of control signals. Because only a single set of control signals are provided to the scalable termination logic  400 , only a single impedance evaluation circuit  102  is required. 
   Although using a stacked fingered transistor pair, which corresponds to a stacked transistor pair included in the resistive element  104 , in the resistive elements  304 ,  402 , may allow impedance to be varied simply, it may be difficult to replace the narrow transistors included in the resistive element  104  with fingered transistors, because dividing a narrow transistor into four transistors, each of which are ¼ the width, for example, may require the width of each transistor included in the fingered transistor to be below a design rule minimum required for optimal model accuracy and therefore, may be impractical. 
     FIG. 5  is a block diagram of a third scalable termination circuit  501  for providing variable impedance on a port of a memory system in accordance with the present invention. The third scalable termination circuit  501 , includes scalable termination logic  500 , which is coupled to the impedance evaluation control circuit  102 , having the resistive element  104 , which includes the upper portion  108  of circuitry and the lower portion  110  of circuitry, coupled to a port  502  of a memory system. The resistive element  104  was described above and is not described again in detail herein. The upper portion  108  of circuitry may be coupled to math logic  504 , and the lower portion  110  of circuitry may be coupled to math logic  506 . The math logic  504 ,  506  is coupled to and receives control signals p 1 -p 7 , n 1 -n 7 , respectively, from the conventional impedance evaluation circuit  102 . The math logic  504 ,  506  may receive an input from a line  508  coupled to a fuse (not shown) or a register (not shown), such as a programmable register, indicating a mathematical operation to be performed on the control signals p 1 -p 7 , n 1 -n 7 . The math logic  504 ,  506  may include combinational and/or sequential logic or may be implemented using an application specific integrated circuit (ASIC). 
   The scalable termination circuit  501  receives control signals p 1 -p 7 , n 1 -n 7  from the impedance evaluation circuit  102  that when applied directly to the resistive element  104  create an original impedance (e.g., a characteristic impedance) on a port  502  to which the resistive element  104  is connected. The scalable termination logic  500  modifies or manipulates one or more of the control signals p 1 -p 7 , n 1 -n 7  using math logic  504 ,  506 , and adjusts the value of the impedance on the port  502  using the modified control signals. More specifically, the math logic  504  may receive a portion of the control signals p 1 -p 7  (e.g., a binary count) output by the impedance evaluation circuit  102  and receive a scaling factor (e.g., a factor by which to modify the control signals) from a fuse (not shown) or register (not shown) via line  508 . The math logic  504  may perform a multiplication and/or division operation on the binary count p 1 -p 7  to modify the control signals p 1 -p 7  appropriately such that they (along with modified control signals n 1 -n 7 ) may be used to modify the impedance on the port  502  as required by the scaling factor. Because the impedance varies in proportion to the inverse of the binary count, a {fraction (4/3)} increase in impedance may be achieved by reducing the binary count by ¾, for example. The math logic  506  modifies the control signals n 1 -n 7  in a similar manner. Although  FIG. 5  illustrates a first math logic  504  that modifies the control signals p 1 -p 7  and a second math logic  506  that modifies the control signals n 1 -n 7 , it should be understood that a single math logic may be used for modifying the control signals p 1 -p 7 , n 1 -n 7 . 
   The math logic  504 ,  506  outputs the modified control signals to the resistive element  104 . The modified signals may serve to selectively activate or de-activate one or more stacked transistor pairs (e.g., P 1 -N 1  and P 2 -N 2 ) included in the resistive element  104 , which modifies the structure and therefore the impedance of the resistive element  104 . Because the resistive element  104  is coupled to the port  502 , a second impedance, which may be scaled or adjusted version of the original impedance, is provided or output on the port  502  based on the modified control signals. Although using math logic  504 ,  506  to modify control signals p 1 -p 7 , n 1 -n 7 , which are used for creating an original impedance on a port, may provide a method of scaling the impedance on the port, because the default transistors P 0 , N 0  included in the resistive element  104  do not receive modified control signals, they are unaffected by the changes made by the math logic  504 ,  506 . Therefore, every transistor included in the resistive element  104  does not receive an adjustment based on the scaling factor. Consequently, the modified impedance output on the port  502  does not accurately reflect the original impedance (e.g., characteristic impedance) modified (e.g., multiplied or divided) by the scaling factor. 
     FIGS. 6A and 6B  are a block diagram of a fourth exemplary scalable termination circuit  601  for providing a fixed termination value on a first port and a variable termination value on a second port using a single set of control signals in accordance with an embodiment of the present invention. Therefore, the scalable termination logic  600  only requires a single impedance evaluation circuit. The fourth scalable termination circuit  601  includes scalable termination logic  600 , which is coupled to the conventional impedance evaluation control circuit  102 , having a first resistive element  104  (e.g., the programmable termination circuit  104  shown in  FIG. 1 ) coupled to a first port  602 . The scalable termination logic  600  may include a second resistive element  104  (e.g., the resistive element  104 , included in the scalable termination logic  500  shown in  FIG. 5 ) coupled to a second port  604 . Because both the programmable termination circuit  104  and the scalable termination logic  500  were described above, they are not described again in detail herein. The scalable termination circuit  104  and the scalable termination logic  500  may be coupled to and receive control signals p 1 -p 7 , n 1 -n 7  from the impedance evaluation circuit  102 . 
   In operation, the scalable termination circuit  601  employs a set of control signals for providing a fixed output impedance on a first port  602 . More specifically, the first resistive element  104  of the scalable termination circuit  601  may receive control signals (e.g., a binary count) p 1 -p 7 , n 1 -n 7  from the impedance evaluation control circuit  102  that serve to selectively activate or de-activate one or more stacked transistor pairs (e.g., P 1 -N 1 , P 2 -N 2 , and P 3 -N 3 ) for creating a resistive element  104  of a fixed impedance. Because the resistive element  104  is coupled to the first port  602 , an output impedance (e.g., the characteristic impedance) is provided on the first port  602  based on the control signals p 1 -p 7 , n 1 -n 7 . 
   The same set of control signals may be employed to provide a variable output impedance on the second port  604 . More specifically, the impedance evaluation circuit  102  may provide control signals p 1 -p 7 , n 1 -n 7  to the scalable termination logic  500 . As stated above while discussing the scalable termination circuit  500 , math logic  504 ,  506  may receive and modify or manipulate the control signals p 1 -p 7 , n 1 -n 7 , respectively, and output the modified control signals to the second resistive element (e.g., the resistive element  104  included in the programmable termination circuit). As stated above, the math logic  504 ,  506  modifies the control signals p 1 -p 7 , n 1 -n 7  based on adjustable scaling factors, which may be provided by a fuse (not shown) or a register (not shown), such as a programmable register, via an input  508 . The modified control signals may serve to selectively activate or de-activate one or more stacked transistor pairs (e.g., P 1 -N 1 , P 2 -N 2 ) included in the second resistive element to create a resistive element having an impedance that is a scaled version of the impedance created on the first port  602 . 
   By modifying the scaling factor provided to the math logic  504 ,  506 , the math logic  504 ,  506  may output a different set of modified control signals. The different set of modified control signals may be used for creating a resistive element (e.g., the resistive element  104  included in the programmable termination circuit) having a different impedance, which is a scaled or adjusted version of the impedance created on the first port  602 . Because the resistive element  104  included in the scalable termination logic  500  is coupled to the second port  604  and the impedance created by the resistive element included in the scalable termination logic  500  may be varied, a variable impedance is provided or output on the second port  604  based on the control signals p 1 -p 7 , n 1 —n 1 . 
   The scalable termination logic  600  uses resistive elements coupled to math logic for providing a fixed termination value on a first port and a variable termination value on a second port using a single set of control signals. Because only a single set of control signals are provided to the scalable termination logic  600 , only a single impedance evaluation circuit is required. 
   Although using math logic  504 ,  506  for modifying control signals, which are used to create an impedance on a port, may provide a method of scaling an impedance on the same or another port, because the default transistors P 0 , N 0  included in the resistive element  104  and the resistive element included in the scalable termination circuit  500  do not receive modified control signals they are unaffected by the changes made by the math logic  504 ,  506 . Therefore, every transistor included in the resistive elements does not receive an adjustment based on the scaling factor. Consequently, the modified impedance output on the port  602 ,  604  does not accurately reflect the characteristic impedance modified (e.g., multiplied or divided) by the scaling factor. 
     FIG. 7  is a block diagram of a fifth exemplary scalable termination circuit for providing a fixed termination value on a first port and a variable termination value on a second port using a single set of control signals in accordance with an embodiment of the present invention. The scalable termination circuit  701  includes scalable termination logic  700 , which is coupled to the impedance evaluation control circuit  102 , having a first resistive element  702  coupled to a first port  704  and a second resistive element  706  coupled to a second port  708 . The first resistive element  702  is coupled to and receives control signals from impedance evaluation control logic  102 . The second resistive element  706  is coupled to and receive control signals from math logic  714 , which is coupled to and receives control signals from the impedance evaluation control logic  102 . The second resistive element  706  may be coupled to and receive control signals (e.g., secondary control signals) from transistor enable logic  707 . The transistor enable logic  707  and the math logic  714  may be coupled to a logic enable signal, ENABLE, via an input  308 , that serves to activate the transistor enable logic  707  and math logic  714 . The math logic  714  may be coupled to a fuse (not shown) or a register (not shown), such as a programmable register, via an input  508 , that indicates a scaling factor (e.g., a factor by which to modify the control signals). 
   The structure of the first resistive element  702  is similar to the structure resistive element  104  included in the programmable termination circuit  100  of FIG.  1 . In contrast to the resistive element  104 , each default transistor of the first resistive element  702  is a fingered transistor set. For example, the default PFET includes transistors P 0 A, P 0 B, P 0 C, and P 0 D connected in parallel to form a fingered transistor set. Each of the transistors P 0 A, P 0 B, P 0 C, P 0 D included in the default PFET are connected to ground such that the default fingered PFET set is always on. Similarly, the default NFET includes transistors N 0 A, N 0 B, N 0 C, and N 0 D connected in parallel to form a fingered transistor set. Each of the transistors N 0 A, N 0 B, N 0 C, N 0 D included in the default NFET are connected to a high voltage level (e.g., a logic “1”) such that the default fingered NFET is always on. A PFET from the default fingered transistor set P 0 A-P 0 D may be coupled to a corresponding NFET from the default fingered transistor set N 0 A-N 0 D to create a plurality of stacked transistor pairs (e.g., P 0 A-N 0 A, P 0 B-N 0 B) and thereby creating a stacked fingered transistor pair (e.g., P 0 A-P 0 D/N 0 A-N 0 D)  710 . 
   The structure of the second resistive element  706  is similar to the first resistive element  702 . In contrast to the first resistive element  702 , one or more stacked transistor pairs (e.g., P 0 D-N 0 D) included in the default stacked fingered transistor pair  712  (e.g., P 0 A-P 0 D/N 0 A-N 0 D), may be coupled to the logic enable signal, ENABLE, and an output of transistor enable logic  707 , respectively. Remaining transistors  718  (e.g., P 1 -P 7 , N 1 -N 7 ) included in the second resistive element  706  may be coupled to an output of the math logic  714 . A PFET from the remaining transistors  718  may be coupled to a corresponding NFET in the remaining transistors  718  to create a plurality of stacked transistor pairs (e.g., P 1 -N 1 , P 2 -N 2 , etc.). 
   In operation, the scalable termination circuit  701  employs a set of control signals p 1 -p 7 , n 1 -n 7  for providing a fixed output impedance on a first port  704 . More specifically, the default stacked fingered transistor pair  710  is always on and is connected in parallel to the remaining transistors  716  (e.g., stacked transistor pairs P 1 -N 1  to P 7 -N 7 ) included in the first resistive element  702 . The remaining transistors  716  included in the first resistive element  702  of the scalable termination circuit  700  may receive control signals (e.g., binary counts) p 1 -p 7 , n 1 -n 7  from the impedance evaluation control circuit  102  that serve to selectively activate or de-activate one or more of the stacked transistors (e.g., P 1 -N 1  to P 7 -N 7 ) to create a resistive element  702  of a fixed impedance. Because the resistive element  702  is coupled to the first port  704 , an output impedance (e.g., the characteristic impedance) is provided on the first port  704  based on the control signals p 1 -p 7 , n 1 -n 7 . 
   The same set of control signals may be employed for providing a variable impedance on the second port  708 . More specifically, the impedance evaluation control circuit  102  may provide the same control signals p 1 -p 7 , n 1 -n 7  provided to the first resistive element  702  to the math logic  714 . When the logic  707  enable signal, ENABLE, coupled to the math logic  714  is asserted, the math logic  714  modifies or manipulates the control signals p 1 -p 7 , n 1 -n 7  output by the impedance evaluation control circuit  102  as indicated by an adjustable scaling factor provided to the math logic  714  (e.g., via the input  508 ). In one embodiment, the math logic  714  may perform a multiplication and/or division operation on the control signals p 1 -p 7 , n 1 -n 7  (e.g., the binary counts) as required by the scaling factor. The math logic  714  outputs modified control signals, which may be used to selectively activate or deactivate one or more stacked transistor pairs (e.g., P 1 -N 1 , P 2 -N 2 ) included in the remaining transistors  718  of the second resistive element  706 , to create a resistive element using the remaining transistor  718  having an impedance that is a scaled version of the impedance created by the remaining transistors  716  of the first resistive element  702 . 
   By modifying the scaling factor provided to the math logic  714 , the math logic  714  may output a different set of modified control signals to the remaining transistors  718  included in the second resistive element  706 . The different set of modified control signals may be used to create a resistive element (using the remaining transistors  718 ) having a different impedance, which is the impedance created by the remaining transistors  716  of the first resistive element  702  modified (e.g., multiplied or divided) by the scaling factor. 
   When the logic enable signal, ENABLE, coupled to the math logic  714  is not asserted the remaining transistors  718  included in the second resistive element  706  may receive an unmodified version the control signals p 1 -p 7 , n 1 -n 7  from the math logic  714  that serve to selectively activate or de-activate one or more of the stacked transistor pairs in the remaining transistors  718  to create a resistive element using the remaining transistors  718  having the same impedance created by the remaining transistors  716  of the first resistive element  702 . 
   The transistor enable logic  707  may receive a secondary control signal (e.g., ENABLE). When the enable logic  707  receives the enable signal, ENABLE, the logic  707  may modify the secondary control signal, and output control signals (e.g., modified secondary control signals) to one or more stacked transistor pairs (e.g., P 0 D-N 0 D) included in the default stacked fingered transistor pair  712 . For example, the enable logic  707  may receive a secondary control signal ENABLE as an input and output modified secondary control signals ENABLE and not ENABLE to the transistors included in the stacked transistor pair P 0 D-N 0 D, respectively. The modified secondary control signals may serve to selectively activate or de-activate one or more stacked transistors (e.g., P 0 D-N 0 D) included in the default stacked fingered transistor pair  712 . The structure of the default stacked fingered transistor pair  712  is changed from that of the default stacked fingered transistor pair  710 , and therefore the impedance of the default stacked fingered transistor pair  712  may be the impedance of the default stacked fingered transistor pair  710  modified by a scaling factor. 
   In one embodiment, the numerator of the scaling factor provided by the enable logic  707  may be the number of stacked transistor pairs (e.g., P 0 A-N 0 A) included in the default stacked fingered transistor pair  712  when the ENABLE signal is low and the denominator of the scaling factor provided by the enable logic  707  may be the number of stacked transistor pairs (e.g., P 0 A-N 0 A) that are activated in the default stacked fingered transistor pair  712  when the ENABLE signal is high. In the exemplary scalable termination logic  700 , the enable logic  707  may deactivate the stacked transistor pair P 0 D-N 0 D and therefore, scale the impedance of the default stacked fingered transistor pair  712  by {fraction (4/3)} when ENABLE is asserted. 
   When the enable logic  707  is not enabled by the logic enable signal, ENABLE, the default stacked fingered transistor pair  712  may receive a second version of the modified secondary control signals from the enable logic  707  that serves to selectively activate or de-activate one or more of the stacked transistor pairs (e.g., stacked P 0 D-N 0 D to create a default stacked fingered transistor pair  712  having the same impedance as the default stacked fingered transistor pair  710  included in the first resistive element  702 . 
   The math logic  714  and the enable logic  707  are activated by the same enable signal, ENABLE. Therefore, the math logic  714  may modify or adjust the impedance of the remaining transistors (e.g., stacked transistor pairs P 1 -N 1  to P 7 -N 7 )  718  included in the second resistive element  706  while the transistor enable logic  707  modifies or adjusts the impedance of the default stacked fingered transistor pair  712  included in the second resistive element  706 . Because the default stacked fingered transistor pair  712  is connected in parallel to the remaining transistors  718 , the value of the scaled impedance provided or output on the second port  708  by the second resistive element  706  may be easily determined by one of skill in the art. In one embodiment, the math logic  714  modifies the impedance of the remaining transistors  718  by the same scaling factor that the enable logic  707  modifies the impedance of the default stacked fingered transistor pair  712 . Therefore, the impedance created on the second port  708  is the impedance created on the first port  704  modified (e.g., multiplied or divided) by the scaling factor. 
   The scalable termination logic  700  may use the first resistive element  702  to provide a fixed termination value on a first port  704 , and a resistive element (e.g., the remaining transistors  718 ) coupled to math logic  714  and resistive element (e.g., the default stacked fingered transistor pair  712 ) coupled to enable logic  707  to provide a variable termination value on a second port  708  using a single set of control signals p 1 -p 7 , n 1 -n 7 . The default stacked fingered transistor pair  712  may be used for providing a scalable maximum termination value on the second port  708 . The remaining transistors  718  may be used for providing a scaled impedance which, when combined with the maximum impedance, serves to reduce the maximum impedance by a certain amount. Because only a single set of control signals p 1 -p 7 , n 1 -n 7  are provided to the scalable termination circuit  701 , only a single impedance evaluation control circuit  102  is required. 
   As stated, the scalable termination circuit  701  may scale the impedance provided by the default stacked fingered transistor pair  712  included in the second resistive element  706  and may scale the impedance provided by the remaining transistors (e.g., stacked transistor pairs P 1 -N 1  to P 7 -N 7 )  718  included in the second resistive element  706  using a same scaling factor. Therefore, the scalable termination circuit  700  may provide a characteristic impedance on a second port  708  that accurately reflects the characteristic impedance on a first port  704  modified (e.g., multiplied or divided) by the scaling factor. 
   As stated above, the first and second resistive elements  702 ,  706  include default fingered transistors (e.g., P 0 A-P 0 D, N 0 A-N 0 D), respectively, that each correspond to the default transistors P 0 , N 0  included in the resistive element  104  of the programmable termination circuit  100 . Because the default transistors included in the programmable termination circuit  100  are large (e.g., wide), the default transistors P 0 , N 0  may be divided into a plurality separate transistors, each of which is 1/N the width of the default transistors P 0 , N 0 , where N is the number of transistors included in the plurality, and included in the scalable termination circuit  700  without approaching the design rule minimum required for optimal model accuracy. 
   The foregoing description discloses only the exemplary embodiments of the invention. Modifications of the above-disclosed embodiments of the present invention which fall within the scope of the invention will be readily apparent to those of ordinary skill in the art. For instance, while the present methods and apparatus disclose the use of fingered transistors that include a group of four transistors (e.g., P 0 A-P 0 D, N 0 A-N 0 D), in other embodiments the fingered transistors may include a larger or smaller group of transistors. Further, while the present methods and apparatus disclose providing control signals to seven PFET transistors P 1 -P 7  and seven NFET transistors N 1 -N 7 , in other embodiments, control signals may be provided to a larger or smaller number of transistors included in the first and second resistive elements  702 ,  706 . Further, while in the above embodiments, a single math logic  714  provides modified control signals to the transistors P 1 -P 7  and N 1 -N 7  included in the second resistive element  706 , separate math logic may be employed to provide portions of the modified control signals to the transistors P 1 -P 7 , N 1 -N 7 , respectively. Although a first termination logic (e.g., the math logic  504 ,  506 ) in the above embodiments was always enabled, in other embodiments the math logic  504 ,  506  may be operatively coupled to and receive an enable signal that serves to activate the math logic  504 ,  506 . Further, the above methods and apparatus may be implemented in a memory system. Although in the above embodiments, control signals, modified control signals, and modified secondary control signals are used to selectively activate or de-activate one or more stacked transistor pairs included in a resistive element, in other embodiments, such signals may be used to selectively activate or de-activate one or more individual transistors included in the resistive element. 
   Accordingly, while the present invention has been disclosed in connection with exemplary embodiments thereof, it should be understood that other embodiments may fall within the spirit and scope of the invention as defined by the following claims.