Patent Document

FIELD OF THE INVENTION 
     Embodiments of the invention relate generally to the field of duty cycle correction, and more particularly to method and apparatus for correcting duty cycle of a CMOS level signal when converted from a Current-Mode-Logic (CML) to a CMOS level signal via a CML to CMOS converter. 
     BACKGROUND 
     As the speed/bandwidth at which the input-output (I/O) circuits operate is increasing, circuits associated with such I/Os become sensitive to duty cycle and voltage swings of associated signals. Timing uncertainty of high speed clock signals, including power supply induced jitter and clock duty cycle error caused by mismatch between adjacent transistors arising from manufacturing variations, limit the overall operating data rate of high speed serial I/Os. To overcome such limitation in overall operating data rate, low signal swing clock signals are used for I/O circuits because such low swing signals are more robust against power supply induced jitter. These low signal swing signals are called Current-Mode-Logic (CML) level signals. CML level signals generally operate at voltage swings which are around 30-40% of the full power supply rail/voltage level. 
     However the end use point of these clock signals is usually CMOS level logic circuits, requiring rail-to-rail voltage swings. Some circuits provide better performance via CMOS level signals with a 50% duty cycle. A signal with 50% duty cycle is a signal that remains high and low for the same duration of time in a signal period. A signal with more than 50% duty cycle is a signal that has a longer high pulse than its corresponding low pulse in a signal period. Similarly, a signal with less than 50% duty cycle is a signal that has a longer low pulse than its corresponding high pulse in a signal period. 
     For such circuits that require CMOS level signals, the CML level signals are converted to CMOS level signals, which are rail-to-rail signals, at the input of these circuits. For example, transmitters and receivers of Peripheral Component Interconnect Express (PCIe) I/O circuits require CMOS level signals with 50% duty cycle and reduced signal jitter for proper operation at high frequencies (e.g., 4 GHz and above). These CML to CMOS level signal converters introduce performance limiting characteristics to the output CMOS level signals. Examples of performance limiting characteristics include higher power consumption, lower timing margin, non-50% duty cycle, increased jitter amplification, power supply induced jitter, etc. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will be understood more fully from the detailed description given below and from the accompanying drawings of various embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments, but are for explanation and understanding only. 
         FIG. 1A  illustrates a processor having a CML to CMOS signal level converter, according to one embodiment of the invention. 
         FIG. 1B  illustrates a CML to CMOS signal level converter, according to one embodiment of the invention. 
         FIG. 2A  illustrates a CML to CMOS signal level converter with differential biasing and gate shielding devices, according to one embodiment of the invention. 
         FIG. 2B  illustrates duty cycle performance of a CML to CMOS signal level converter with an embedded differential biasing unit and gate shielding devices, according to one embodiment of the invention. 
         FIG. 3A  illustrates an embedded differential biasing unit, according to one embodiment of the invention. 
         FIG. 3B  illustrates an alternative embedded differential biasing unit, according to one embodiment of the invention. 
         FIG. 4  illustrates a compensation scheme for compensating duty cycle via a CML to CMOS signal level converter, according to one embodiment of the invention. 
         FIG. 5  illustrates a compensation scheme for compensating duty cycle via a CML to CMOS signal level converter having an embedded differential biasing unit and gate shielding devices, according to one embodiment of the invention. 
         FIG. 6  illustrates a flow chart for generating a 50% duty cycle CMOS level signal via a CML to CMOS signal level converter, according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention discuss method and apparatus for correcting duty cycle of a CMOS level signal when converted from a Current-Mode-Logic (CML) to a CMOS level signal via a CML to CMOS signal converter. 
     Reference in the specification to “an embodiment,” “one embodiment,” “some embodiments,” or “other embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least some embodiments, but not necessarily all embodiments. The various appearances of “an embodiment,” “one embodiment,” or “some embodiments” are not necessarily all referring to the same embodiments. If the specification states a component, feature, structure, or characteristic “may,” “might,” or “could” be included, that particular component, feature, structure, or characteristic is not required to be included. If the specification or claim refers to “a” or “an” element, that does not mean there is only one of the element. If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element. 
       FIG. 1A  illustrates a processor  100  having a CML to CMOS signal level converter, according to one embodiment of the invention. In one embodiment, a phase locked loop (PLL) unit  101  generates a clock signal. In one embodiment, the clock signal is converted to low swing signal such as a CML level signal by a converter (not shown) which resides inside the PLL unit  101  or outside the PLL unit  101 . In one embodiment, the CML level signal is received by an I/O buffer  102 . In one embodiment, the I/O unit  102  includes a CML to CMOS signal level converter  103  which is discussed later. In one embodiment the processor  100  includes a memory  104  (e.g., a Random Access Memory) to store computer executable instructions which are executed by the core logic  105 . In one embodiment, the computer executable instructions when executed perform a method which is discussed in  FIG. 6 . 
       FIG. 1B  illustrates a CML to CMOS signal level converter  110 , according to one embodiment of the invention. In one embodiment, the converter  110  receives differential CML level signals cml_p and cml_n via a receiver  111 . In one embodiment, the receiver  111  includes an adjustable high pass filter. In one embodiment, the high pass filter includes adjustable capacitors C 1 -C 2  and adjustable resistors R 1 -R 2 . In one embodiment, the receiver  111  includes a duty cycle adjustment circuit  111   a . In other embodiments, the duty cycle adjustment circuit  111   a  is outside the receiver  111 . 
     In one embodiment, the duty cycle adjustment unit  111   a  adjusts the DC bias levels of signals at the gates of transistors M 1 , M 6 , M 2 , and M 5 . The adjusted DC bias levels further adjust the duty cycle of the CMOS level signals at the output. The term unit is interchangeably called circuit. In one embodiment, the duty cycle adjustment unit  111   a  in  111  is a digital to analog converter (DAC). The adjustment of the DC bias levels of signals at the gates of transistors M 1 , M 6 , M 2 , and MS are based on the duty cycle of the CMOS output signal CLK_OUT (or buffered version of that signal). 
     In one embodiment, the capacitors C 1 -C 2  and resistors R 1 -R 2  are adjusted by a digital logic (not shown) having multiplexers to select appropriate combination of capacitance and resistance for a particular input CML level signal frequency. In one embodiment, the capacitors C 1 -C 2  and resistors R 1 -R 2  are configured to have fixed values that are predetermined for a particular CML level signal frequency range. In one embodiment, the values for C 1 -C 2  and R 1 -R 2  are configured to be 200 fF and 10 KOhms respectively for CML level signals having frequency range of 4-5 GHz. 
     In one embodiment, the duty cycle control bits to the duty cycle adjustment block  111   a  lowers the difference between the DC bias levels of signals at the gate of transistors M 1  and M 6  and the DC bias levels of signals at the gate of transistors M 2  and M 5 . In one embodiment, the lower difference in the DC bias levels increases the duty cycle of the output CMOS level signals 
     In another embodiment, the duty cycle control bits to the duty cycle adjustment block  111   a  raise the DC bias levels of the signals at the gate of transistors M 1  and M 6  and lower DC bias levels of the signals at the gate of transistors M 2  and M 5 . In one embodiment, the above adjustment of DC bias levels reduces the duty cycle of the output CMOS level signals. In one embodiment, the duty cycle control bits are set by hardware or software or both. In one embodiment, the duty cycle control bits are generated by a compensation unit discussed later in reference to  FIG. 4 . 
     Referring back to  FIG. 1B , as discussed above the CML level signals cml_p and cml_n are coupled with a pair of differential amplifiers/units  112  and  113  via the receiver block  111 . In one embodiment, the two differential pair amplifier/units are based on a current mirror architecture having transistors M 1 -M 4  and M 5 -M 8 . Other implementations of differential pair amplifiers/units can be used without changing the principle of operation of the described embodiments. In one embodiment, each differential pair amplifier/unit  112  and  113  has its independent current sources  112   a  and  113   a  respectively. 
     In one embodiment, the differential amplifiers/units  112  and  113 , are configured to receive complementary CML level input signals, i.e. transistor M 1  of  112  receives cml_p while corresponding transistor M 5  of  113  receives cml_n. Such configuration generates complementary output signals outp 1  and outp 2  of  112  and  113  respectively. 
     In one embodiment, the complementary output signals outp 1  and outp 2  are received by a differential-to-single-ended converter  114  that converts the differential outputs outp 1  and outp 2  to a single-ended output. In one embodiment, the differential-to-single-ended converter  114  is implemented as a current mirror architecture having transistors M 9 -M 12 . Other implementations of differential-to-single-ended converter can be used without changing the principle of operation of the described embodiments. 
     In one embodiment, the complementary differential outputs outp 1  and outp 2  compensate for non-50% duty cycle in the CML level signals by controlling the current in the differential-to-single-ended converter  114 . 
     In one embodiment, the transistors M 10  and M 12  that receive the signals outp 1  and outp 2  are biased by the DC levels of the signals outp 1  and outp 2 . The DC bias levels of the signals outp 1  and outp 2  also depend on the duty cycle adjustment unit  111   a . In one embodiment, the DC bias level of outp 1  increases when the DC bias level of the signals at the gate of transistors M 1  and M 6  increases. In one embodiment, the DC bias level of outp 1  decreases when the DC bias level of the signals at the gate of transistors M 1  and M 6  decreases. In one embodiment, the DC bias level of outp 2  increases when the DC bias level of the signals at the gate of transistors M 2  and M 5  increases. In one embodiment, the DC bias level of outp 2  decreases when the DC bias level of the signals at the gate of transistors M 2  and M 5  decreases. The complementary nature of the signals outp 1  and outp 2  adjusts the duty cycle of the single-ended output CLK 3  to be of 50% duty cycle. 
     The single-ended output from the differential-to-single-ended converter  114  is also in CMOS level, i.e. with rail-to-rail signal swing. In one embodiment, the differential-to-single-ended converter  114  is configured by appropriate device sizes to behave as a final driver of the CMOS level output signal. In such an embodiment, the differential-to-single-ended converter  114  amplifies the CMOS level output and thus limits the power supply induced jitter on the CMOS level output. 
     In one embodiment, the single-ended output CLK 3  is driven out by a driver  115  coupled with the differential-to-single-ended converter  114 . In one embodiment, the driver  115  is implemented as an inverter having transistors M 13 -M 14 . Other implementations of the driver  115  can be used without changing the principle of operation of the described embodiments. In one embodiment, the driver  115  inverts the polarity of the CMOS level signal from the differential-to-single-ended converter  114 . The driver  115  amplifies the signal strength of the CMOS level signal and also limits the power supply induced jitter on the CMOS level output. 
       FIG. 2A  illustrates a CML to CMOS signal level converter  200  with differential biasing and gate shielding devices, according to one embodiment of the invention. Compared to the embodiment of  FIG. 1B , the CML to CMOS signal level converter of  FIG. 2A  includes an embedded duty cycle controller  205  (instead of independent current sources) that allows the CML level signals cml_p and cml_n to directly connect with a pair of differential amplifiers/units  201  and  202  while providing 50% duty cycle at the output of the converter  200 . 
     In one embodiment, jitter amplification is reduced by shielding devices in the differential pairs  201  and  202 , and differential-to-single-ended converter  203 . In one embodiment, the shielding devices are inserted between the diode connection of the diode connected transistors M 3 , M 7 , and M 9 . In one embodiment, the shielding devices  206 - 208  are resistors R 1 -R 3 . In one embodiment, the resistors R 1 -R 3  have adjustable resistance controlled by a logic circuit (not shown) which controls the amount of jitter amplification of the converter  200  at the cost of active area of the converter  200 . 
     The shielding devices shield the gates of transistors M 3 , M 7 , and M 9  from noise generated by high speed nodes outn 1 , outn 2 , and CLK 2 . In one embodiment, the nodes outn 1 , outn 2 , and CLK 2  operate at the same frequencies as the frequencies of the input CML level signals cml_p and cml_n. By shielding the gates of transistors M 3 , M 7 , and M 9 , the effective dynamic capacitance, i.e. switching capacitance, of the converter  200  is reduced compared to when no shielding devices are inserted between the diode connection of the diode connected transistors M 3 , M 7 , and M 9 . A person skilled in the art knows that power consumption of a CMOS based circuit depends on power supply level, frequency of the signal, and the capacitance of the circuit. By reducing the active capacitance via the shielding devices, active power consumption is reduced for the converter  200 . Furthermore, by reducing the active capacitance seen by outn 2 , outn 1 , and CLK 2  nodes bandwidth of these nodes is increased. 
     An alternative way to understand the bandwidth enhancement of the converter  200  is via the low frequency output impedance of the diode connected transistors M 3 , M 7 , and M 9 . The low frequency (near DC) output impedance of the diode connected transistors M 3 , M 7 , and M 9  is the inverse of their trans-conductance (1/gm) while the output impedance at high frequencies (e.g., 4 GHz and above) is 1/gds. The output impedance of 1/gds is higher than the output impedance of 1/gm. This means that at high CML level signal frequency (e.g., 4 GHz and above), the output impedance increases gain of the converter  200 . The shielding resistors  206 - 208  enhance bandwidth of the converter  200  because the amount of capacitance observed by nodes outn 2 , outn 1 , and CLK 2  is reduced. 
     In one embodiment, the gate shielding resistors R 1 -R 3  are set to 10 K Ohms for an input CML level signal frequency of 4-5 GHz to generate a CMOS level signal having 50% duty cycle and reduced jitter amplification. In such an embodiment, the bandwidth of the nodes outn 2 , outn 1 , and CLK 2 , and hence the bandwidth of the converter  200 , is increased by 25% versus when no shielding devices are used. Furthermore, the 25% increase in bandwidth is realized without any additional power consumption because it is not based on the resistance values. Instead, in one embodiment, active power consumption is reduced by 5% because the nodes outn 1 , outn 2 , and CLK 2  now see less gate capacitance (of transistor gates M 3 , M 7 , and M 9 ). 
     The embedded duty cycle converter  205  controls the amount of current that flows through the differential amplifiers/units  201  and  202 . In one embodiment, the CML level signals cml_p and cml_n are directly connected with the pair of differential amplifiers/units  201  and  202 . In one embodiment, the two differential pair amplifiers/units  201  and  202  are based on a current mirror architecture having transistors M 1 -M 4  and M 5 -M 8 . Other implementations of differential pair amplifiers/units can be used without changing the principle of operation of the described embodiments. 
     In one embodiment, the differential amplifiers/units  201  and  202 , are configured to receive complementary CML level input signals, i.e. transistor M 1  of  201  receives cml_p while corresponding transistor M 5  of  202  receives cml_n. Such configuration generates complementary output signals outp 1  and outp 2  of  201  and  202 , respectively. 
     In one embodiment, the complementary output signals outp 1  and outp 2  are received by a differential-to-single-ended converter  203  that converts the differential outputs outp 1  and outp 2  to a single-ended output CLK 3 . In one embodiment, the differential-to-single-ended converter  203  is implemented as a current mirror architecture having transistors M 9 -M 12 . Other implementations of differential-to-single-ended converter can be used without changing the principle of operation of the described embodiments. 
     In one embodiment, the complementary differential outputs outp 1  and outp 2  compensate for non-50% duty cycle in the CML level signals by controlling the current in the differential-to-single-ended converter  203 . 
     In one embodiment, the differential-to-single-ended converter  203  behaves as a gain stage in the CML to CMOS level signal converter. The gain generated by the differential-to-single-ended converter  203  enhances the overall gain of the converter  200 . A higher gain of the converter  200  limits power supply induced jitter because the gain increases the slew rate of the CMOS level signal. In one embodiment, the differential-to-single-ended converter  203  also operates as a driver of the CMOS level signal. The complementary differential outputs outp 1  and outp 2  compensate for non-50% duty cycle in the CML level signals by controlling the current in the differential-to-single-ended converter  203 . 
     In one embodiment, transistors M 10  and M 12  of the differential-to-single-ended converter  203  receive signals outp 1  and outp 2  from the differential pairs  201  and  202 . In one embodiment, the signals outp 1  and outp 2  are indirectly biased by the differential current biasing circuit  205 . The complementary nature of the signals outp 1  and outp 2  adjusts the duty cycle of the single-ended output CLK 3  to be of 50% duty cycle. 
     In one embodiment, the single-ended output CLK 3  is driven out by a driver  204  coupled with the differential-to-single-ended converter  203 . In one embodiment, the driver  204  is implemented as an inverter having transistors M 13 -M 14 . Other implementations of the driver  204  can be used without changing the principle of operation of the described embodiments. In one embodiment, the driver  204  inverts the polarity of the CMOS level signal from the differential-to-single-ended converter  203 . The driver  204  amplifies the signal strength of the CMOS level signal and also limits the power supply induced jitter on the CMOS level output. In one embodiment, the driver  204  inverts the polarity of the CMOS level signal from the differential-to-single-ended converter  203 . 
     In one embodiment, the signal strength of node outp 1  verses that of node outp 2  is scaled by controlling the bias current of the differential pair units  201  and  202 . The scaling of the current is performed by the differential current biasing unit  205 . In one embodiment, the difference in signal strength of outp 1  and outp 2  and the DC voltage levels (bias level) at nodes outp 1  and outp 2  determine the duty cycle of the output CLK 3  and CLK_OUT. 
     In one embodiment, the tail current to the differential pair  201  is increased by the differential current biasing unit  205  while the tail current to the differential pair  202  is decreased by the differential current biasing unit  205 . As a result of the difference in the tail currents of the differential pair units  201  and  202 , the signal swing at node outp 2  becomes larger than it was before the increase in the tail current of the differential pair unit  201  and before the decrease in tail current of the differential pair unit  202 . In one embodiment, the increase in signal swing at node outp 2  causes the duty cycle at node CLK 3  (output of the differential-to-single-ended converter  203 ) to be increased. In one embodiment, the differential-to-single-ended converter  203  is followed by a driver  204 . In such an embodiment, the duty cycle of the output CLK_OUT of the driver  204  is decreased. 
     In one embodiment, the tail current to the differential pair unit  201  is decreased by the differential current biasing unit  205  while the tail current to the differential pair unit  202  is increased by the differential current biasing unit  205 . As a result of the difference in the tail currents of the differential pair units  201  and  202 , the signal swing at node outp 2  becomes smaller than it was before the decrease in the tail current of the differential pair unit  201  and before the increase in tail current of the differential pair unit  202 . In one embodiment, the decrease in signal swing at node outp 2  causes the duty cycle at node CLK 3  (output of the differential-to-single-ended converter  203 ) to be decreased. In one embodiment, the differential-to-single-ended converter  203  is followed by a driver  204 . In such an embodiment, the duty cycle of the output CLK_OUT of the driver  204  is increased. 
     In one embodiment, the embedded differential current biasing unit  205  is controlled by bias current control bits. In one embodiment, the values of the current control bits are controlled by software. In another embodiment, the values of the current control bits are controlled by hardware. In yet another embodiment, both hardware and software are used to control the values of the current control bits. The control mechanism is later discussed in reference to  FIG. 5 . 
     In one embodiment, the embedded differential current biasing unit  205  does not interfere with the high speed path of the CML to CMOS signal level converter. Details of an embodiment of the embedded differential current biasing unit  205  are later discussed in reference to  FIG. 3A . The embodiment  200  of  FIG. 2A  consumes less power than the embodiment of  FIG. 1B  because the duty cycle adjustment circuit and capacitors C 1 -C 2  are not used. 
       FIG. 2B  illustrates via a graph  210  the duty cycle performance of a CML to CMOS signal level converter, according to the embodiment described in  FIG. 2A . The x-axis of the graph list values of the bias current control bits. These bits control the currents of embedded differential current biasing unit  205  which in turn control the tail currents of the differential pair units  201  and  202 . The y-axis of the graph lists the duty cycle in percentage at node CLK_OUT. The CML to CMOS level signal converter  200  of  FIG. 2A  provides a monotonic response  211  to duty cycle for the control bits. In this example, 50% duty cycle is achieved near control bit setting of 16. In one embodiment, the setting of the control bit is achieved by a compensation unit discussed later in reference to  FIG. 5 . 
       FIG. 3A  illustrates an embedded differential biasing circuit/unit  300 , according to one embodiment of the invention. In one embodiment, the embedded current source  301  provides current sources to both the differential pair units  201  and  202  of  FIG. 2A . The outputs of the embedded current source  301  are shown as ioutp and ioutn which are connected with tail current nodes of differential pair units  201  and  202  of  FIG. 2A . In one embodiment, the current strength for these outputs is independently controlled by control bits. In one embodiment, the control bits are complementary bits, i.e. control bits (biasdac[N:0]) are of opposite polarity to the control bits biasdacb[N:0], where N is the number of control bits that are individually controlling transistors M 4  and M 5 . In one embodiment, transistors M 1  and M 3  are configured to provide a minimum static current to the differential pairs  201  and  202 . The embedded current source  301  has a maximum current sourcing capacity set by transistor M 2  which is biased by nbias. In one embodiment, nbias is an analog voltage generated by a bias circuit (not shown) such as a band-gap circuit. 
       FIG. 3B  illustrates an embedded differential biasing circuit/unit  310 , according to one embodiment of the invention. This circuit is explained later in detail as an alternative to the embedded differential biasing circuit/unit  300  of  FIG. 3A . 
       FIG. 4  illustrates a compensation scheme  400  for compensating duty cycle via a CML to CMOS signal level converter, according to one embodiment of the invention. In one embodiment, a CML to CMOS level signal converter  401   a  is compensated for a 50% duty cycle over process, voltage, and temperature (PVT) conditions. In one embodiment, the converter  401   a  is the same as the converter  110  of  FIG. 1B . In one embodiment, the CMOS level signal from the converter  401   a  is converted to differential signals by single-to-differential converter  401   b . In one embodiment, the output of the single-to-differential converter  401   b  is detected by a duty cycle detector  402 . In one embodiment, the output of the duty cycle detector  402  is received by a finite state machine (FSM)  403 . The FSM  403  determines the appropriate DC control bit setting for the DC adjustment circuit  405  of the receiver  111  of  FIG. 1B . In one embodiment, the loop ( 401   a → 401   b → 402 → 403 → 404 → 401   a ) is repeated regularly to provide duty cycle control bit settings for a PVT condition so that the CML to CMOS level signal converters provide 50% duty cycle CMOS level signals. 
       FIG. 5  illustrates a compensation scheme  500  for compensating duty cycle via a CML to CMOS level signal converter  501   a  having an embedded differential biasing unit and gate shielding devices, according to one embodiment of the invention. In one embodiment, a CML to CMOS level signal converter  501   b  is compensated for a 50% duty cycle over PVT conditions. In one embodiment, the converter  501   b  is the same as the converter  200  of  FIG. 2A . In one embodiment, the CMOS level signal from the converter  501   a  is converted to differential signals by a single-to-differential converter  501   b . In one embodiment, the output of the single-to-differential converter  501   b  is detected by a duty cycle detector  502 . In one embodiment, the output of the duty cycle detector  502  is received by a finite state machine (FSM)  503 . The FSM  503  determines the appropriate biasdac bit setting for the embedded differential bias current source of  FIG. 2A . In one embodiment, the loop ( 501   a → 501   b → 502 → 503 → 501   a ) is repeated regularly to provide biasdac settings for a PVT condition so that the CML to CMOS level signal converters provide 50% duty cycle CMOS level signals. 
       FIG. 6  illustrates a flow chart  600  for generating a 50% duty cycle CMOS level signal via a CML to CMOS signal level converter, according to one embodiment of the invention. The flow chart  600  is applicable to converters  110  and  200  of  FIG. 1B  and  FIG. 2A  respectively. At block  601 , CML level signals are received by the first differential pair ( 111  of  FIG. 1B ,  201  of  FIG. 2A ). At block  602 , CML level signals are received by the second differential pair ( 112  of  FIG. 1B ,  202  of  FIG. 2A ). At block  603 , compensation control bits to adjust drive strengths of the first and the second differential pairs are received. In one embodiment, the compensation control bits are the duty cycle control bits generated by the compensation unit  400  of  FIG. 4  for the converter  110  of  FIG. 1B . In one embodiment, the compensation control bits are duty cycle control bits generated by the compensation unit  500  of  FIG. 5  for the converter  200  of  FIG. 2A . At block  604 , the duty cycle of the CMOS level signal is adjusted to be 50% duty cycle based on the compensation control bits. At block  605 , the converter generates a 50% CMOS level signal. 
     Elements of embodiments are also provided as a machine-readable medium (e.g.,  104  of  FIG. 1A ) for storing the computer-executable instructions (e.g., setting the control bits for biasing the embedded differential biasing circuits  300  and  310  of  FIG. 3A  and  FIG. 3B , setting the duty cycle adjustment bits in the CML to CMOS level signal converter  200  of  FIG. 2A ). The machine-readable medium may include, but is not limited to, flash memory, optical disks, CD-ROMs, DVD ROMs, RAMs, EPROMs, EEPROMs, magnetic or optical cards, or other type of machine-readable media suitable for storing electronic or computer-executable instructions. For example, embodiments of the invention may be downloaded as a computer program (e.g., BIOS) which may be transferred from a remote computer (e.g., a server) to a requesting computer (e.g., a client) by way of data signals via a communication link (e.g., a modem or network connection). 
     While the invention has been described in conjunction with specific embodiments thereof, many alternatives, modifications and variations will be apparent to those of ordinary skill in the art in light of the foregoing description. 
     For example, in one embodiment, the embedded differential biasing unit/circuit  300  of  FIG. 3A  can be implemented as the circuit shown in  FIG. 3B .  FIG. 3B  illustrates an embedded differential biasing circuit  310 , according to one embodiment of the invention. This embodiment biases the source transistor M 5  by pbias and the biasdac[N:0]. In one embodiment, to increase sourcing of current at connections ioutp and ioutn (which are both connected to differential pair  201  and  202  respectively of  FIG. 2A ) biasdac[N:0] controls the current strength of transistor M 7 . In one embodiment, the DC level of pbias which is connected with the gate of transistor M 8  determines the maximum amount of current that the embedded differential biasing circuit  300  can sink via transistor M 5 . In one embodiment, pbias is generated by a bias circuit (not shown) such as a band-gap circuit. 
     In one embodiment, transistors M 4  and M 5 , which in  FIG. 3A  were N transistors each controlled by biasdac[N:0] and its complementary control bits, are no longer controlled by biasdac[N:0] control bits. Instead, the complementary function of biasdac[N:0] of  FIG. 3A  is implemented via the bias enable signals biasp_en and biasn_en. In one embodiment, both biasp_en and biasn_en are complementary to one another. In one embodiment, transistors M 1  and M 3  are configured to provide a minimum static current to the differential pair units  201  and  202  of  FIG. 2A . By having the ability to control the tail current of differential pairs units  201  and  202  of  FIG. 2A  via ioutp and ioutn, the duty cycle of the CMOS level signal is adjusted. 
     While the CML to CMOS level signal converter of  FIG. 1B  and  FIG. 2A  are implemented as NMOS input based differential amplifiers/units, PMOS input based differential amplifiers/units can easily replace the NMOS based differential amplifiers/units without changing the essence of the embodiments of the invention. 
     Similarly, the NMOS bias generation circuits/units of  FIG. 3A  and  FIG. 3B  can be easily replaced with PMOS bias generation circuits/units without changing the essence of the embodiments of the invention. Embodiments of the invention are intended to embrace all such alternatives, modifications, and variations as to fall within the broad scope of the appended claims.

Technology Category: 5