Abstract:
An apparatus comprising a reference circuit, a resistor ladder, and an output circuit. The reference circuit may be configured to generate a reference signal in response to (i) a clock signal, (ii) a first phase signal and (iii) a second phase signal. The resistor ladder circuit may be configured to generate a tap voltage in response to the reference signal. The tap voltage may be generated by enabling one or more of a plurality of tap resistors. The output circuit may be configured to generate an adjusted clock signal in response to (i) the tap voltage, (ii) the clock signal, (iii) the first phase signal, (iv) the second phase signal, and (v) a reset signal. The adjusted clock signal may have an adjusted phase with respect to the clock signal.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to timing circuits generally and, more particularly, to a method and/or apparatus for implementing a resistor ladder based phase interpolation circuit. 
       BACKGROUND OF THE INVENTION 
       [0002]    Conventional timing circuits often have phase variations that need to be adjusted by phase interpolation circuits. Phase interpolation circuits are used in clocking circuits such as Clock/Data Recovery circuits (CDRs), Spread Spectrum Phase Locked Loops (PLLs), etc. The desirable specifications of the phase interpolation circuits are that they should be monotonic and linear in operation. 
         [0003]    It would be desirable to implement a resistor ladder based phase interpolation. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention concerns an apparatus comprising a reference circuit, a resistor ladder, and an output circuit. The reference circuit may be configured to generate a reference signal in response to (i) a clock signal, (ii) a first phase signal and (iii) a second phase signal. The resistor ladder circuit may be configured to generate a tap voltage in response to the reference signal. The tap voltage may be generated by enabling one or more of a plurality of tap resistors. The output circuit may be configured to generate an adjusted clock signal in response to (i) the tap voltage, (ii) the clock signal, (iii) the first phase signal, (iv) the second phase signal, and (v) a reset signal. The adjusted clock signal may have an adjusted phase with respect to the clock signal. 
         [0005]    The objects, features and advantages of the present invention include providing phase interpolation that may (i) implement a resistor ladder, (ii) provide a phase adjustment, (iii) be implemented as part of a Phase Locked Loop (PLL) circuit, (iv) be implemented as part of a clock and data recovery (CDR) circuit and/or (v) provide a monotonic and/or linear operation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    These and other objects, features and advantages of the present invention will be apparent from the following detailed description and the appended claims and drawings in which: 
           [0007]      FIG. 1  is a block diagram of the present invention; 
           [0008]      FIG. 2  is a circuit diagram of the circuit of  FIG. 1 ; 
           [0009]      FIG. 3  is a timing diagram of the various signals of  FIGS. 1 and 2 ; 
           [0010]      FIG. 4  is a timing diagram of a reference voltage charging and discharging; 
           [0011]      FIG. 5  is a timing diagram of an input voltage charging and discharging; 
           [0012]      FIG. 6  is a timing diagram of an interpolated clock generation process; 
           [0013]      FIG. 7  is a timing diagram of two phases showing an interpolated clock; and 
           [0014]      FIG. 8  is a timing diagram of an interpolated clock signal over a number of taps. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]    Referring to  FIG. 1 , a block diagram of a circuit  100  is shown in accordance with an embodiment of the present invention. The circuit  100  generally comprises a block (or circuit)  102 , a block (or circuit)  104  and a block (or circuit)  106 . The block  102  may be implemented as a reference circuit. The block  104  may be implemented as a resistor ladder circuit. The block  106  may be implemented as an output circuit. The circuit  102  may have an input  110  that may receive a signal (e.g., PHE), an input  112  that may receive a signal (e.g., PHLB), an input  114  that may receive the signal PHE, an input  116  that may receive a signal (e.g., PHL), an input  118  that may receive a signal (e.g., COUT), an input  120  that may receive a signal (e.g., IREF), and an output  122  that may present (or generate) a signal (e.g., VREFBUF). 
         [0016]    The circuit  104  may have an input  130  that may receive the signal the signal VREFBUF, and an output  132  that may present a signal (e.g., VTAP). The circuit  106  may have an input  140  that may receive a signal (e.g., PHEB), an input  142  that may receive a signal PHL, an input  144  that may receive the signal PHE, an input  146  that may receive the signal VTAP, an input  148  that may receive a signal (e.g., RESET), an input  150  that may receive a signal (e.g., IINTP), and an output  160  that may present a signal (e.g., ICLK). 
         [0017]    The signal PHE may represent a phase early signal. The signal PHL may represent a phase late signal. The signal PHE and the signal PHL may have a phase difference (e.g., Tp), where Tp is 1/8  of the total period of the clock signal ICLK. The signal PHLB may be an inverted version (e.g., digital complement) of the signal PHL. The signal PHEB may be an inverted version (e.g., digital complement) of the signal PHE. The signal VREFBUF may be implemented as a reference voltage. The signal VTAP may be implemented as a tap voltage. 
         [0018]    Referring to  FIG. 2 , a more detailed diagram of the circuit  100  is shown. The circuit  102  generally comprises a switch (e.g., SW 1 ), a switch (e.g., SW 2 ), a switch (e.g., SW 3 ), a capacitor (e.g., CREF), a gate  200 , a gate  202 , a gate  204 , a device  206  and a device  208 . The gate  200  and/or the gate  202  may be implemented as one or more AND gates. The gate  204  may be implemented an NOR gate. The particular implementation of the gates  200 ,  202  and/or  204  may be varied to meet the design criteria of a particular implementation. The device  206  may be implemented as a current source configured to generate a current (e.g., IREF). The device  208  may be implemented as a differential amplifier. 
         [0019]    The circuit  104  generally comprises a number of circuits TAP 1 -TAP 7  and a number of resistors R 0 -R 7 . The circuits TAP 1 -TAP 7  may be implemented as tap circuits. One or more of the circuits TAP 1 -TAP 7  may be enabled (e.g., turned “ON”) in response to the signal VREFBUF. The circuit  106  generally comprises a device  220 , a switch (e.g., SW 4 ), a switch (e.g., SW 5 ), a switch (e.g., SW 6 ), a capacitor (e.g., CINTP), a device  220 , a device  222 , a device  224 , a device  226 , a device  228 , and a device  230 . In one example, the device  220  and/or the device  222  may be implemented as one or more AND gates. In one example, the device  224  may be implemented as a current source configured to generate a current (e.g., IINTP). In one example, the device  226  may be implemented as a comparator circuit. In one example, the device  228  may be implemented as an inverter. In one example, the device  230  may be implemented as a latch. In one example, the device  230  may be implemented as a D-type latch. 
         [0020]    Referring to  FIG. 3 , the phase signals PHE (phase early) and PHL (phase late) are shown. The respective complimentary phases PHEB and PHLB are also shown. The two phases PHE and PHL have a phase difference of Tp where Tp, is 1/8  of total clock period T. 
         [0021]      FIGS. 4-8  show various simulation graphs of various waveforms of the circuit  100 . Referring to  FIG. 4 , the bottom graph shows the signal PHE and the signal PHL across time. The mid-graph shows a waveform representing the signal VREF. This signal VREF starts ramping from 0 when the signal PHE transitions from low to high. The signal VREF stops ramping when the signal PHL transitions from low to high. At the time when the signal COUT is high, and both the signal PHE and the signal PHL are low, the signal VREF is discharged to ground (or 0). Until this time, the signal VREF remains at a particular value (e.g., IV in this case). The operation is shown across multiple cycles of the signal PHE and the signal PHL. 
         [0022]    Referring to  FIG. 5 , the bottom graph shows the signal PHE and the signal PHL. The top graph shows a waveform representing the signal VINTP. The signal VINTP starts ramping from 0 when the signal PHE transitions from high to low. The signal VINTP stops ramping when the signal PHL transitions from high to low. At the time when both the signal PHE and the signal PHL are low, the signal VINTP is discharged to ground (e.g., 0). Until this time, the signal VINTP remains at a particular value (e.g., 1V in this case). The operation is shown across multiple cycles of the signal PHE and the signal PHL. 
         [0023]    Referring to  FIG. 6 , the top graph shows the signal PHE and the signal PHL. The bottom graph shows a waveform representing the signal VREF. The signal VREF starts ramping from 0 when the signal PHE transitions from low to high. The signal VREF stops ramping when the signal PHL transitions from low to high. At the time when the signal COUT is high, and the signal PHE and the signal PHL are low, the voltage VREF is discharged to ground (e.g., 0). Until this time, the signal VREF remains at a particular value (e.g., 1V in this case). The graph also shows a waveform representing the signal VINTP. The signal VINTP starts ramping from 0 when the signal PHE transitions from high to low. The signal VINTP stops ramping when the signal PHL transitions from high to low. At the time when both the signal PHE and the signal PHL are low, the voltage VINTP is discharged to ground (e.g., 0). Until this time, the signal VINTP remains at a particular value (e.g., 1V in this case). The bottom graph also shows a waveform of the voltage VTAP- 1 , which is the waveform of voltage VTAP when the TAP  1  is selected in resistor ladder  104 . As the signal VINTP transitions above VTAP- 1 , the signal COUT is high and stays high till the signal VINTP transitions below VTAP- 1 . In general, the signal COUT is a phase shifted waveform with phase shift equal to Tp/8. The operation is shown across multiple cycles of the signal PHE and the signal PHL. 
         [0024]    Referring to  FIG. 7 , the top graph shows the signal PHE and the signal PHL. The bottom graph shows a number of signals ICLK- 1  to ICLK- 7 . The signals ICLK- 1  to ICLK- 7  represent a set of interpolated waveforms. The signal ICLK- 1  is a waveform of the signal ICLK when the TAP 1  is selected. The signal ICLK- 7  represents the signal ICLK when the TAP 7  is selected. 
         [0025]    Referring to  FIG. 8 , the zoomed picture of the interpolated waveforms ICLK- 1  to ICLK- 7  with spacing between consecutive waveform equal to Tp/8 is shown. 
         [0026]    Until the signal RESET transitions high, the output interpolated clock ICLK (and all the respective intermediate voltages) are zero. When the signal RESET is released, the phase interpolation begins. When both the signal PHE and the signal PHLB are high (e.g., during the period Tp), the switch SW 2  turns ON. The current source IREF starts charging the capacitor CREF. The charging stops when the signal PHLB transitions low. At this time, the switch SW 2  turns OFF and the switch SW 1  turns ON. At the end of this period, the voltage on the capacitor CREF is generally the reference voltage VREF. The reference voltage VREF is buffered as the voltage VREFBUF and then divided into eight equal parts using the resistor ladder R 0 -R 7 . 
         [0027]    When the signal PHEB and the signal PHL both are high, the switch SW 5  turns ON. The current source IINTP starts charging the capacitor CINTP. The charging stops when the signal PHL transitions low. The switch SW 5  then turns OFF and the switch SW 4  turns ON. At the end of this period, the voltage on the capacitor CINTP is generally the voltage VINTP. At such time, the value of current sources IREF and IINTP are equal. The two capacitors CREF and CINTP are also normally substantially equal in value. By virtue of these equalities, and the phase relationship between the signal PHE and the signal PHL, the time for which the switches SW 4  and SW 5  are ON, is normally equal and is Tp. The two voltages VREF and VINTP are also equal. The phase difference may be interpolated by selecting one of the taps of resistor ladder R 0 -R 7  to compare to the voltage VINTP. The particular number of resistors R 0 -R 7  implemented may be varied (e.g., increased or decreased) to meet the design criteria of a particular implementation. 
         [0028]    The interpolated phase may have resolution of Tp/8 as the voltage VREF is divided into eight equal parts. When the voltage VINTP crosses the voltage VTAP, the comparator output COUT transitions high. When the signal COUT is high, and both the signal PHE and the signal PHL are low, the reference capacitor CREF is discharged to ground. The next time both the signal PHE and the signal FHLB are high, the discharging stops and the capacitor CREF is once again charged to VREF. The other capacitor CINTP is generally discharged to ground when both the signal PHE and the signal PHL are high. During this time, the output COUT of the comparator  226  transitions low. The next time both the signal PHEB and the signal PHL are high, the discharging stops and the capacitor CINTP is once again charged to the voltage VINTP and again the output of the comparator  226  transitions high. This way the cycle repeats. 
         [0029]    The various signals of the present invention are generally “on” (e.g., a digital HIGH, or 1) or “off” (e.g., a digital LOW, or 0). However, the particular polarities of the on (e.g., asserted) and off (e.g., de-asserted) states of the signals may be adjusted (e.g., reversed)to meet the design criteria of a particular implementation. Additionally, inverters may be added to change a particular polarity of the signals. 
         [0030]    While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the scope of the invention.