Patent Publication Number: US-10790808-B2

Title: Variable delay circuits

Description:
TECHNICAL FIELD 
     Embodiments herein relate to variable delay circuits. In particular, they relate to passable latch circuits and variable delay chains for delaying or time aligning data in an electronic device. 
     BACKGROUND 
     Variable delay circuits are used in a wide variety of electronic devices including wireless communication devices such as base stations and other network nodes, as well as portable devices such as smartphones and other user equipment. Variable delay circuits are generally digital logic circuits and they may also be found in computer and other processors. 
     In the fifth generation (5G) wireless communication systems, one of the challenging issues is the variable delay unit, which must be able to operate at very high speed, e.g. 2.5 Gb/s or higher. A variable delay unit is usually used to delay the time of a data or clock signal. The power consumption of the variable delay unit should also be as low as possible otherwise it will lead to issues of heat dissipation and short battery time. Further, the time delay should also be as accurate as possible otherwise the signal correlation could be reduced or lost. 
     One part of such a wireless communication system where variable delays are required is an antenna array system where signals are sent and received at differing time instants e.g. when forming a directional beam. 
     There are some options for implementing a variable delay unit. One option is using standard logics such as shift latches or D-type Flip Flops (DFFs) and multiplexer (MUX). Static logic DFFs have higher power consumption, and MUX which may be used to select one of the delayed version of data among all possible delays will have a limit on operating speed when signal frequency is high. Another option is to use a barrel shifter based on an address decoder. A barrel shifter is a digital circuit that can shift a data word by a specified number of cycles without the use of any sequential logic, only pure combinatorial logic. However, the barrel shifter speed is even slower than the standard logics. A further option is to use an analog delay line, but the analog delay line cannot deal with large delay range. 
     So far there are no good solutions which can meet the data delay requirements for 5G wireless communication systems. 
     SUMMARY 
     Therefore, it is an object of embodiments herein to provide a variable delay circuit with improved performance. 
     According to one aspect of embodiments herein, the object is achieved by a passable latch circuit. The passable latch circuit comprises a dynamic latch comprising a first P-transistor, a first N-transistor, a second P-transistor, a second N-transistor and a clock input circuitry. Sources of the first and second P-transistors are connected to a first voltage, and sources of the first and second N-transistors are connected to a second voltage. Further, gates of the first P-transistor and the first N-transistor are connected to a data input, and drains of the second P-transistor and the second N-transistor are connected to a data output. Furthermore, a drain of the first P-transistor is connected to a gate of the second P-transistor, and a drain of the first N-transistor is connected to a gate of the second N-transistor. 
     The clock input circuitry is connected between the gates of the second P-transistor and the second N-transistor such that a first node of the clock input circuitry is connected to the drain of the first P-transistor and the gate of the second P-transistor, a second node of the clock input circuitry is connected to the drain of the first N-transistor and the gate of the second N-transistor. 
     The passable latch circuit further comprises a control switch connected between the gates of the second P-transistor and the second N-transistor. The control switch has an on state and an off state, and the passable latch circuit is configured to have different delays by controlling the state of the switch. 
     According to one aspect of embodiments herein, the object is achieved by a variable delay chain comprising two or more passable latch circuits according to the embodiments described above. 
     The passable latch circuit according to the embodiments herein uses a dynamic latch with a pass control. The dynamic latch is a True Single Phase Clock Logic (TSPC) dynamic latch or so called “Split-output latch” which comprises only 5 transistors. This dynamic latch is considered as the most power efficient latch. By adding an additional control switch which may be implemented by a transistor, the latch function is controlled by the switch. If the switch is on, the latch function is no long existed but becomes a pure delay instead due to that the dynamic latch circuit turns into two equivalent inverters cascaded. When the switch is off, the passable latch circuit is the same as the normal “Split-output latch”. A normal latch delays half clock cycle in retiming function. So the passable latch circuit  100  may be used as a delay unit or a retiming unit when the control switch is off to provide a latch delay or clocked delay or as a bypass unit when the control switch is on to have a pass delay. The pass delay is an intrinsic delay or propagation delay of the latch circuit. In this way, the passable latch circuit according to the embodiments herein may be configured to have different delays by controlling the state of the control switch, e.g. either be set as a bypass unit to have intrinsic delay or be set as a normal latch to have a half clock period delay. 
     A variable delay chain may be implemented by using many passable latch circuits as basic variable delay unit. A variation of the passable latch circuit may also be used as retiming unit. The variable delay chain built with such basic variable delay unit and the retiming unit has lower power consumption, higher speed, and less area comparing to the standard static logic. The implementation is also very simple and regular and the delay range is long, e.g. as long as several clock periods. 
     The passable latch circuit and the variable delay chain according to embodiments herein may be used in 5G wireless communication systems where digital beamforming is required in transceivers, and in a multiple-bit data path where multiple bits, e.g. 8 bits, are used. They may also be used in applications where data alignment is required, and the data rate is very high. 
     Thus, embodiments herein provide variable delay circuits with improved performance on e.g. power consumption, speed, delay range and area etc. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Examples of embodiments herein are described in more detail with reference to attached drawings in which: 
         FIG. 1  is a general block view of a passable latch circuit according to embodiments herein; 
         FIG. 2  is a schematic block view illustrating example implementations of the control switch in the passable latch circuit according to embodiments herein; 
         FIG. 3  is a schematic block view illustrating a first example embodiment of the passable latch circuit according to embodiments herein; 
         FIG. 4  is a schematic diagram illustrating a second example embodiment of the passable latch circuit according to embodiments herein; 
         FIG. 5  is a schematic diagram illustrating a third example embodiment of the passable latch circuit according to embodiments herein; 
         FIG. 6  is a general block view illustrating a variable delay chain according to embodiments herein; 
         FIG. 7  is a block view illustrating one example of a variable delay chain according to embodiments herein; 
         FIG. 8  is a block view illustrating another example of a variable delay chain according to embodiments herein; 
         FIG. 9  is a diagram illustrating delay scheme for a variable delay chain according to embodiments herein; 
         FIG. 10 ( a ) ( b ) ( c )  are block views illustrating example implementations of multiple-bit delay paths according to embodiments herein; 
         FIG. 11 ( a ) ( b ) ( c )  are example embodiments for creating clock phases for fine delay adjustment unit; and 
         FIG. 12  is a block diagram illustrating an electronic device in which embodiments herein may be implemented. 
     
    
    
     DETAILED DESCRIPTION 
     A general schematic block view of a passable latch circuit  100  according to embodiments herein is shown in  FIG. 1 . The passable latch circuit (PL)  100  comprises a dynamic latch which comprises a first P-transistor MPI, a first N-transistor MNI, a second P-transistor MPD, a second N-transistor MND and a clock input circuitry  110 . 
     As shown in  FIG. 1 , the transistors in the dynamic latch are connected in the following way: 
     sources of the first and second P-transistors MPI, MPD are connected to a first voltage e.g. a power supply Vdd; sources of the first and second N-transistors MNI, MND are connected to a second voltage, e.g. a ground Gnd; 
     gates of the first P-transistor MPI and the first N-transistor MNI are connected to a data input D; 
     drains of the second P-transistor and the second N-transistor are connected to a data output Q; 
     a drain of the first P-transistor MPI is connected to a gate of the second P-transistor MPD; and 
     a drain of the first N-transistor MN 1  is connected to a gate of the second N-transistor MND. 
     Further, the clock input circuitry  110  is connected between the gates of the second P-transistor and the second N-transistor in the following way: 
     a first node  111  of the clock input circuitry  110  is connected to the drain of the first P-transistor MPI and the gate of the second P-transistor MPD, and a second node  112  of the clock input circuitry  110  is connected to the drain of the first N-transistor MNI and the gate of the second N-transistor MND. 
     The passable latch circuit  100  further comprises a control switch  120  connected between the gates of the second P-transistor MPD and the second N-transistor MND. The control switch  120  has an on state and an off state. 
     When the control switch is off, the passable latch circuit  100  is a normal dynamic latch, which is a True Single Phase Clock Latch (TSPC) or so called a “Split-output latch”. This dynamic latch is considered as the most power efficient latch. By adding an additional control switch  120 , the latch function may be controlled by a control signal PASS. If the control signal PASS turns on the control switch  120 , then the latch function no longer exists but becomes a pure delay instead. This is due to that the passable latch circuit  100  turns into two cascaded equivalent inverters, where the first P-transistor and N-transistor forms a first inverter and the second P-transistor and N-transistor forms a second inverter. When the control signal PASS turns off the control switch, the passable latch circuit  100  is the same as the “Split-output latch” and has normal latch function. A normal latch delays input data half clock cycle in retiming function. So, the passable latch circuit  100  may be used as a delay unit or a retiming unit when the control switch is off to provide a latch delay or clocked delay or as a bypass unit when the control switch is on to have a pass delay. The pass delay is an intrinsic delay or propagation delay of the latch circuit. 
     Therefore, according to embodiments herein, the passable latch circuit  100  may be configured to have different delays by controlling the state of the control switch  120 . 
     The passable latch circuit shown in  FIG. 1  may be configured as a single phase retiming unit, where only one clock signal Ck is used. 
     According to some embodiments herein, the control switch  120  may be implemented by an N-transistor, a P-transistor or a transmission gate comprising an N-transistor and a P-transistor connected in parallel, as shown in  FIG. 2 . 
     According to some embodiments, the clock input circuitry  110  may comprise an N-transistor.  FIG. 3  shows an example embodiment of the passable latch circuit  100 , wherein the clock input circuitry  110  comprises an N-transistor MN 1 . A drain of the N-transistor MN 1  is connected to the first node  111  of the clock input circuitry  110 , a source of the N-transistor MN 1  is connected to the second node  112  of the clock input circuitry  120  and a gate of the N-transistor MN 1  is connected to a clock input Ck. In this embodiment, the control switch  120  is implemented by an N-transistor MPass. The control switch  120  may also be implemented by a P-transistor or a transmission gate as discussed above. The dynamic latch with N-transistor in the clock input circuitry  110  is an N-dynamic latch. In order to simplify the connections of circuits built by the passable latch circuit  100 , a symbol of the passable latch circuit, denoted as PL, with input/output and control terminals is shown to the right in  FIG. 3 . 
     Alternatively, the clock input circuitry  110  may comprise a P-transistor (not shown), a source of the P-transistor is connected to the first node  111  of the clock input circuitry  110 , a drain of the P-transistor is connected to the second node  112  of the clock input circuitry  110  and a gate of the P-transistor is connected to a clock input Ck. The dynamic latch with P-transistor in the clock input circuitry  110  is a P-dynamic latch. 
     According to some embodiments, the clock input circuitry  110  may comprise two clock input branches, as shown in  FIG. 4 . Each clock input branch comprises a first N-transistor MN 11 , MN 21  and a second N transistor MN 12 , MN 22  connected in series between the first and second nodes  111 ,  112  of the clock input circuitry  110 . 
     A gate of the first N-transistor MN 11 , MN 21  is connected to a respective clock signal Ck 1 , Ck 2  input terminals and a gate of the second N-transistor MN 12 , MN 22  is connected to a respective control signal S 1 , S 2  input terminals. 
     The passable latch circuit shown in  FIG. 4  may be configured to be a two-phase retiming unit, denoted as  400 , by controlling the two clock input branches and the state of the control switch MPass. Two opposite clock phase signals Ck 1 , Ck 2  may be applied on the clock signal input terminals, and controlled by the respective control signals S 1  and S 2 . The clock signals Ck 1  and Ck 2  only take effect when the clock and control input terminals are both logic high. The passable control PASS has the same function as in the passable latch circuit  100 ,  300  shown in  FIGS. 1 and 3 , meaning that the two-phase retiming unit  400  can also be by-passed. In order to simplify the connections of circuits built by the two-phase retiming unit  400 , a symbol of the two-phase retiming unit  400 , denoted as C 2 , with input/output and control terminals is shown to the right in  FIG. 4 . 
     According to some embodiments, the clock input circuitry  110  may comprise a plurality of M clock input branches, as shown in  FIG. 5 . Each clock input branch comprises a first N-transistor MN 11 , MN 21  . . . MNm 1  and a second N-transistor MN 12 , MN 22 , MNm 2  connected in series between the first and second nodes of the clock input circuitry  110 . 
     A gate of the first N-transistor MN 11 , MN 21 , . . . MNm 1  is connected to a respective clock signal Ck 1 , Ck 2 , . . . Ckm input terminals and a gate of the second N-transistor MN 12 , MN 22 , . . . MNm 2  is connected to a respective control signal S 1 , S 2 , . . . Sm input terminals. 
     The passable latch circuit shown in  FIG. 5  may be configured to be a multiple M-phase retiming unit, denoted as  500 , by controlling the M clock input branches and the state of the control switch MPass. Except that the M-phase retiming unit  500  has more clock input terminals and control terminals, it works similarly to the PL  100 ,  300  and two-phase retiming unit C 2   400 . When multiple phase clocks are appeared at the clock input terminals, and proper control signals are set at the S 1 , S 2 , . . . Sm input terminals, one may get a required delay among the M possible phases to achieve a fine delay retiming function. 
     In order to simplify the connections of circuits built by the M-phase retiming unit  500 , a symbol of the M-phase retiming unit  500 , denoted as Cm, with input/output and control terminals is shown to the right in  FIG. 5 . 
     A single phase retiming unit, e.g. PL  100 ,  300 , consumes the least power in operation comparing to C 2  and Cm units  400 ,  500 . The passable latch circuits may be built with Complementary Metal Oxide Semiconductor (CMOS) transistors. As circuit speed improves as the CMOS technology develops, the PL  100 ,  300 ,  400 ,  500  according to embodiments herein may reach very high speed for latch operation. 
     As discussed above, the passable latch circuit  100 ,  300 ,  400 ,  500  according to embodiments herein comprises a dynamic latch which latches data for a short time, e.g. a clock period. If the data needs to be kept or stored for a longer time, the passable latch circuit  100 ,  300 ,  400 ,  500  may comprise a logic holding circuit (not shown), so the dynamic latch becomes a static one. 
     All of the passable latch circuits  100 ,  300 ,  400 ,  500  described above with relation to  FIGS. 1-5  may be used as a bypass unit to provide an intrinsic delay or propagation delay when the control switch  120  is on, or as a retiming unit to provide a clocked delay or latch delay when the control switch  120  is off. The passable latch circuits  100 ,  300 ,  400 ,  500  may be combined into a chain comprising multiple passable latch circuits or bypass/retiming units. The chain may be controlled using a controller which sets the control switch  120  in each unit. 
     The delay obtained using a chain will depend on the state of the control switch in each unit and the applied and/or selected clock for each unit. 
     A variable delay chain may be divided into one or more segments.  FIG. 6  shows a top view of a variable delay chain VDC  600  comprising one or more segments Seg( 1 ), Seg( 2 ) . . . Seg(k). Each segment may comprise at least one variable delay unit VD and at least one retiming unit RT after the VD, and in each segment, except for the last one, the last stage must be a retiming unit. In the last segment Seg (k), RT may be replaced by a fine adjustment unit FA. 
     The VD may be implemented by the passable latch circuit  100 ,  300 , two-phase and/or M-phase retiming unit  400 ,  500  according to the embodiments described above in  FIGS. 1-5 . 
     The retiming unit RT may e.g. be a passable latch circuit shown in  FIGS. 1-5  but it should be configured such that the control switch  120  is switched off. Alternatively, the RT may be an equivalent circuit without the control switch  120  which would always provide a latch delay or a clocked delay, i.e. the last retiming unit of each segment is configured to provide a clocked delay. 
     A VD unit may be configured as a variable delay unit or re-configured as a combination of variable delay units separated by retiming units. 
     For instance, a VD may comprise 10 passable latch circuit  100 ,  300  in serial. The VD may be configured as a variable delay unit by configuring 9 of the passable latch circuits  100 ,  300  as bypass but not the last one. This VD may be re-configured as 2 VDs separated by a retiming unit, where each VD have 5 passable latch circuits  100 ,  300  in serial, 4 of them may be configured as bypass but not the 5th and 10th of the passable latch circuits  100 ,  300 . The 5th and 10th of the passable latch circuits  100 ,  300  may be used as the retiming unit. 
     The variable delay chain VDC  600  further comprises a controller  610  which is used as a delay control generator, configured to generate delay control signals PASS for controlling the variable delay chain  600  by controlling the state of the control switches in the passable latch circuits  100 ,  300  and/or M-phase retiming units  400 ,  500 . The controller  610  may be further configured to select a clock input branch by controlling the states of the control signal inputs S 1 , S 2  . . . Sm in the multiple M-phase retiming units  400 ,  500 . 
     In other words, the variable delay chain  600  may be controlled by a digital vector Delay Value as shown in  FIG. 6 . Based on this Delay Value, the controller  610  creates a control word PASS, and may also generate clock control signals S 1 , S 2  . . . Sm if multiple M-phase retiming units  400 .  500  are used, and control the VDC  600  to reach the desired delay. 
     According to some embodiments, the first segment may comprise one or more two-phase retiming units C 2   400 , and the last segment may comprise a two-phase retiming unit C 2   400  and a passable latch circuit PL  100 ,  300 .  FIG. 7  shows an example of a multiple segment VDC  700  formed by the passable latch PL  100 ,  300  shown in  FIGS. 1 and 3  and the two-phase retiming unit C 2   400  shown in  FIG. 4 . As shown in  FIG. 7 , all VD units are built with two-phase retiming unit C 2   400 , except that the last one which is a PL  100 ,  300  and triggered by Ckm to get accurate delay, where Ckm is the m-th clock signal with a delay at desired accuracy. 
     According to some embodiments, the first segment may comprise one or more passable latch circuits PLs  100 ,  300  and a two-phase retiming unit C 2   400 , and the last segment may comprise one or more M-phase retiming units Cm  500 . 
       FIG. 8  shows an example embodiment of a multiple segment VDC  800 . In this example, all main delay latches are built with the passable latch PL  100 ,  300  shown in  FIGS. 1 and 3 . In the last segment, the last retiming unit is a fine delay unit built with a multiple-phase retiming unit Cm  500  shown in  FIG. 5 . Of course the last retiming unit may be built with C 2   400  or PL  100 ,  300 , but triggered by Ckm. 
     One design option is that the PLs are configured to be triggered by Ck 1  and Ck 2  along the delay chain in an interleaved manner, as shown in  FIG. 8 . 
     In general, according to embodiments herein, a VDC may be built with different options, chosen among the basic delay circuits PL  100 ,  300 , C 2   400 , and Cm  500  described above. Inserting one latch operating at an opposite clock phase to a clock applied in a preceding latch, with a clock period Tp will delay data signal a half clock period Tp/2 if a timing criteria is met, which will be discussed in the following. Two latches operating at opposite clock phases can form a flip-flop function, and delay the data one clock period Tp. 
     Inserting one latch operating at the same clock phase to a clock applied in a preceding latch, with a clock period Tp will delay data signal a full clock period Tp if a timing criteria is met, which will be described in the following. 
     If a very long variable delay time is required, for instance the total delay time is reaching 100 clock periods, then the VDC must be cut into several segments, Seg(i), i=1, 2, . . . , k, each comprise a VD and a RT. Each segment can be re-configured as multiple VDs separated by RTs, and ended with an RT stage. But for a short variable delay one segment may be enough. 
     In the following, variable time delay of a VDC will be discussed. 
     In general, the delay of a VDC is 
     
       
         
           
             
               
                 
                   Delay 
                   = 
                   
                     
                       td 
                       fine 
                     
                     + 
                     
                       
                         ∑ 
                         i 
                       
                       ⁢ 
                       
                         Delay 
                         ⁡ 
                         
                           ( 
                           
                             Seg 
                             ⁡ 
                             
                               ( 
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                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
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     Where td fine  is a delay from the fine adjustment unit. The delay range is defined as the time from the minimum delay to the maximum delay. 
     In the following, the delay from a segment is discussed. As described earlier, a segment may comprise a VD followed by an RT. The VD may comprise one or more PLs, as shown in  FIG. 8 , in Seg ( 1 ). When the PL delay units or retiming units use N-dynamic latch based circuits, data is transparent at the output when the clock terminal is logic high. 
     For keeping timing correctly, there are should be allowed upper and lower limits of intrinsic delay for a PL. Assume tdp_max and tdp_min are the maximum and the minimum intrinsic time delay allowed for a PL, respectively, between two retiming units triggered by two clock phases. Also assume that the incoming data is triggered with the second clock rising edge Ck 2 . 
     For a first case, i.e. when the incoming data is triggered with the second clock rising edge Ck 2  and the first clock Ck 1  rising edge is used in the retiming unit. Assume that ni stages of PL latches are set as in a bypass state, i.e. each PL has a delay of tdp, where ni is an odd number. When the ni latches are set as passable, they have delay D 1 =ni·tdp. For keeping the output data in the correct time, the edge of the output of the delayed data is retimed by the first clock Ck 1 , the following timing criteria is sufficient condition for correct timing:
 
max( D 1)= ni·tdp _max&lt;0.5· Tp   (2)
 
     In such a delay segment, Seg(i), the minimum delay is a half clock period, i.e. Delay(i)_min(i)=0.5·Tp, and the maximum delay is ni times the half clock period, that is Delay_max(i)=0.5·ni·Tp. As such the delay segment may be implemented only using the PL  100 ,  300 , which gives minimum power consumption. Then the delay of the segment Seg(i) is 
     
       
         
           
             
               
                 
                   
                     
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                       ⁡ 
                       
                         ( 
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                         ) 
                       
                     
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                                 ( 
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                         · 
                         Tp 
                       
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                   ⁢ 
                   
                     
                       where 
                       ⁢ 
                       
                           
                       
                       ⁢ 
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                       ⁡ 
                       
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                     ≤ 
                     ni 
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     C(i) is a control word for the delay segment Seg(i). 
     For a second case, i.e. when the incoming data is triggered with the second clock rising edge Ck 2  and the same rising edge of the second clock Ck 2  is used in the retiming unit. When the ni latches are set as passable, they have delay D 2 =ni·tdp. For keeping the output data in the correct time, the edge of the delayed output data is retimed by the second clock Ck 2 . The following timing criteria are sufficient condition for correct timing:
 
max( D 2)= ni·tdp _max&lt; Tp  
 
min( D 2)= ni·tdp _min&gt; Tw   (4)
 
     Where Tw is the clock pulse width and Tw is typically less than Tp/2. In such a delay segment, the minimum delay is one clock period, and the maximum delay is ni times half clock period. Where ni is an even number. 
     Comparing Eq. (4) with Eq. (2), using the same clock edge in the retiming unit as the clock edge triggering incoming data could allow twice of delay units, i.e. PLs, used in one segments as in the first case, thus increases the delay time. But the minimum delay is also twice of that for the first case. 
     The same clock phase may only be asserted after the data signal has passed though sufficient PLs configured as bypass to provide the minimum intrinsic delay of Tw or Tp/2. 
     In case that the delay provided by the PL chain is too short i.e. is less than half of Tp, an opposite clock should be asserted. 
     There are many variations when building a delay segment. For example, all PLs in  FIG. 8  may be replaced by C 2 . This is shown in  FIG. 7 . 
     In this way, one could implement any delay, 
                 i   .   e   .           ⁢   td     =       n   ·   Tp     2       ,         
where n is an integer, and n&gt;1.
 
     The functions and different delays of the VDC may be further illustrated by a delay scheme shown in  FIG. 9 . In  FIG. 9 , the input data shown on the top of the figure is triggered by Ck 2  and the output data is shown at the bottom of the figure. Two cycles, R 1  and R 2 , shown in the middle of the figure, represent the retiming units, and the texts Ck 1 , Ck 2  inside the cycles indicate which clock rising edge is used for latching the retiming units. The multiple paths denoted by numbers  901 - 908  with different delay times D 1  and D 2  represent varies delay paths possible from the input node to the output node to achieve different delay time. For example, the paths  901 →R 2 → 907  and  901 →R 2 → 903 →R 1 → 904 →R 2 → 907  have different delay time. 
     For a segment but not the last one, the output can always come from R 2 , i.e. from a retiming unit triggered by the second clock edge. This is shown by the solid path lines e.g.  901 →R 2 → 907 , or  901 →R 2 → 903 →R 1 → 904 →R 2 → 907 , or  901 →R 2 → 905 →R 2 → 907 , from the input node to the output node. 
     For the last segment, the output may come from either R 1  or R 2  and pass the Fine Adjust unit F. This is shown in either solid path line, e.g.  907  or dashed path line, e.g.  908 . 
     When the path goes through D 1  one time, it will contribute  1  unit delay, and when it goes though D 2  one time, it will contribute  2  unit delay. Total delay is the sum of all contributions as the path goes through from the input node to the output node, excluding the fine delay adjust in the fine adjustment, which is less than half of clock period. Note that one unit delay is half of clock period and two unit delay is one clock period. 
     So for a segment, but not the last one, the delay is: 
     
       
         
           
             
               
                 
                   
                     Delay 
                     ⁡ 
                     
                       ( 
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                       ) 
                     
                   
                   = 
                   
                     
                       
                         ∑ 
                         
                           k 
                           = 
                           1 
                         
                         
                           Ni 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                       
                       ⁢ 
                       
                         Tp 
                         2 
                       
                     
                     + 
                     
                       
                         ∑ 
                         
                           k 
                           = 
                           1 
                         
                         
                           Ni 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                       
                       ⁢ 
                       Tp 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     Where Ni 1  is the number of paths go through D 1  and Ni 2  is the number of paths go through D 2 . 
     And for the VDC, the delay is 
     
       
         
           
             
               
                 
                   Delayvdc 
                   = 
                   
                     
                       td 
                       fine 
                     
                     + 
                     
                       
                         ∑ 
                         
                           j 
                           = 
                           1 
                         
                         k 
                       
                       ⁢ 
                       
                         Delay 
                         ⁡ 
                         
                           ( 
                           k 
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     Where td fine  is the delay from the fine adjustment unit and k is the number of segments in the VDC. 
     With the delay scheme shown in  FIG. 9 , It is easy to see if the output comes from R 1  or R 2  and finally to node F. If the output comes from R 2  and finally to node F, the data is delayed by N times of the clock period, where N is an integer. If the output comes from R 1  and finally to node F, the data is delayed by (2N+1)/2 times of the clock period. This may be used as design rules to make the circuit function properly, i.e, the delay is correct and logic is correct. 
     According to some embodiments, a multiple-bit data delay path may be built which comprises a plurality of variable delay chains VDCs. Each variable delay chain VDC may be configured to delay one bit of the multiple-bit data.  FIG. 10( a ) ( b ) ( c )  show three examples of multiple-bit data delay path. In  FIG. 10( a ) , the VDC, for instances, comprises two segments each comprises a VD and an RT. In the first segment, the VD comprises a plurality of PLs, e.g. 4 PLs and the RT comprises a two-phase retiming unit C 2 . In the second segment, the VD also comprises a plurality of PLs, 3 PLs and the RT comprises a two-phase retiming unit C 2  and an M-phase retiming unit Cm. 
     Multiple phase clocks CK={Ck 1 , . . . , Ck 2 , . . . , CkM} are needed for the VDC shown in  FIG. 10( a ) , where Ck 1  and Ck 2  are two-phase non-overlapping clocks among the M-phase clocks. A required phase Ckm is created among the M-phase clocks, and used in the last stage for fine delay adjustment according to the required time delay Td: 
     
       
         
           
             
               
                 
                   
                     Td 
                     = 
                     
                       
                         j 
                         · 
                         Tp 
                       
                       + 
                       
                         t 
                         dk 
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     an 
                     ⁢ 
                     d 
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       
                         k 
                         · 
                         Tp 
                       
                       M 
                     
                     ≤ 
                     
                       t 
                       dk 
                     
                     ≤ 
                     
                       
                         
                           ( 
                           
                             k 
                             + 
                             1 
                           
                           ) 
                         
                         · 
                         Tp 
                       
                       M 
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     Where Tp is clock period, and k=0, 1, . . . , M−1. t dk  is delay difference between two successive clocks, and j is the number of period of the VDC delays. 
     In  FIG. 10( b ) , the retiming unit RT in the second segment is built with three 2-phase retiming units C 2   400 . Then the VDC only needs 3 clock phases, that is two-phase clocks Ck 1  and Ck 2 , and an additional fine delayed clock phase Ckm, to delay the data according to a required delay at a required accuracy. 
     In  FIG. 10( c ) , the retiming unit RT in the first segment is built with a PL  100 ,  300  and the retiming unit RT in the second segment is built with two 2-phase retiming units C 2   400 . If timing error can be avoided between VDs, the C 2   400  may be replaced by a PL 100 ,  300  and make circuit simple. 
     In general, for implementing a fine delay tuning or adjustment in the last stage, at least three latches may be needed, for example in  FIG. 10( b ) . Two of them deliver required delay at an accuracy of half clock period, and the last one creates an extra delay which is less than half of clock period, at the required accuracy. 
     The fine delay adjustment may be implemented in many other ways. One way is to delay the data by an analog variable delay circuit. The other alternative is to delay the clock rather than the data itself, and use the accurate delayed clock to trigger the data by latches implemented by, e.g. the passable latch circuit PL  100 ,  300 , two- or M-phase retiming unit C 2   400 , Cm  500 . 
     Therefore, according to some embodiments, the fine delay adjustment unit may be implemented by any one of the passable latch circuit  100 ,  300 , two-phase or M-phase retiming unit  400 ,  500 . Then the fine delay adjustment is realized by delaying the clock signal used to trigger the data in the fine adjustment unit. 
     The ways to generate the required clock edge for fine delay adjustment are shown in  FIG. 11 , where (a) is via variable capacitance connected to the output of delay buffers, while (b) is via a multiple phase clock generator implemented by, for instance a Digital Logic Latch (DLL), and a MUX to select the desired clock phase; and (c) is a quadrature clock generator, which first creates quadrature clocks, and then via a phase rotation circuit to interpolate the desired clock phase. 
     To summarise the discussions above, advantages of various embodiments of the passable latch circuit PL  100 ,  300 , two- or M-phase retiming unit C 2   400 , Cm  500  and variable delay chain VDC  600  include: 
     The passable latch circuit  100 ,  300  is power efficient and can operate at high speed. 
     The variable delay chain  600 ,  700 ,  800  implemented by the passable latch circuit  100 ,  300  and the two-phase or M-phase retiming unit  400 ,  500  has lower power consumption, higher speed, and less area comparing to the prior art solution built with the standard static logic. The implementation is also very simple and regular and the delay range is long, e.g. as long as several clock periods. 
     The passable latch circuit  100 ,  300 ,  400 ,  500  and the variable delay chain  600 ,  700 ,  800  according to embodiments herein may be used in 5G wireless communication systems where digital beamforming is required in transceivers, and in a multiple-bit data path where the bit-width is at least 8 bits. They may also be used in applications where data alignment is required, and the data rate is very high. 
     The passable latch circuit PL  100 ,  300 , two- or M-phase retiming unit C 2   400 , Cm  500  and the variable delay chain VDC  600 ,  700 ,  800 , the multiple-bit data delay path  1000  according to the embodiments herein may be employed in digital logic circuits or various electronic devices.  FIG. 12  shows a block diagram for an electronic device  1200 . The electronic device  1200  may be a receiver, a transmitter, a transceiver, a wireless communication device, such as a user equipment or a mobile device and/or a base station, a multi-antenna systems in a radio base station, or any general electronic circuit or equipment, such as computers, processors etc. which use digital logic circuits and need variable delay circuits. The electronic device  1200  may comprise other units, where a processing unit  1210  is shown, which may interact with the delay control generator  610 ,  710 ,  810  in the variable delay chain VDC  600 ,  700 ,  800  and the multiple-bit data delay path  1000  for different delay settings. 
     Those skilled in the art will understand that although N-transistor, P-transistor and control switch transistor as shown in  FIGS. 1-5  are Field-Effect Transistors (FET), any other types of transistors, e.g. Metal-Oxide-Semiconductor FET (MOSFET), Junction FET (JFET), etc., may be comprised in the passable latch circuit PL  100 ,  300 , two- or M-phase retiming unit C 2   400 , Cm  500 . 
     When using the word “comprise” or “comprising” it shall be interpreted as non-limiting, i.e. meaning “consist at least of”. 
     The embodiments herein are not limited to the above described preferred embodiments. Various alternatives, modifications and equivalents may be used. Therefore, the above embodiments should not be taken as limiting the scope of the invention, which is defined by the appended claims.