Patent Publication Number: US-11640184-B2

Title: Phase synchronized LO generation

Description:
INCORPORATION BY REFERENCE 
     This present disclosure claims the benefit of U.S. Provisional Application No. 62/869,149, “PHASE SYNCHRONIZED LO GENERATION” filed on Jul. 1, 2019, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     A transceiver may have multiple transmitter (TX) paths. Each path may have its own local oscillator (LO), which may be generated from a divide-by-2 (Div2) frequency divider associated with the path. Although these frequency dividers can output the same frequency LO, the phases of these LO may be different and varied, which may cause a meta-stability issue. In order to avoid the meta-stability issue, it may be desirable to synchronize different dividers so that their phase difference can be kept constant. 
     SUMMARY 
     Aspects of the disclosure provide an apparatus for controlling an internal reset signal that is synchronous to a clock signal. The apparatus includes a clock switch circuit and a plurality of serially coupled D flip-flops (DFFs). The clock switch circuit receiving the clock signal can output the clock signal in an on state and block the clock signal in an off state. The plurality of serially coupled DFFs are coupled to the clock switch circuit and driven by the clock signal. In response to an external reset signal being enabled, the plurality of serially coupled DFFs can enable the internal reset signal. After a predefined number of clock signal cycles after the external reset signal is disabled, the plurality of serially coupled DFFs can disable the internal reset signal. 
     In an embodiment, a data terminal of a first DFF in the plurality of DFFs is coupled to a logic voltage at which the internal reset signal is enabled, and an inverse output terminal of a last DFF in the plurality of DFFs is coupled to the internal reset signal. 
     In an embodiment, an input terminal of the clock switch circuit is coupled to the clock signal, an output terminal of the clock switch circuit is coupled to a clock terminal of each DFF in the plurality of DFFs, and a control terminal of the clock switch is coupled to the external reset signal. 
     In an embodiment, the clock switch circuit is in the off state so that the clock signal is not output to the plurality of DFFs in response to the external reset signal being enabled, and in the on state so that the clock signal is output to the plurality of DFFs in response to the external reset signal being disabled. 
     In an embodiment, the external reset signal is coupled to a reset terminal of each DFF in the plurality of DFFs. 
     In an embodiment, each DFF is set in a reset mode and in a standby mode in response to the external reset signal being enabled and disabled, respectively. 
     In an embodiment, the predefined number of clock signal cycles is dependent on a total number of DFFs. 
     Aspects of the disclosure also provide a method for controlling an internal reset signal that is synchronous to a clock signal. In the method, the clock signal is received through a clock switch circuit that is configured to output the clock signal in an on state and block the clock signal in an off state. The internal reset signal is enabled through a plurality of serially coupled D flip-flops (DFFs) in response to an external reset signal being enabled, and disabled through the plurality of serially coupled DFFs after a predefined number of clock signal cycles after the external reset signal is disabled. The plurality of serially coupled DFFs are coupled to the clock switch circuit and driven by the clock signal. 
     Aspects of the disclosure also provide an electronic system including a local divider reset (LORST) circuity. The LORST circuitry receives a clock signal through a clock switch circuit that is configured to output the clock signal in an on state and block the clock signal in an off state. The LORST circuitry enables an internal reset signal through a plurality of serially coupled D flip-flops (DFFs) in response to an external reset signal being enabled, and disables the internal reset signal through the plurality of serially coupled DFFs after a predefined number of clock signal cycles after the external reset signal is disabled. The plurality of serially coupled DFFs are coupled to the clock switch circuit and driven by the clock signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of this disclosure that are proposed as examples will be described in detail with reference to the following figures, wherein like numerals reference like elements, and wherein: 
         FIG.  1    shows an exemplary local divider reset (LORST) circuitry according to an embodiment of the disclosure; 
         FIGS.  2 A- 2 D  show detailed operations of the LORST circuitry according to an embodiment of the disclosure; 
         FIG.  3    shows an exemplary timing diagram of controlling an internal reset signal RST_INT according to an embodiment of the disclosure; 
         FIG.  4    shows an exemplary timing diagram according to an embodiment of the disclosure; 
         FIG.  5    shows an exemplary divider circuit block and the corresponding detailed circuitry according to an embodiment of the disclosure; 
         FIG.  6    shows an example of controlling two divider circuit blocks according to an embodiment of the disclosure; 
         FIG.  7 A  shows an exemplary local LORST configuration according to an embodiment of the disclosure; 
         FIG.  7 B  shows an exemplary cascade LORST configuration according to an embodiment of the disclosure; 
         FIGS.  8 A- 8 D  show detailed operations of the cascade LORST configuration according to an embodiment of the disclosure; 
         FIG.  8 E  show an alternative of the cascade LORST configuration according to an embodiment of the disclosure; 
         FIG.  9    shows a flowchart of an exemplary process according to an embodiment of the disclosure; and 
         FIGS.  10 A- 10 C  show some exemplary transceivers according to embodiments of the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG.  1    shows an exemplary local divider reset (LORST) circuitry  100  according to an embodiment of the disclosure. The LORST circuitry  100  includes a clock switch circuit  101  and a plurality of (e.g., M) serially coupled D flip-flops (DFFs). 
     The clock switch circuit  101  receives a clock signal CLK and is controlled by a reset signal RST. The clock signal CLK can be either a global clock signal or a local clock signal. The reset signal RST can be either a global reset signal or a local reset signal. In the disclosure, the clock signal CLK and the reset signal RST can be also referred to as external clock signal and external reset signal, respectively. It is noted that the external reset signal RST does not have timing information of the external clock signal CLK. That is, the external reset signal RST may not be synchronous to the external clock signal CLK. 
     The clock switch circuit  101  includes a switch  102  and an inverter  103 . An input terminal of the switch  102  is coupled to the external clock signal CLK and an output terminal of the switch  102  is coupled to a clock terminal of each DFF in the plurality of DFFs of the LORST circuitry  100 . In addition, a control terminal of the switch  102  is coupled to an output terminal of the inverter  103 . An input terminal of the inverter  103  is coupled to the external reset signal RST. 
     The operation of the clock switch circuit  101  is controlled by the external reset signal RST. When the external reset signal RST is enabled, the clock switch circuit  101  can block the external clock signal CLK so that no clock signal triggers the plurality of DFFs. When the external reset signal RST is disabled, the clock switch circuit  101  can pass the external clock signal CLK to be an internal clock signal CLK_INT that triggers the plurality of DFFs. 
     In  FIG.  1   , it is assumed that the external reset signal RST is enabled at logic 1 (e.g., a supply voltage VCC) and disabled at logic 0 (e.g., a ground voltage GND), and the switch  102  is turned on and off when the control terminal of the switch  102  is set as logic 1 and logic 0, respectively. Accordingly, when the external reset signal RST is enabled, the inverter  103  receives logic 1 and outputs logic 0, setting the control terminal of the switch  102  as logic 0. Therefore, the switch  102  is turned off, and then the external clock signal CLK is blocked by the switch  102 . When the external reset signal RST is disabled, the inverter  103  receives logic 0 and outputs logic 1, setting the control terminal of the switch  102  as logic 1. Therefore, the switch  102  is turned on, and then the external clock signal CLK can be passed through the switch  102  to be the internal clock signal CLK_INT that is coupled to the clock terminal of each DFF. 
     It is noted, in other embodiments, the external reset signal RST can be enabled at logic 0 and disabled at logic 1. Accordingly, the switch  102  is turned on and off when the control terminal of the switch  102  is set as at logic 0 and at logic 1, respectively. 
     Referring back to  FIG.  1   , each DFF in the LORST circuitry  100  includes a data terminal D, an output terminal Q (and/or an inverse output terminal QB), a reset terminal R, and a clock terminal. The plurality of DFFs is serially coupled from a first DFF  104  to a last DFF  107 . That is, a data terminal D of a current DFF (e.g. DFF  105 ) is coupled to an output terminal Q of a previous DFF (e.g., DFF  104 ), and an output terminal Q of the current DFF is coupled to a data terminal D of a next DFF (e.g., DFF  106 ). 
     According to aspects of the disclosure, the LORST circuitry  100  can output an internal reset signal RST_INT through the inverse output terminal QB of the last DFF  107 . 
     In the  FIG.  1    example, it is assumed that the internal reset signal RST_INT is enabled at logic 1 and disabled at logic 0, thus a data terminal D of the first DFF  104  is coupled to logic 1. However, it is noted that if the internal reset signal RST_INT is enabled at logic 0 and disabled at logic 1, the data terminal D of the first DFF  104  can be coupled to logic 0 (e.g., GND) in another example. In such an example, when the external reset signal RST is enabled, each DFF outputs logic 1 so that the internal reset signal RST_INT is set as logic 0 through the inverse output terminal QB of the last DFF  107 . When the external reset signal RST is disabled, after M clock signal cycles, the internal reset signal RST_INT is disabled at logic 1 due to the data terminal D of the first DFF  104  being coupled to logic 0. Therefore, the data terminal D of the first DFF is coupled to a logic voltage at which the internal reset signal RST_INT is enabled. 
     In addition, in the LORST circuitry  100 , a clock terminal and a reset terminal of each DFF are coupled to the output terminal of the switch  102  and the external reset signal RST, respectively. The inverse output terminal QB of the last DFF  107  is coupled to a plurality of (e.g., N) divider blocks DIV-1˜DIV-N in the  FIG.  1    example. However, in another example, the inverse output terminal QB of the last DFF  107  can be coupled to various circuit blocks with a reset function. 
     According to aspects of the disclosure, the internal reset signal RST_INT can be synchronous to the external clock signal CLK in order to reset the divider blocks DIV-1˜DIV-N in the  FIG.  1    example or various circuit blocks in other examples. An operation of resetting the divider blocks DIV-1˜DIV-N (or various circuit blocks in other examples) includes: (1) resetting all internal nodes of the divider blocks with predefined voltages; (2) using the LORST circuity to generate internal reset signal RST_INT that is synchronous to the external clock signal CLK; and (3) unlocking the divider blocks, so that the divider blocks can be free running. 
     Specifically, when the external reset signal RST is enabled, the external clock signal CLK is blocked, and each DFF is set in a reset mode and thus outputs logic 0, so that the internal reset signal RST_INT is set as logic 1 through the inverse output terminal QB of the last DFF  107 . When the external reset signal RST is disabled, each DFF is set in a standby mode and the external clock signal CLK is passed through the switch  102  to each DFF. After a number of clock signal cycles, the internal reset signal RST_INT is flipped from logic 1 to logic 0. Accordingly, the internal reset signal RST_INT can be synchronous to the external clock signal CLK. The number of clock signal cycles is dependent on a total number of the DFFs. In  FIG.  1   , the total number of the DFFs is M. 
       FIGS.  2 A- 2 D  show detailed operations of the LORST circuitry  100  according to an embodiment of the disclosure. It is assumed that all DIV-1˜DIV-N blocks are set in the reset mode and in the standby mode when the internal reset signal is enabled at logic 1 and disabled at logic 0, respectively. 
     In a first step, as shown in  FIG.  2 A , the external reset signal RST is enabled at logic 1 and the inverter  103  outputs logic 0, so that the switch  102  is turned off and the external clock signal CLK is blocked by the switch  102 . The internal clock signal CLK_INT is set as logic 0 and each DFF is set in the reset mode and outputs logic 0. Accordingly, the internal reset signal RST_INT is enabled and set as logic 1 through the inverse output terminal QB of the last DFF  107 . 
     In a second step, as shown in  FIG.  2 B , the external reset signal RST transits from logic 1 to logic 0 after a certain number of clock signal cycles after the first step. That is, the external reset signal RST transits from the enable status to the disable status, and thus each DFF transits from the reset mode to the standby mode. However, due to the propagation delay, the inverter  103  still outputs logic 0 and thus the external clock signal CLK is still blocked by the switch  102  so that the internal clock signal CLK_INT is still set as logic 0. 
     In a third step, as shown in  FIG.  2 C , the LORST circuitry  100  has completed the transition from the reset mode to the standby mode. Accordingly, the external reset signal RST is disabled at logic 0 and the inverter  103  outputs logic 1, so that the switch  102  is turned on and the clock signal CLK is passed through the switch  102  to initiate the internal clock signal CLK_INT. After a first clock signal cycle of the internal clock signal CLK_INT, the first DFF  104  is flipped from logic 0 to logic 1 since the data terminal D of the first DFF  104  is coupled to logic 1 (e.g., VCC). However, the outputs of other DFFs including the last DFF  107  are not flipped and still set as logic 0. Accordingly, the internal reset signal RST_INT is still enabled and set as logic 1 through the inverse output terminal QB of the last DFF  107 . 
     In a fourth step, as shown in  FIG.  2 D , the LORST circuitry  100  has completed M clock signal cycles after the transition from the reset mode to the standby mode. That is, the LORST circuit completes first M clock signal cycles of the internal clock signal CLK_INT. Accordingly, all outputs of the plurality of DFFs including the last DFF  107  are flipped and set as logic 1, the internal reset signal RST_INT is disabled and flipped from logic 1 to logic 0 through the inverse output terminal QB of the last DFF  107 . 
     Through this operation, the internal reset signal RST_INT can be synchronous to the external clock signal CLK to reset the divider blocks (or various circuit blocks in other examples) that are coupled to the internal reset signal RST_INT. 
       FIG.  3    shows an exemplary timing diagram  300  of controlling the internal reset signal RST_INT according to an embodiment of the disclosure. For simplicity, the external reset signal RST is set to be synchronous to the clock signal CLK. However, it is noted that the external reset signal RST may not be synchronous to the clock signal CLK and can be randomly enabled and disabled. 
     In the timing diagram  300 , a waveform  301  depicts the external reset signal RST, a waveform  302  depicts the external clock signal CLK, and a waveform  303  depicts the internal reset signal RST_INT. At T 1 , the external reset signal RST is enabled at logic 1, and thus the LORST circuitry  100  is set in the reset mode, enabling the internal reset signal RST_INT. At T 2 , the external reset signal RST is disabled at logic 0 and thus the LORST circuitry  100  is set in the standby mode, but the internal reset signal RST_INT is still enabled at logic 1 since the inverse output terminal QB of the last DFF  107  is not flipped. At T 3 , the internal reset signal RST_INT is disabled at logic 0 since the inverse output terminal QB of the last DFF  107  is flipped after a number of clock signal cycles. The number of clock signal cycles between T 2  and T 3  is dependent on a total number of the plurality of DFFs. 
       FIG.  4    shows an exemplary timing diagram  400  according to an embodiment of the disclosure. In the timing diagram  400 , a waveform  410  depicts a clock signal CLK_IN with a frequency of freq input to a divide-by-2 (DIV2) divider circuitry, which can output a divided clock signal with a frequency of freq/2. The DIV2 divider is a rising edge triggered circuitry. 
     It is noted that the DIV2 divider is used in the  FIG.  4    example just for simplicity, in another example, other types of divider, such as divide-by-3 (DIV3) or divide-by-4 (DIV4), can be used. 
     In the timing diagram  400 , a waveform  420  depicts a reset signal RST 1  for resetting the DIV2 circuit block. When the reset signal RST 1  is enabled at logic 1, the DIV2 circuit block is set in the reset mode and suspends the output of the divided clock signal. When the reset signal RST 1  is disabled at logic 0, the DIV2 circuit block is set in the standby mode and resumes the output of the divided clock signal. The dived clock signal is referred to as CLK_OUT 1  and depicted in a waveform  430 . It is noted that the reset signal RST 1  is synchronous to the input clock signal CLK_IN, and thus a falling edge  421  at which the reset signal RST 1  is disabled is synchronous to a falling edge  411  of the input clock signal CLK_IN. After the reset signal RST 1  is disabled at the falling edge  421 , the divided clock signal CLK_OUT 1  starts free running at a next rising edge, i.e., rising edge  412 , of the input clock signal CLK_IN. Accordingly, a rising edge  431  of a first clock of the divided clock signal CLK_OUT 1  is synchronous to the rising edge  412  of the input clock signal CLK_IN. 
     Still referencing to the timing diagram  400 , a waveform  440  depicts a reset signal RST 2  for resetting the same DIV2 circuit block. When the reset signal RST 2  is enabled at logic 1, the DIV2 circuit block is set in the reset mode and suspends the output of the divided clock signal; when the reset signal RST 2  is disabled at logic 0, the DIV2 circuit block is set in the standby mode and resumes the output of the divided clock signal. The divided clock signal is referred to as CLK_OUT 2  and depicted in a waveform  450 . It is noted that the reset signal RST 2  is not synchronous to the input clock signal CLK_IN, and thus a falling edge  441  at which the reset signal RST 2  is disabled is not synchronous to any rising or falling edge of the input clock signal CLK_IN. After the reset signal RST 2  is disabled at the falling edge  441 , the divided clock signal CLK_OUT 2  starts free running at a next rising edge, i.e., rising edge  413 , of the input clock signal CLK_IN. Accordingly, a rising edge  451  of a first clock of the divided clock signal CLK_OUT 2  is synchronous to the rising edge  413  of the clock signal CLK_IN. 
     It is noted that, in the timing diagram  400 , the divided clock signals CLK_OUT 1  and CLK_OUT 2  have complementary phases. It indicates that, a phase of a divided clock signal (e.g., CLK_OUT 2 ) that is controlled by a randomly reset signal (e.g., RST 2 ) can be different from or even complementary to a phase of a divided clock signal (e.g., CLK_OUT 1 ) that is controlled by a synchronously reset signal (e.g., RST 1 ). Therefore, for multiple divided clock signals, it is desirable to use synchronously reset signal, so that each divided clock signal can have a pre-determined phase. 
       FIG.  5    shows an exemplary divider circuit block  500  and the corresponding detailed circuitry according to an embodiment of the disclosure. The divider circuit block  500  is a DIV2 block that receives through an input terminal CLK_IN an input clock signal with a frequency of freq (e.g., the input clock signal CLK_IN  410  in the  FIG.  4    example) and outputs through an output terminal CLK_OUT a divided clock signal with a frequency of freq/2 (e.g., the divided clock signal CLK_OUT 1   430  in the  FIG.  4    example). A reset terminal RST is used for receiving a reset signal (e.g., the reset signal RST 1   420  in the  FIG.  4    example) that is enabled and disabled to set the divider circuit block  500  in the reset mode and in the standby mode, respectively. That is, when the reset signal (e.g., RST 1   420 ) is enabled (e.g., at logic 1), the divided clock signal (e.g., CLK_OUT 1   430 ) is turned off and each internal nodes of the divider circuit block  500  is set with a respective predefined charge. When the reset signal (e.g., RST 1   420 ) is disabled (e.g., at logic 0), the divided clock signal (e.g., CLK_OUT 1   430 ) is turned on and an initial phase of the divided clock signal is defined by the preset charges of the internal nodes of the divider circuit block  500 . 
     Specifically, when the reset signal is enabled at logic 1 (i.e., the inverse reset signal is disabled at logic 0), transistors M 1 -M 4  are turned on. Thus, the logic voltages at nodes  501  and  505  are logic 0, and the logic voltages at nodes  502  and  506  are logic 1, respectively. The internal terminals CLK and CLKB are logic 0 and logic 1, respectively. Since the logic voltages at nodes  505  and  506  are logic 0 and logic 1, the logic voltages at nodes  507  and  508  are logic 1 and logic 0, respectively, due to the inverters  560  and  561 . Since the logic voltages at nodes  501  and  502  are logic 0 and logic 1, transmission gates  540  and  541  are turned off, and transmission gates  570  and  571  are turned on, causing the logic voltages at nodes  509  and  510  being logic 1 and logic 0, respectively. Therefore, the logic voltages at the output terminal CLK_OUT and the inverse output terminal CLKB_OUT of the divider circuit block  500  is logic 0 and logic 1, respectively. Due to the inverters  530  and  531 , the logic voltages at nodes  503  and  504  are logic 1 and logic 0, respectively. 
     It is shown that when the reset signal of the divider circuit block  500  is enabled, the output terminal CLK_OUT is clapped to be logic 0. When the reset signal is disabled, the divided clock signal can be output from the output terminal CLK_OUT. Therefore, a first clock signal cycle of the divided clock signal is always from logic 0 to logic 1, indicating a predefined phase for the divided clock signal. 
       FIG.  6    shows an example of controlling two divider circuit blocks according to an embodiment of the disclosure. In  FIG.  6   , the two divide-by-2 (DIV2) circuit blocks, i.e., a first DIV2 block  610  receives a first input clock signal CLK  601  and outputs a first divided clock signal  604 , and a second DIV2 block  620  receives a second input clock signal CLK_ 60   602  and outputs a second divided clock signal  605 . It is noted that both the first input clock signal CLK  601  and the second input clock signal CLK_ 60   602  have the same frequency. However, in some embodiments, the phases of these two input clock signals may be different. For example, a phase difference between the first input clock signal CLK  601  and the second input clock signal CLK_ 60   602  is 60°. 
     In  FIG.  6   , the reset terminals of the first DIV2 block  610  and the second DIV2 block  620  are coupled to an internal reset signal RST_INT  603  that is synchronous to the first clock signal CLK  601 . When the internal reset signal RST_INT  603  is enabled at logic 1, both the first DIV2 block  610  and the second DIV2 block  620  are set in the reset mode, in which all internal nodes of the first DIV2 block  610  and the second DIV2 block  620  are set with predefined voltages. When the internal reset signal RST_INT  603  is disabled at logic 0, both the first DIV2 block  610  and the second DIV2 block  620  are set in the standby mode, and the first clock cycles of both the first divided clock signal  604  and the second divided clock signal  605  are from logic 0 to logic 1. In addition, the phase difference between the first divided clock signal  604  and the second divided clock signal  605  is the same as the phase difference between the first input clock signal CLK  601  and the second input clock signal CLK_ 60   602 . 
       FIG.  7 A  shows an exemplary local LORST configuration according to an embodiment of the disclosure. In  FIG.  7 A , two DIV2 blocks  701  and  702  are placed close to each other, thus a single LORST block  703  is used to control both DIV2 blocks  701  and  702  by generating the internal reset signal RST_INT. However, if the two DIV2 blocks  701  and  702  are placed far from each other, using a single LORST to control both DIV2 blocks  701  and  702  may cause a meta-stability issue. 
       FIG.  7 B  shows an exemplary cascade LORST configuration according to an embodiment of the disclosure. In  FIG.  7 B , two DIV2 blocks  711  and  712  are separated far from each other so that each block is associated with a respective local LORST block. For example, the DIV2 block  711  is associated with a local LORST block  713 , and the DIV2 block  712  is associated with a local LORST block  714 . Each DIV2 block receives an internal clock signal and outputs a divided clock signal. For example, the DIV2 block  711  receives an internal clock signal CLK_IN 1  and outputs a divided clock signal CLK_D 1 , and the DIV2 block  712  receives an internal clock signal CLK_IN 2  and outputs a divided clock signal CLK_D 2 . Both internal clock signals CLK_IN 1  and CLK_IN 2  are derived from an external clock signal CLK. However, in some related cases, at least one of the internal clock signals may not be synchronous to the external clock CLK, leading to the meta-stability issue. 
     To avoid the meta-stability issue, a buffer circuit block  718  is inserted between the external clock signal CLK and the DIV2 blocks  711  and  712 . The buffer circuit block  718  includes a buffer  716 , a switch  717 , and a local LORST block  715 . The LORST block  715  can output a switch control signal CLK_SW used for controlling the switch  717 . In an embodiment, the switch  717  is turned off when the switch control signal CLK_SW is set as logic 1 and on when the switch control signal CLK_SW is set as logic 0. Accordingly, the switch control signal CLK_SW is synchronized with the external clock signal CLK through the LORST  715 . 
     When the switch  717  is turned on, the internal clock signals CLK_IN 1  and CLK_IN 2  can be synchronous to the external clock signal CLK. Accordingly, initial waveforms (or phases) of both internal clock signals are same, e.g., the first clock cycles of both internal clock signals are always from logic 0 to logic 1. In addition, each internal clock signal is input to a respective DIV2 block and a local LORST associated with the DIV2 block. For example, the internal clock signal CLK_IN 1  is input to the DIV2 block  711  and the LORST block  713 , and the internal clock signal CLK_IN 2  is input to the DIV2 block  712  and the LORST block  714 . Therefore, for different LORST blocks, the internal clock signals have the same predefined initial waveforms (or phases). The meta-stability issue among different transmitter paths can be avoided. 
     It is noted that the reset terminals of all local LORST blocks  713 - 715  are coupled to a same external reset signal RST. 
       FIGS.  8 A- 8 D  show detailed operations of the cascade LORST configuration according to an embodiment of the disclosure. It is noted that each LORST block in the cascade LORST configuration includes M DFFs. 
     In a first step, as shown in  FIG.  8 A , the external reset signal RST is enabled at logic 1, so that all local LORST blocks are set in the reset mode and output logic 1. That is, the internal reset signal RST_INT 1  output from the LORST block  713  is set as logic 1, the internal reset signal RST_INT 2  output from the LORST block  714  is set as logic 1, and the switch control signal CLK_SW output from the LORST block  715  is set as logic 1. Therefore, both the DIV2 blocks  711  and  712  are set in the reset mode; the switch  717  is turned off; both the internal clock signals CLK_IN 1  and CLK_IN 2  are set as logic 0; and both the divided clock signals CLK_D 1  and CLK_D 2  are set as logic 0. 
     In a second step, as shown in  FIG.  8 B , the external reset signal RST transits from logic 1 to logic 0 after a certain number of clock cycles after the first step. That is, the external reset signal RST transits from the enable status to the disable status. Then all the LORST blocks  713 - 715  are set in the standby mode and the LORST block  715  starts free running, but the LORST blocks  713  and  714  do not start free running since the internal clock signals CLK_IN 1  and CLK_IN 2  are still set as logic 0. Due to the propagation delay, the outputs of the local LORST blocks are still set as logic 1. Therefore, the internal reset signals RST_INT 1  and RST_INT 2  and the switch control signal CLK_SW are still set as logic 1; both the DIV2 blocks  711  and  712  are set in the reset mode; the switch  717  is still turned off; and the internal clock signals CLK_IN 1  and CLK_IN 2  and the divided clock signals CLK_D 1  and CLK_D 2  are still set as logic 0. 
     In a third step, as shown in  FIG.  8 C , the external reset signal RST is still disabled and set as logic 0 after M clock cycles after the second step, i.e., the transition of the external reset signal RST from the enable status to the disable status. Since the LORST block  715  includes M DFFs, the switch control signal CLK_SW output from the LORST block  715  flips from logic 1 to logic 0. Then the switch  717  is turned off, and the external clock signal CLK can be passed through the switch  717  to initiate the internal clock signals CLK_IN 1  and CLK_IN 2 . Accordingly, both the internal clock signals CLK_IN 1  and CLK_IN 2  start to rise from logic 0 to logic 1. However, both the internal reset signals RST_INT 1  and RST_INT 2  are still set as logic 1. Therefore, both the DIV2 blocks  711  and  712  are set in the reset mode; both the divided clock signals CLK_D 1  and CLK_D 2  are still set as logic 0. 
     In a fourth step, as shown in  FIG.  8 D , the external reset signal RST is still disabled and set as logic 0 after M clock cycles after the third step, i.e., 2 M clock signal cycles after transiting from the enable status to the disable status. That is, both the internal clock signals CLK_IN 1  and CLK_IN 2  have been operated with M clock cycles. Since each of the LORST blocks  713  and  714  includes M DFFs, the corresponding internal reset signal RST_INT 1 /RST_INT 2  is disabled and flipped from logic 1 to logic 0, and then the divided clock signal CLK_D 1 /CLK_D 2  starts to rise from logic 0 to logic 1. 
     Through this operation, initial waveforms (or phases) for different LORST blocks can be same even these LORST blocks are separated far from each other. 
       FIG.  8 E  shows an alternative of the cascade LORST configuration according to an embodiment of the disclosure. In  FIG.  8 E , the LORST  715  in the buffer circuit block  718  and the local LORST blocks (e.g., LORST blocks  713  and  714 ) are controlled by different external reset signals. For example, RST 1  controls the LORST block  715  and RST 2  controls the LORST blocks  713  and  714 . The control timing of RST 1  and RST 2  can be different. That is, RST 1  can be later than or earlier than RST 2 . RST 1  can be a delayed version of RST 2  or RST 2  can be a delayed version of RST 1 . It is noted that the timing relationship between RST 1  and RST 2  is not limited in this disclosure. 
       FIG.  9    shows a flowchart of an exemplary process  900  according to an embodiment of the disclosure. The process  900  is used for controlling an internal reset signal that is synchronous to a clock signal. The process  900  may start from step S 910 , where the process  900  receives the clock signal through a clock switch circuit. The clock switch circuit outputs the clock signal in an on state and blocks the clock signal in an off state. Then the process  900  proceeds to step S 920 . 
     At step S 920 , the process  900  enables the internal reset signal through a plurality of serially coupled D flip-flops (DFFs) in response to an external reset signal being enabled. Then the process  900  proceeds to step S 930 . 
     At step S 930 , the process  900  disables the internal reset signal through the plurality of serially coupled DFFs after a predefined number of clock signal cycles after the external reset signal is disabled. The plurality of serially coupled DFFs are coupled to the clock switch circuit and driven by the clock signal. Then the process  900  terminates. 
     In an embodiment, a data terminal of a first DFF in the plurality of DFFs is coupled to a logic voltage at which the internal reset signal is enabled, and an inverse output terminal of a last DFF in the plurality of DFFs is coupled to the internal reset signal. 
     In an embodiment, an input terminal of the clock switch circuit is coupled to the clock signal, an output terminal of the clock switch circuit is coupled to a clock terminal of each DFF in the plurality of DFFs, and a control terminal of the clock switch is coupled to the external reset signal. 
     In an embodiment, the clock switch circuit is in the off state so that the clock signal is not output to the plurality of DFFs in response to the external reset signal being enabled, and in the on state so that the clock signal is output to the plurality of DFFs in response to the external reset signal being disabled. 
     In an embodiment, the external reset signal is coupled to a reset terminal of each DFF in the plurality of DFFs. 
     In an embodiment, each DFF is set in a reset mode and in a standby mode in response to the external reset signal being enabled and disabled, respectively. 
     In an embodiment, the predefined number of clock signal cycles is dependent on a total number of DFFs. 
     In general, the processes  900  can be suitably adapted by modifying one or more operations in the respective process, adding one or more operations, omitting one or more operations, and the like. For example, an additional operation can be added where a selection of one or more specific processes is made in a process set including the processes  900 . 
       FIGS.  10 A- 10 C  show some exemplary transceivers according to embodiments of the disclosure. As shown in  FIG.  10 A , a first transceiver  1010  configuring with a phase rotation (PR) architecture employs an LORST block  1011  to synchronize phases of two path dividers  1012  and  1013 . As shown in  FIG.  10 B , a second transceiver  1020  configuring with a low power mode (LPM) architecture for gain slicing employs an LORST block  1021  to synchronize phases of different divider slices, for example, the divider slice  1022  and the divider slice  1023 . As shown in  FIG.  10 C , a third transceiver  1030  configuring with multiple transmitters (TX) for multi-input multi-output (MIMO) employs multiple LORST block (e.g., LORST blocks  1031  and  1032 ) to synchronize phases of different TXs (e.g., TXA  1033  and TXB  1034 ). 
     It is noted that both the LORST blocks  1011  and  1021  can be with the local LORST configuration, while the LORST blocks  1031  and  1032  can be with the cascade LORST configuration. 
     According to aspects of the disclosure, at least some of the various processes, operations, and techniques described above may be implemented utilizing hardware, a processor executing firmware instructions, a processor executing software instructions, or any combination thereof. Also, some of the various processes, operations, and techniques may be performed in a different order (and/or concurrently) and still achieve desirable results. When implemented utilizing a processor executing software or firmware instructions, the software or firmware instructions may be stored in any computer readable memory such as on a magnetic disk, and optical disk, or other storage medium, in a random access memory (RAM) or read-only memory (ROM) or flash memory, processor, hard disk drive, optical disk drive, tape drive, etc. Likewise, the software or firmware instructions may be delivered to a user or a system via any known or desired delivery method including, for example, on a computer readable disk or other transportable computer storage mechanism or via communication media. Communication media typically embodies computer readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave or other transport mechanism. The term “modulated data signal” means signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wire media such as a wired network or direct-wired connection, and wireless media such as acoustic, radio frequency, infrared and other wireless media. Thus, the software or firmware instructions may be delivered to a user or a system via communication channels such as telephone line, a DSL line, cable television line, a fiber optics line, wireless communication channels, the Internet, etc. (which are viewed as being the same as or interchangeable with providing such software via a transportable storage medium). The software or firmware instructions may include machine readable instructions that, when executed by the processor, cause the processor perform various acts. 
     When implemented in hardware, the hardware may comprise one or more of discrete components, an integrated circuit, an application-specific integrated circuit (ASIC), etc. 
     While aspects of the present disclosure have been described in conjunction with the specific embodiments thereof that are proposed as examples, alternatives, modifications, and variations to the examples may be made. Accordingly, embodiments as set forth herein are intended to be illustrative and not limiting. There are changes that may be made without departing from the scope of the claims set forth below.