Abstract:
A method and a circuit are described for recovery of video clocks for a DisplayPort receiver. The disclosure includes two clock dividers, a direct digital synthesis (DDS), a fixed multiplier Phase-Locked Loop (PLL) on a DisplayPort video system. A DisplayPort receiver link clock is divided to a lower frequency as the input of the DDS which can lower the performance requirement on a DDS circuit. The output from a time stamp value indirectly controls a direct digital synthesis device, which then drives a PLL to generate the recovery clock signal. The technique is suitable for implementation on an integrated circuit and Field Programmable Gate array system.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to International Application No. PCT/CN2010/070256 entitled, “METHOD AND CIRCUIT FOR DISPLAY PORT VIDEO CLOCK RECOVERY”, filed on Jan. 19, 2010, and all the benefits accruing therefrom under 35 U.S.C. 371, the contents of which are herein incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention is generally related to the field of stream video clock recovery in digital communication and, more specifically, to video clock recovery for DisplayPort receivers. 
       Discussion of Related Art 
       [0003]    The DisplayPort standard delivers high performance digital connectivity for many video applications at home or at work. The DisplayPort standard allows for multiple video streams over single physical connection and supports internal chip-to-chip communication by driving display panels directly, allowing for cheaper and slimmer displays. Therefore, The DisplayPort standard allows the production of ultra-thin monitors and displays to be a reality due to its exclusive direct drive capability, enabling monitors to take up less desk space while delivering high quality visual performance. Recently, a high-definition multimedia interface DisplayPort for transmitting digital video and audio contents have been standardized. The standard is available through Video Electronics Standard Association (VESA). 
         [0004]    In accordance with the DisplayPort standard the clock signal is embedded in the data signal. The task of recreating the video pixel clock rate or the audio sample clock rate is called stream clock recovery. There are a variety of stream clock recovery methods that can be implemented, each method having a different set of performance characteristics. 
         [0005]    A common clock recovery or regeneration method is based on a fractional-N phased locked loop (PLL). For example, U.S. Pat. No. 7,088,398 discloses an audio clock regeneration method by applying fractional-N PLL. However, a fractional-N PLL circuitry typically is a complicated and costly structure. 
         [0006]    Previous work on clock recovery using Direct Digital Synthesis (DDS), for example as disclosed in U.S. Pat. No. 6,157,646 and No. 6,363,073, itself does not reduce the bandwidth performance requirement and the display cost in a DisplayPort system. 
         [0007]    Accordingly, it is desirable to provide for satisfactory and practical solutions for stream clock recovery in a DisplayPort system. 
       SUMMARY 
       [0008]    Consistent with embodiments of the present invention, a stream clock recovery device for the DisplayPort standard is provided. The stream clock recovery device includes a receiver receiving a clock data along with its time stamped values; the receiver further comprising: a first clock divider to divide the clock frequency to a lower frequency; a fixed multiplier phase locked loop (PLL) device for synthesizing an output clock; a digital control logic circuit that converts the time stamped values into a reference clock and a divider factor; a direct digital synthesis (DDS) device, wherein the DDS takes the frequency-divided signal from the first clock divider and the reference clock, and synthesizes an input signal for the PLL device; and a second clock divider that applies the divider factor to the PLL output clock to create a recovered clock signal. 
         [0009]    Systems according to the present invention do not need a fractional-N PLL, which is hard to implement in a Field Programmable Gate Array (FPGA) system and is expensive to implement. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Some embodiments of the present invention will be described more fully below with reference to the accompanying drawings. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. 
           [0011]      FIG. 1  shows an overview of a general high definition multimedia digital content transport system. 
           [0012]      FIG. 2  shows a clock regeneration model including a DisplayPort (DP) receiver, consistent with some of the disclosed embodiments. 
           [0013]      FIG. 3  describes an embodiment of the disclosed DP clock recovery circuit including a DDS and a fixed multiplier PLL, consistent with the present invention. 
           [0014]      FIG. 4  describes an exemplary embodiment of the disclosed DP clock recovery circuit including a DDS and a 16× PLL, consistent with the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    In the following description, specific details are set forth describing some embodiments of the present invention. It will be apparent, however, to one skilled in the art that some embodiments may be practiced without some or all of these specific details. The specific embodiments disclosed herein are meant to be illustrative but not limiting. One skilled in the art may realize other material that, although not specifically described here, is within the scope and the spirit of this disclosure. 
         [0016]      FIG. 1  shows a general architecture of a high definition multimedia digital content transport system  100 . Such a system typically includes a source device  110  that includes a transmitter  115 , a sink device  150  that includes a receiver  155 , and perhaps a number of additional repeaters or other devices, interconnected by one or more uni-directional, high-speed, and low-latency serial main link channel  121  (link channels  121   a ,  121   b , and  121   c  are specifically shown) designed to transport isochronous streams such as uncompressed digital video data  111 , digital audio data  112 , and control/status data  113 . At the sink  150 , receiver  155  receives and outputs a video signal  151 , an audio signal  152 , and a control/status signal  153 . In addition, an auxiliary link channel  124  can be utilized to transport data  154 , which may include Extended Digital Identification Data (EDID). 
         [0017]    In many stream clock recovery systems, the video data  111  and audio stream data  112  being carried across the main serial link channel  121  may not retain the original video pixel clock rate or audio sample clock rate. The serial link channel  121  is driven by a high speed clock running at a different rate than the original video pixel clock rate or the audio sample clock rate. For example, in the DisplayPort standard, the main serial link channel  121  clock rate is fixed at either 1.62 Gbps or 2.7 Gbps, irrespective of the input video or audio clock rates. In the HDMI standard, the main serial link channel  121  clock runs at the TMDS (Transition Minimized Differential Signaling) clock rate, which corresponds to the video pixel rate, but is independent of the audio sample clock rate. In both HDMI and DisplayPort standards, a fractional relationship time-stamped clock regeneration model  200  such as that shown in  FIG. 2  can be defined. The values M and N are integers representing frequencies in clock recovery. M is a dynamic parameter that is counted based on the incoming daa signals, and N is a static parameter that depends on the relevant communication standard of the system. Source  110  includes a cycle time counter  210  to count the frequency parameter M from input video clock  111  and audio clock  112 . A DisplayPort (DP) link symbol clock  203 , which typically runs at  10  percent of the clock rate of the main link channel  121 , drives a “divide by N” operation, performed by a divider  220 , to the counted frequency parameter M from the cycle time counter  210 . The stream data including M and N is carried by main link channel  121  to DisplayPort (DP) receiver sink  250 , which provides a recovered DP clock signal  225 . 
         [0018]    A problem occurs for the above stream clock recovery method when the M &amp; N values at sink  150  are not equal to the original M &amp; N values at the source  110 , which is typically caused by bit error in link channel  121  or imperfect measurement mechanisms at source  110 . In the ideal case, where the received M &amp; N values are equal to the original M &amp; N values, fractional long-term accuracy of the recovered stream clock is the same as that of the original. But in the non-ideal case where the received M &amp; N values are not equal to the original M &amp; N values, frequency deviation between source  110  and sink  150  in  FIG. 1  will accumulate over time, leading to a phase offset between read and write operations. 
         [0019]      FIG. 3  illustrates a block diagram for a DisplayPort receiver  300  according to some embodiments of the present invention. The DisplayPort receiver  300  includes: a clock divider circuit  310  receiving a DP link clock data from a main link channel  121  and dividing it to a lower frequency input data for a Direct Digital Synthesizer (DDS)  320 . DDS is used here as a means to generate a frequency- and phase-tunable output signal referenced to a fixed-frequency precision clock source. 
         [0020]    The DisplayPort receiver  300  also receives a time stamped value  122 , which is processed by a Digital Control Process Logic module  330 . Two digital control signals are generated by the Digital Control Process Logic  330 : Digital control A  333  and Digital control B  335 . Digital control A  333  provides a reference frequency signal to DDS  320 . DDS  320  synthesizes a clock signal with a frequency closest to the reference frequency signal provided by the Digital control A. This output signal from DDS is used as an input to the fixed multiplier PLL  340  to be phase locked at a clock signal closest to the reference frequency. Digital control B  335  provided by the Digital Control Process Logic  330  is sent to the Clock Divider  350  to control the multiplexer factor in the Clock Divider  350 . Therefore, the final recovered clock signal  325  is an accurate counting of the DP video clock rate  121   a  in the source in  FIG. 1 . The received time stamp data M and N in  FIG. 2  change in continuous video frames, therefore Digital control A and Digital control B change accordingly. 
         [0021]      FIG. 4  shows an exemplary embodiment when the input DP link clock has a frequency of 270 MHz. The Clock Divider  410  divides the link clock frequency by 4 and sends the divided signal at 67.5 MHz to DDS  420  for synthesizing clocks at a much lower frequency than the link clock rate in the DDS circuitry, controlled by the reference signal input Digital control A  433  from the Digital Control Process Logic  430 . DDS output is sent to PLL  440  at a fixed multiple of 16. The output of  440  is frequency-divided by Clock Divider  450  at a divider factor provided by the Digital control B  435  from the Digital Control Process Logic  430 . At the end, Clock Divider  450 &#39;s output  425  is delivered as the required video clock for the DisplayPort transport system. The clock divider  410  lowers the input frequency of DDS by a factor of 4 in this example, thus DDS performance requirement is significantly simplified, and the overall cost of the video transport is reduced as well. 
         [0022]    The foregoing description is intended to illustrate but not to limit the scope of the invention, which is defined by the scope of the appended claims. Other embodiments are within the scope of the disclosure.