System and method to improve the efficiency of synchronous mirror delays and delay locked loops

CROSS REFERENCE TO RELATED APPLICATION

0001This application is a division of U.S. patent application Ser. No. 09/921,614, filed Aug. 3, 2001, which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

0002The present invention relates generally to the field of integrated circuits. More particularly, the invention relates to circuits that will synchronize the internal timing or clock signals within an integrated circuit such as a synchronous dynamic random access memory (SDRAM) to external timing or clock signals.

0003Most digital logic implemented on integrated circuits is clocked syncfhronous sequential logic. In electronic devices such as synchronous dynamic random access memory circuits (SDRAMs), microprocessors, digital signal processors, and so forth, the processing, storage, and retrieval of information is coordinated with a clock signal. The speed and stability of the clock signal determines to a large extent the data rate at which a circuit can function. Many high-speed integrated circuit devices, such as SDRAMs, microprocessors, etc., rely upon clock signals to control the flow of commands, data, addresses, etc., into, through and out of the devices.

0004A continual demand exists for devices with higher data rates; consequently, circuit designers have begun to focus on ways to increase the frequency of the clock signal. In SDRAMs, it is desirable to have the data output from the memory synchronized with the system clock that also serves the microprocessor. The delay between a rising edge of the system clock (external to the SDRAM) and the appearance of valid data at the output of the memory circuit is known as the clock access time of the memory. A goal of memory circuit designers is to minimize clock access time as well as to increase clock frequency.

0005One of the obstacles to reducing clock access time has been clock skew, that is, the delay time between the externally supplied system clock signal and the signal that is routed to the memory's output circuitry. An external system clock is generally received with an input buffer and then further shaped and redriven to the internal circuitry by an internal buffer. The time delay of the input buffer and the internal buffer will skew the internal clock from the external clock. This clock skew will cause signals that are to be transferred from the integrated circuit to be out of synchronization with the external system clock. This skew in the clock signal internal to the integrated circuit is furthered by the delays incurred in the signal passing through the clock input buffer and driver and through any associated resistive-capacitive circuit elements. One solution to the problem of clock skew is the use of a synchronous mirror delay, and another is the use of delay-locked loops.

0006Delay-locked loops (DLL) are feedback circuits used for synchronizing an external clock and an internal clock with each other. Typically, a DLL operates to feed back a phase difference-related signal to control a delay line, until the timing of one clock signal is advanced or delayed until its rising edge is coincident with the rising edge of a second clock signal.

0007A synchronous mirror delay circuit (SMD) is a circuit for synchronizing an external clock and an internal clock with each other. The SMD can acquire lock generally within two clock cycles. The SMD has a period of delay, known as a delay range. The delay range of the SMD determines the actual operating range, or clock frequency, within which the integrated circuits (ICs) can operate. In other words, it is desired to reduce the number of delay stages required in the SMD while maintaining the lock delay range. One goal is to improve the efficiency of the SMD to maintain the proper operating range and to reduce the required area and power consumption of the SMD.

0008For the conventional SMD implementations, two delay lines are required, one for delay measurement, one for variable mirrored delay. The effective delay length for both delay lines is defined as: <br /><i>t</i>_delay<i>=t</i>_ck<i>−t</i>_mdl<br /> where t_ckis the clock period, t_mdlis the delay of an input/output (“I/O”) model, including clock input buffer, receiver, clock tree and driver logic. The delay stages required for each delay line is given by: <maths id="MATH-US-00001" num="1"><math overflow="scroll"><mrow><mi>N</mi><mo>=</mo><mrow><mfrac><msub><mi>t</mi><mi>delay</mi></msub><mi>t</mi></mfrac><mo>=</mo><mfrac><mrow><msub><mi>t</mi><mi>ck</mi></msub><mo>-</mo><msub><mi>t</mi><mi>md1</mi></msub></mrow><mi>t</mi></mfrac></mrow></mrow></math></maths><br /> where t_dis the delay per stage. The worst case number is given by: <maths id="MATH-US-00002" num="2"><math overflow="scroll"><mrow><msub><mi>N</mi><mi>worst</mi></msub><mo>=</mo><mfrac><mrow><mrow><msub><mi>t</mi><mi>ck</mi></msub><mo>⁡</mo><mrow><mo>(</mo><mi>long</mi><mo>)</mo></mrow></mrow><mo>-</mo><mrow><msub><mi>t</mi><mi>md1</mi></msub><mo>⁡</mo><mrow><mo>(</mo><mi>fast</mi><mo>)</mo></mrow></mrow></mrow><mrow><msub><mi>t</mi><mi>d</mi></msub><mo>⁡</mo><mrow><mo>(</mo><mi>fast</mi><mo>)</mo></mrow></mrow></mfrac></mrow></math></maths>

0009For example, where t_ck(long)=15 ns (as in a 66 MHz bus), t_mdl(fast)=1 ns and t_d(fast)=110 ps, <maths id="MATH-US-00003" num="3"><math overflow="scroll"><mrow><msub><mi>N</mi><mi>worst</mi></msub><mo>=</mo><mrow><mfrac><mrow><mrow><mn>15</mn><mo>⁢</mo><mi>ns</mi></mrow><mo>-</mo><mrow><mn>1</mn><mo>⁢</mo><mi>ns</mi></mrow></mrow><mrow><mn>110</mn><mo>⁢</mo><mstyle><mtext> </mtext></mstyle><mo>⁢</mo><mi>ps</mi></mrow></mfrac><mo>≈</mo><mn>128</mn></mrow></mrow></math></maths>

0010For two delay lines in an SMD, a total of 256 stages are needed to adjust the delay.

0011When locking, t_lock=d_in+t_mdl+(t_ck−t_mdl) (measured)+(t_ck−t_mdl) (variable)+d_out. This is the conventional equation to calculate the lock time of the SMD, which is generally two clock cycles, based on sampling from one rising edge to the next rising edge of the internal clock signal.

0012Therefore, one goal of the present invention is to reduce the effective delay stages used in the SMD while maintaining the lock range.

SUMMARY OF THE INVENTION

0013The present invention solves the aforementioned problems, and improves the efficiency of the synchronous circuitry for the internal clock signal to lock with the external clock signal.

0014In one aspect of the invention, a phase detection and selection circuit includes a phase detector for receiving a clock input signal CIN and a clock delay signal CDLY. Each signal has timing conditions and generates a plurality of output signal combinations, each combination corresponding to pre-defined phases of the signals based upon the timing characteristics. Logic is associated with the phase detector to select one of the output signal combinations corresponding to the timing conditions of the signals. The timing characteristics define a period of CIN as t_ckand also define a period from a rising edge in CIN to a rising edge in CDLY as t_mdl, and wherein when t_mdl>t_ck/2, CIN is input into the SMD, and when t_mdl<t_ck/2, an inverted clock signal CIN′ is input into the SMD to reduce the number of delay stages in the SMD.

0015In another aspect of the invention, a method of improving the efficiency of a synchronous mirror delay circuit comprises the steps of providing a clock input signal CIN, an inverted clock signal (CIN′) and a clock delay signal CDLY, each having timed characteristics. The method includes interposing a phase detector and selection system between an external clock signal and a synchronous mirror delay circuit, and determining which of a number of phases the signals are in based on the timing characteristics, and directing the signals based upon the phase of the signals.

0016In another aspect of the invention, a phase detection and selection circuit for a delay-locked loop (DLL) includes a phase detector for receiving a clock input signal CIN and a clock feedback signal CKFB. Each signal has timing conditions and generates a plurality of output signal combinations, each combination corresponding to pre-defined phases of the signals based upon the timing characteristics. Logic is associated with the phase detector to select one of the output signal combinations corresponding to the timing conditions of the signals. The timing characteristics define a period of CIN as t_ckand also define a period from a rising edge in CIN to a rising edge in CKFB as t_e, and wherein when t_e&lt;t_ck/2, the effective delay of the DLL is less than t_ck/2.

BRIEF DESCRIPTION OF THE DRAWINGS

0017The drawings illustrate the best mode presently contemplated for carrying out the invention.

0018In the drawings:

0019<figref idref="DRAWINGS">FIG. 1</figref> is a block diagram of a synchronous mirror delay with phase detection in accordance with the present invention.

0020<figref idref="DRAWINGS">FIG. 2</figref> is a circuit diagram illustrating the phase detector in accordance with one aspect of the present invention.

0021<figref idref="DRAWINGS">FIG. 3</figref> is a signal timing diagram showing the timing of a clock input signal and a clock delay signal in accordance with one aspect of the present invention.

0022<figref idref="DRAWINGS">FIG. 4</figref> is a signal timing diagram showing the timing of a clock input signal and a clock delay signal in accordance with one aspect of the present invention.

0023<figref idref="DRAWINGS">FIG. 4</figref><i>a </i>is a signal timing diagram showing the timing of a clock input signal and a clock delay signal under lock conditions in accordance with one aspect of the present invention.

0024<figref idref="DRAWINGS">FIG. 5</figref> is a chart illustrating the logic combinations of the signals in <figref idref="DRAWINGS">FIG. 2</figref> based upon the timing characteristics of <figref idref="DRAWINGS">FIGS. 3 and 4</figref>.

0025<figref idref="DRAWINGS">FIG. 6</figref> is a flowchart illustrating a method in accordance with one aspect of the present invention.

0026<figref idref="DRAWINGS">FIG. 7</figref> is a block diagram of a delay-locked loop with phase detection in accordance with the present invention.

0027<figref idref="DRAWINGS">FIG. 8</figref> is a signal timing diagram showing the timing of a clock input signal and a clock feedback signal in accordance with one aspect of the present invention.

0028<figref idref="DRAWINGS">FIG. 9</figref> is a signal timing diagram showing the timing of a clock input signal and a clock feedback signal in accordance with one aspect of the present invention.

0029<figref idref="DRAWINGS">FIG. 10</figref> is a block diagram illustrating a system in which the present invention may be used.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

0030Referring now to <figref idref="DRAWINGS">FIG. 1</figref>, a system in accordance with the present invention is shown generally by the numeral <b>10</b>. The system <b>10</b> includes a synchronous mirror delay (SMD) circuit <b>12</b> and a phase detector control block <b>14</b>. An external clock signal <b>16</b> is input into receiver and buffer <b>18</b>. This produces clock input signal <b>20</b> (CIN), inverted clock input signal (CIN′) <b>21</b> and clock delay signal <b>22</b> (CDLY). Clock delay signal <b>22</b> is delayed by an I/O system delay t_mdlillustrated by block <b>24</b>. CDLY <b>22</b> is also directly fed via line <b>23</b> into the SMD <b>12</b>.

0031Phase detector control block <b>14</b> includes phase detector <b>26</b> and associated logical circuitry. The goal of the present invention is to take clock input signal <b>20</b> and clock delay signal <b>22</b> and, by defining certain characteristics and relationships about the timing of the signals, delineate specific conditions under which the circuit is operating, and direct the signal accordingly. Ultimately, the phase of the signals will determine whether CIN <b>20</b> or CIN′ <b>21</b> is used as the input to the SMD, or whether the SMD is bypassed altogether. Although a specific logic arrangement is shown, it is contemplated that any suitable control logic may be used to define the conditions of the signals and select them accordingly. Associated with the phase detector is a multiplexor <b>28</b> which is used as an input selection multiplexor, that is to determine which selection input (CIN or CIN′), based on the difference between CIN signal <b>20</b> and CDLY signal <b>22</b>, to send to the SMD <b>12</b>. The outputs (collectively 32) of phase detector <b>26</b>, which will be described in further detail with respect to <figref idref="DRAWINGS">FIG. 2</figref> are fed into circuitry control block <b>30</b>. Circuitry block <b>30</b> may be, for instance, a decoder, although any suitable logic is contemplated. The outputs <b>38</b> and <b>40</b> of phase detection circuitry block <b>30</b> will be used to select the outputs for multiplexors <b>28</b> and <b>46</b>, respectively. Based on the signal <b>38</b> from control circuitry block <b>30</b>, input multiplexor <b>28</b> will select either CIN <b>20</b> or CIN′ <b>21</b> to be placed on line <b>48</b>. The output multiplexor <b>46</b> is used in combination with the control circuitry block <b>30</b> to select which signal is to be put on output line <b>50</b>. Line <b>48</b> (either CIN signal <b>20</b> or CIN′ signal <b>21</b>) is directed into the SMD <b>12</b>. Line <b>48</b> is also directed via connection <b>34</b> to an input of output selection multiplexor <b>46</b>. As is known in the art, the SMD <b>12</b> includes a measurement delay line composed of a plurality of serially cascaded delay elements (not shown), the measurement delay line having a measurement delay line input and a measurement delay line output. Each delay stage is a delay element with control gates. An output of the measurement delay line is used as the input to a variable delay line. The variable delay line is also a plurality of serially connected delay elements (not shown), the variable delay line having a variable delay line input and a variable delay line output. The output of the variable delay line of the SMD <b>12</b> is output signal SMDOUT <b>44</b>. Output signal SMDOUT <b>44</b> is used as the input to output multiplexor <b>46</b>. In some circumstances, it is desired to entirely bypass SMD <b>12</b>, and in such a case, control circuitry block <b>30</b> will send a control signal <b>40</b> selecting line <b>34</b> rather than SMDOUT <b>44</b> as the output <b>50</b> of output selection multiplexor <b>46</b>. As a result, line <b>48</b> (either CIN signal <b>20</b> or CIN′ signal <b>21</b>) will be used as the input for output selection multiplexor <b>46</b>. In other cases, the control circuitry block <b>30</b> will send a control signal <b>40</b> selecting signal SMDOUT <b>44</b> from SMD <b>12</b>. Having selected one of the signals <b>34</b> or signal SMDOUT <b>44</b>, output signal <b>50</b> is used as the input to clock tree <b>54</b>. As is known, a clock tree is a circuit used for distributing a local clock signal. A clock tree may include an internal buffer in order to amplify, buffer and delay the signal in order to form internal clock signal CLKIN <b>56</b>. Although not shown, it is contemplated that an inverter may be placed before the clock tree <b>54</b> in order to invert the clock signal if desired. In this manner, internal clock signal CLKIN <b>56</b> will be matched to the external clock <b>16</b>.

0032Referring now to <figref idref="DRAWINGS">FIG. 2</figref>, phase detector <b>26</b> is described in more detail. Phase detector <b>26</b> receives clock input signal <b>20</b> and clock delay signal <b>22</b>. Clock delay signal <b>22</b> is used as clock inputs <b>58</b> and <b>60</b> into registers <b>64</b> and <b>62</b> respectively. Although D flip-flops are used as registers <b>62</b> and <b>64</b>, it is contemplated that any suitable logic device suitable for the application may be employed. Signal <b>22</b> is input into the clock inputs for the D flip-flops. Clock input signal CIN <b>20</b> is input as the D inputs <b>66</b> and <b>68</b> of flip-flops <b>62</b> and <b>64</b>, respectively. Input <b>68</b> is delayed from clock input signal <b>20</b> by t_d<b>70</b>, which is representative of the delay per stage, and therefore there is a delay between input signals <b>66</b> and <b>68</b>, by a period t_d<b>70</b>. Each flip-flop <b>62</b> and <b>64</b> respectively outputs a signal <b>34</b> and <b>32</b>. The logical level, i.e., a logical 1 or a logical 0, of signal branches <b>32</b> and <b>34</b> determine the condition under which the relationship of the CIN signal <b>20</b> and CDLY signal <b>22</b> are operating in. The signal conditions are based on their individual timing characteristics.

0033Referring now to <figref idref="DRAWINGS">FIG. 3</figref>, a clock diagram is shown illustrating one possible combination of timing characteristics of CIN signal <b>20</b> and CDLY signal <b>22</b>. CIN signal <b>20</b> fires first, and the characteristic t_mdl, which is the delay of the I/O model, is measured from the rising edge <b>23</b>A to the rising edge <b>25</b>A of CDLY signal <b>22</b>. The entire period of CIN signal <b>20</b>, that is the measurement of rising edge <b>23</b>A to the next rising edge <b>23</b>B is defined as the clock period or t_ck. Therefore, the time defined from the rising edge <b>25</b>A of CDLY signal <b>22</b> to the next rising edge <b>23</b>B of the CIN signal <b>20</b> defines a delay, t_delay<b>27</b>A, which may be defined by t_ckminus t_mdl. This series of timing characteristics would occur when CDLY signal <b>22</b> fires after the first falling edge <b>29</b>A of CIN signal <b>20</b>. This sampling of CIN from rising edge to rising edge requires a given number of delay stages to accomplish, where the total delay of these delay stages is t_delay, which is less than half of t_ck.

0034Referring now to <figref idref="DRAWINGS">FIG. 4</figref>, an alternate timing diagram is shown for CIN signal <b>20</b> and is CDLY signal <b>22</b>. These timing characteristics would occur when the rising edge <b>25</b>B of CDLY signal <b>22</b> occurred prior to the falling edge <b>29</b>B of CIN signal <b>20</b>. Again, the delay between the firing at the rising edge <b>23</b>B of CIN signal <b>20</b> and rising edge <b>25</b>B defines the period of delay for the I/O model t_mdl. Because the period of time from rising edge <b>23</b>B to falling edge <b>29</b>B represents half of the clock period t_ck, that portion of the signal may be represented by t_ck/2. Therefore, that distance minus the delay period for the I/O model t_mdlresults in the delay period <b>27</b>B, in this case defined as t_ck/2 minus t_mdl. Therefore, if the phase detector analyzes when the rising edge of CDLY signal <b>22</b> occurs with respect to the falling edge of CIN signal <b>20</b>, a distinction can be made with respect to the timing characteristics of the individual signals <b>20</b> and <b>22</b>. Since the total delay required from the SMD for synchronization is reduced from (t_ckminus t_mdl) to (t_ck/2 minus t_mdl) where t_mdlis less than t_ck/2, more than half of the delay stages can be saved with this invention. The present invention takes advantage of the ability to sample from a rising edge <b>23</b><i>b </i>to falling edge <b>29</b><i>b, </i>resulting in fewer delay stages in the SMD to accomplish.

0035Referring now to <figref idref="DRAWINGS">FIG. 4</figref><i>a, </i>the timing diagram is shown illustrating the lock conditions. CIN signal is shown as well as CIN plus t_d, where t_drepresents the delay between the two signals. In lock condition 3, signal CDLY is shown rising between the rising of CIN and CIN plus t_d, and falling between the falling of CIN and CIN plus t_d, respectively. Under this circumstance, a lock condition exists and the synchronous mirror delay is bypassed. Under lock condition 4, CDLY signal rises between the falling edge of CIN and the falling edge of CIN plus t_d. And CDLY falls between the rising edge of CIN and the rising edge of CIN plus t_d. Again, a lock condition exists and again the synchronous mirror delay is bypassed.

0036Referring now to <figref idref="DRAWINGS">FIG. 5</figref>, the four possible combinations of the logical levels of PH<b>1</b> signal <b>32</b> and PH<b>2</b> signal <b>34</b> are illustrated. Based on the logical levels of each of these signals, such that the condition of the signals may be determined from the logic levels on these lines.

0037Condition (1): <br />t_mdl>t_ck/2

0038For condition (1), the effective delay length in the SMD is equal to t_ck−t_mdl. When locking, t_lock=d_in+t_mdl+(t_ck−t_mdl) (measured)+(t_ck−t_mdl) (variable)+d_out=2t_ck+d_in+d_out−t_mdl≈2t_ck, where d_inand d_outare I/O intrinsic delays on which t_mdlis represented or modeled.

0039This is the conventional equation to calculate the lock time of the SMD, which is two clock cycles.

0040Condition (2): <br />t_mdl&lt;t_ck/2

0041Under this condition, a multiplexor is used to select a different phase of CIN to feed in the SMD and the effective delay length is equal to t_ck/2−t_mdl.

0042Again, t_lock=d_in+t_mdl+(t_ck/2−t_mdl)+(t_ck/2−t_mdl)+d_out=t_ck+d_in+d_out−t_mdl≈t_ck.

0043The lock time is decreased to only one clock cycle. From the previous example, <maths id="MATH-US-00004" num="4"><math overflow="scroll"><mrow><msub><mi>N</mi><mi>worst</mi></msub><mo>=</mo><mrow><mfrac><mrow><mrow><mn>15</mn><mo>⁢</mo><mrow><mi>ns</mi><mo>/</mo><mn>2</mn></mrow></mrow><mo>-</mo><mrow><mn>1</mn><mo>⁢</mo><mi>ns</mi></mrow></mrow><mrow><mn>110</mn><mo>⁢</mo><mstyle><mtext> </mtext></mstyle><mo>⁢</mo><mi>ps</mi></mrow></mfrac><mo>=</mo><mrow><mn>59</mn><mo>⁢</mo><mstyle><mtext> </mtext></mstyle><mo>⁢</mo><mi>stages</mi></mrow></mrow></mrow></math></maths><br /> compared to 128 stages without the invention.

0044Condition (3):

0045When t_mdl=t_ck, the phase detector would declare a lock condition and the clock signal CIN is output directly without even passing into the SMD. The SMD may be disabled to save power.

0046Condition (4):

0047When t_mdl=t_ck/2, the CIN is inverted and the SMD may be disabled to save power.

0048It is contemplated that the present invention will reduce the effective delay elements used in the SMD, as a function of the signals being found under the condition 2, saving both silicon area and power in the memory device, which is the primary goal.

0049For conditions (2) and (4), if there is a severe duty cycle distortion, the falling edges of CIN cannot provide a correct reference to adjust the delay, which would result in a large skew (phase error) at the output.

0050Referring now to <figref idref="DRAWINGS">FIG. 6</figref>, a flowchart illustrating a methodology associated with the present invention is disclosed. At the start <b>70</b>, the present invention is used for those circuits in which it is desired to reduce the number of delay stages and there is negligible duty-cycle distortion. Therefore, signal CIN, inverted CIN and CDLY are provided in step <b>72</b>. CDLY is delayed by the delay of the I/O system. In step <b>74</b>, a phase detector is interposed between the synchronous mirror delay and CIN and CDLY signals. Both CIN and CDLY are input into the phase detector <b>76</b>, after which it is necessary to determine based on the timing characteristics and relationships of CIN to CDLY, which condition or phase the timing signals are in <b>78</b>. This leads to a series of four decisions <b>80</b><i>a </i>through <b>80</b><i>d </i>used to determine the relationship of the particular timing characteristics t_mdlversus t_ck. Although the series of decisions are shown made in a serial fashion, that is, <b>80</b><i>a </i>prior to <b>80</b><i>b </i>and so on, these operations could also be rearranged to run in other serial fashions or in parallel, so long as the determinations are made. In decision <b>80</b><i>a, </i>it is determined whether t_mdlis greater than t_ck/2 but less than t_ck. If so, <b>82</b><i>a </i>condition 1 is triggered <b>84</b><i>a </i>in which the lock time is equal to two clock cycles, which is the conventional synchronous mirror delay lock time. In a conventional manner, CIN is then fed into the synchronous mirror delay. The SMDOUT signal is input into the clock tree. If condition 1 is not satisfied <b>81</b>, it is determined whether t_mdlis less than t_ck/2 in decision <b>80</b><i>b. </i>If so <b>82</b><i>b, </i>condition 2 is implicated in which the lock time is equal to approximately one clock cycle, or approximately half of the conventional synchronous mirror delay lock time. CIN is then inverted and fed into the synchronous mirror delay. The SMDOUT signal is input into the clock tree. If condition 2 is not satisfied <b>83</b>, it is determined whether t_mdlis equal to t_ckin decision <b>80</b><i>c. </i>If so <b>82</b><i>c, </i>condition 3 84<i>c </i>is implicated, and lock has already occurred so a lock is declared and the synchronous mirror delay is bypassed. The CIN signal is input directly into the clock tree for internal production of the clock. If none of these conditions are true <b>85</b> and decision <b>80</b><i>d </i>is determined whether t_mdlis equal to t_ck/2. If so <b>82</b><i>d, </i>condition 4 <b>84</b><i>d </i>is implicated and it is merely necessary to invert the CIN signal or use an inverted CIN to be input into the clock tree. Again, since there is no need to further delay, the synchronous mirror delay is bypassed and, in a preferred embodiment may be disabled in order to save power. The CIN′ signal is input into the clock tree again to distribute the internal clock signal. The result of all four conditions <b>84</b><i>a</i>-<i>d </i>is that lock <b>86</b> occurs with an overall reduction in delay stages, which is the purpose of the circuit while maintaining the desired operating range.

0051Referring now to <figref idref="DRAWINGS">FIG. 7</figref>, the present invention is shown being used in a delay-locked loop or DLL, which is shown generally by the numeral <b>200</b>. An external clock signal <b>216</b> is input into receiver and buffer <b>218</b>. This produces clock input signal (CIN) <b>220</b>. The delay in the signal as it passes through buffer receiver <b>218</b> is represented by d_in<b>219</b>. CIN signal <b>220</b> is input via branch <b>222</b> into phase detector <b>226</b>. CIN signal <b>220</b> is also directed via branch <b>224</b> into delay line <b>228</b>. Phase detector <b>226</b> may include any associated logical circuitry. The goal of the present invention is to take CIN signal <b>220</b> as well as a clock feedback signal <b>230</b> (CKFB) and, by defining particular characteristics and relationships about the timing of CIN signal <b>220</b> and CKFB <b>230</b>, to delineate specific conditions under which the signals are operating, and selecting and directing the signals accordingly. Although a specific logic arrangement is shown, it is contemplated that any suitable control logic may be used to define the conditions of the signals and then selecting them accordingly. CKFB feedback signal <b>230</b> is a typical feedback loop as is found in a common delayed-lock loop (DLL). Phase detector <b>226</b> compares the timing of signal CIN and signal CKFB. Based on timing conditions and characteristics of each signal, control signals are sent via control lines <b>232</b> to control block <b>234</b> and output via lines <b>236</b> to delay line <b>228</b>. The period of the delay is represented by t_delay<b>230</b>. Associated with the delay line <b>228</b> is selector <b>238</b> which receives an input <b>240</b> from the phase detector <b>226</b> as well as inputs <b>242</b> and <b>244</b> representative of the clock CLK and inverted clock signals respectively. Selector <b>238</b> selects, based on the input <b>240</b> from the phase detector <b>226</b>, whether to put signal <b>242</b> or <b>244</b> to input <b>246</b> into clock tree driver <b>248</b>. The period of delay by the driver is represented by t_tree<b>250</b>. The output <b>252</b> of the clock tree driver <b>248</b> is sent to an output buffer <b>254</b> which has an input data line <b>256</b> and a data output line <b>258</b>. The delay by the output of data is represented by the parameter d_out<b>260</b>. Clock tree driver <b>248</b>, as part of the delay-locked loop, feeds back into phase detector <b>226</b> via line <b>230</b>. The delay associated with the I/O model <b>262</b> is represented by the parameter d_inand d_out.

0000Generally speaking,

00521. In order to synchronize XCLK with DQs, <br /><i>t</i>_delay<i>=t</i>_ck<i>−t</i>_tree−(<i>d</i>_in<i>+d</i>_out)

0053In traditional DLLs, the delay stages required are: <maths id="MATH-US-00005" num="5"><math overflow="scroll"><mtable><mtr><mtd><mrow><mi>N</mi><mo>=</mo><mi /><mo>⁢</mo><mrow><mfrac><msub><mi>t</mi><mi>delay</mi></msub><msub><mi>t</mi><mi>d</mi></msub></mfrac><mo>=</mo><mfrac><mrow><msub><mi>t</mi><mi>ck</mi></msub><mo>-</mo><msub><mi>t</mi><mi>tree</mi></msub><mo>-</mo><mrow><mo>(</mo><mrow><msub><mi>d</mi><mi>in</mi></msub><mo>+</mo><msub><mi>d</mi><mi>out</mi></msub></mrow><mo>)</mo></mrow></mrow><msub><mi>t</mi><mi>d</mi></msub></mfrac></mrow></mrow></mtd></mtr><mtr><mtd><mrow><msub><mi>N</mi><mi>worst</mi></msub><mo>=</mo><mi /><mo>⁢</mo><mfrac><mrow><mrow><msub><mi>t</mi><mi>ck</mi></msub><mo>⁡</mo><mrow><mo>(</mo><mi>long</mi><mo>)</mo></mrow></mrow><mo>-</mo><mrow><msub><mi>t</mi><mi>tree</mi></msub><mo>⁡</mo><mrow><mo>(</mo><mi>short</mi><mo>)</mo></mrow></mrow><mo>-</mo><mrow><mrow><mo>(</mo><mrow><msub><mi>d</mi><mi>in</mi></msub><mo>+</mo><msub><mi>d</mi><mi>out</mi></msub></mrow><mo>)</mo></mrow><mo>⁢</mo><mrow><mo>(</mo><mi>fast</mi><mo>)</mo></mrow></mrow></mrow><mrow><msub><mi>t</mi><mi>d</mi></msub><mo>⁡</mo><mrow><mo>(</mo><mi>fast</mi><mo>)</mo></mrow></mrow></mfrac></mrow></mtd></mtr><mtr><mtd><mrow><mo>=</mo><mi /><mo>⁢</mo><mrow><mfrac><mrow><mrow><mn>15</mn><mo>⁢</mo><mi>n</mi></mrow><mo>-</mo><mi>ln</mi></mrow><mrow><mn>110</mn><mo>⁢</mo><mstyle><mtext> </mtext></mstyle><mo>⁢</mo><mi>ps</mi></mrow></mfrac><mo>≈</mo><mn>128</mn></mrow></mrow></mtd></mtr></mtable></math></maths>

00542. Use same method, and adding a selector: <br /><i>t</i>_e<i><t</i>_ck/2<i>, t</i>_delay<i>=t</i>_ck/2<i>−t</i>_e<br /><i>t</i>_e<i>>t</i>_ck/2<i>, t</i>_delay<i>=t</i>_ck<i>−t</i>_e

0055For both cases, <ul id="ul0001" list-style="none"><li id="ul0001-0001" num="0000"><ul id="ul0002" list-style="none"><li id="ul0002-0001" num="0056"> t_delayis less than or equal to t_ck/2 <maths id="MATH-US-00006" num="6"><math overflow="scroll"><mrow><msub><mi>N</mi><mi>worst</mi></msub><mo>=</mo><mrow><mfrac><mrow><mrow><mrow><msub><mi>t</mi><mi>ck</mi></msub><mo>/</mo><mn>2</mn></mrow><mo>⁢</mo><mrow><mo>(</mo><mi>long</mi><mo>)</mo></mrow></mrow><mo>-</mo><mi>other</mi></mrow><mrow><msub><mi>t</mi><mi>d</mi></msub><mo>⁡</mo><mrow><mo>(</mo><mi>fast</mi><mo>)</mo></mrow></mrow></mfrac><mo>=</mo><mrow><mfrac><mrow><mrow><mn>7.5</mn><mo>⁢</mo><mi>n</mi></mrow><mo>-</mo><mi>ln</mi></mrow><mrow><mn>110</mn><mo>⁢</mo><mstyle><mtext> </mtext></mstyle><mo>⁢</mo><mi>ps</mi></mrow></mfrac><mo>≈</mo><mn>59</mn></mrow></mrow></mrow></math></maths></li></ul></li></ul>

0057Referring now to <figref idref="DRAWINGS">FIG. 8</figref>, a timing diagram for signals CIN and CKFB are shown in a particular arrangement. The period from the rising edge <b>300</b> to rising edge <b>302</b> is designated as t_ck. The amount of time from rising edge <b>300</b> of CIN and rising edge <b>304</b> of CKFB is represented by the parameter t_e. Additionally, the parameter from the rising edge <b>304</b> of CKFB and the falling edge <b>306</b> of CIN is represented by the parameter t_delay. In this case, t_delayis less than or equal to half of t_ck.

0058Referring now to <figref idref="DRAWINGS">FIG. 9</figref>, the second case is illustrated where CKFB does not fire until after the first pulse of CIN. Again, t_ckis represented by the rising edge <b>308</b> of CIN and the next rising edge <b>310</b> of CIN. Additionally, the length of time from the rising edge <b>308</b> to the rising edge <b>312</b> of CKFB is shown by the parameter t_e. However, in this instance, t_delayis measured from the rising edge <b>312</b> of CKFB until the next rising edge <b>310</b> of CIN. Similarly, in this case, t_delayis less than or equal to one-half of the clock period t_ck.

0059<figref idref="DRAWINGS">FIG. 10</figref> is a block diagram of a computer system <b>100</b>. The computer system <b>100</b> utilizes a memory controller <b>102</b> in communication with SDRAMs <b>104</b> through a bus <b>105</b>. The memory controller <b>102</b> is also in communication with a processor <b>106</b> through a bus <b>107</b>. The processor <b>106</b> can perform a plurality of functions based on information and data stored in the SDRAMs <b>104</b>. One or more input devices <b>108</b>, such as a keypad or a mouse, are connected to the processor <b>106</b> to allow an operator to manually input data, instructions, etc. One or more output devices <b>110</b> are provided to display or otherwise output data generated by the processor <b>106</b>. Examples of output devices include printers and video display units. One or more data storage devices <b>112</b> may be coupled to the processor <b>106</b> to store data on, or retrieve information from, external storage media. Examples of storage devices <b>112</b> and storage media include drives that accept hard and floppy disks, tape cassettes, and CD read only memories.

0060While the present invention has been described in conjunction with preferred embodiments thereof, many modifications and variations will be apparent to those of ordinary skill in the art. For example, although the present invention is directed to synchronous mirror delay systems, the present invention is contemplated to be used with any implementable logic devices and in other arrangements, such as in a digital delay locked loop (DDLL), to improve the efficiency in that arrangement. The foregoing description and the following claims are intended to cover all such modifications and variations.