Implementing memory read data eye stretcher

[0001] The present invention relates generally to the data processing field, and more particularly, relates to a method and data receiver apparatus for implementing high speed storage, such as double data rate (DDR) memory, read data eye stretcher.

[0002] Capture of data and commands in any electrical bus includes a certain percentage of the usable data eye taken up with transition time on both the rising and falling edges that may exist around the time that the values are captured.

[0003] In all systems, it is desirable to maximize the data eye being captured in order to improve setup and hold time, and to center the capture of the data within the usable data eye. The ability to maximize this time is critical to improving how fast a particular signal, bus, or interface can operate.

[0004] Referring to FIGS. 1A, 1B, 1C, and 1D, prior art aspects of conventional data latching schemes are illustrated. FIG. 1A illustrates a typical data eye with a mid point reference level indicated by a dotted line labeled VREF. The reference level VREF is supplied externally, as in many known systems that use a voltage reference signal. The reference level VREF typically is set to be a mid-point between a low and high level.

[0005] FIG. 1B illustrates a typical first read data stage including one path latching even data and another path latching odd data. An odd latch 102 provides an output of odd data, and an even latch 104 provides an output of even data. A data signal DQ is applied to comparator 106 receiving a midpoint voltage reference level VREF. A source synchronous clock strobe DQS is applied to comparator 108 receiving either a midpoint voltage reference level VREF, or an inverted DQS signal (/DQS). The strobe DQS is applied to a ½ cycle delay 110 and the delayed strobe DQS output of the ½ cycle delay 110 is applied to clock the even latch 104. The delayed output strobe DQS of the ½ cycle delay 110 is applied to an inverter 112 inverted and applied to clock the odd latch 102.

[0006] FIG. 1C illustrates a conventional data latching timing diagram for double-data-rate two (DDR2) and double-data-rate three (DDR3) synchronous dynamic random access memory (SDRAM) interfaces. FIG. 1D illustrates conventional data latching timing diagram for graphics double-data-rate three (GDDR3) SDRAM interfaces. SDRAM interfaces are implemented by a random access memory technology used for high-speed storage of working data of a computer or other data processing system.

[0007] A need exists for an effective receiver mechanism that maximizes the data eye being captured to provide improved setup and hold time, and to center the capture of the data within the usable data eye.

[0008] Principal aspects of the present invention are to provide a method and data receiver apparatus for implementing high speed, such as double data rate (DDR), memory read data eye stretcher. Other important aspects of the present invention are to provide such a method and data receiver apparatus substantially without negative effect and that overcome many of the disadvantages of prior art arrangements.

[0009] In brief, a method and apparatus are provided for implementing memory read data eye stretcher. A plurality of input data paths is provided, each receiving input data and a respective reference level. A pair of data latches is coupled to each of the input data paths, an even data latch latching even data and an odd data latch latching odd data. A reference level is selected for the current data being latched based upon the previous data state of latched data to maximize the data eye.

[0010] In accordance with features of the invention, the data eye is improved by effectively reducing the amount of time that a signal spends in transition, from 1 to 0 and 0 to 1, by altering the receiver reference level. Altering the reference level enables in essence that the setup time is improved or stretched earlier in time.

[0011] In accordance with features of the invention, the data state of the latched even data is coupled to logic controlling a multiplexer coupled to the odd data latch. The data state of the latched odd data is coupled to logic controlling a multiplexer coupled to the even data latch. Knowing this previous data state, a portion of the transition time can effectively be reduced allowing the data receiver apparatus to operate at higher speeds.

[0021] In accordance with features of the invention, a method improves the data eye by effectively reducing the amount of time that a signal spends in transition (from 1 to 0 or 0 to 1) in systems by effectively altering the receiver reference level. A plurality of reference levels are provided including, a VREF signal, that is set to be a mid-point between a low and high level, such as VDD/2, a VREF+ signal, that is set above the VREF signal, and a VREF− signal, that is set below the VREF signal.

[0022] Having reference now to the drawings, FIGS. 2, 3, and 4 illustrate respective data eyes in accordance with the preferred embodiment. Improved data eyes are provided by effectively reducing the amount of time that a signal spends in transition (from 1 to 0 or 0 to 1) in data receivers by effectively altering the receiver input voltage reference level.

[0023] FIG. 2 illustrates a data eye generally designated by the reference character 200 in accordance with the preferred embodiment with the voltage reference level VREF− signal, that is set below the VREF signal. The setup time for the data eye 200 is implemented with the voltage reference level VREF− signal with a previous “0” or tri-state “0” in accordance with the preferred embodiment. The voltage reference level is set to VREF for the hold side of the data eye 200.

[0024] FIG. 3 illustrates a data eye generally designated by the reference character 300 in accordance with the preferred embodiment with the VREF+ signal, that is set above the VREF signal. The setup time for the data eye 300 is implemented with the VREF+ signal with a previous “1” or tri-state “1” in accordance with the preferred embodiment. The voltage reference level is set to VREF for the hold side of the data eye 300.

[0025] FIG. 4 illustrates a data eye generally designated by the reference character 400 in accordance with the preferred embodiment with the VREF or VDD/2 signal. The setup time and the hold time for the data eye 400 are implemented with the VREF signal with a previous VDD/2 or midpoint tri-state in accordance with the preferred embodiment.

[0026] Referring now to FIG. 5A, there is shown a receiver apparatus generally designated by the reference character 500 in accordance with the preferred embodiment. Receiver apparatus 500 includes an eye stretching first read data stage including one path latching even data and another path latching odd data.

[0027] An odd latch 502 provides a latched data output of odd data, and an even latch 504 provides a latched data output of even data. A respective one of a pair of odd and even multiplexers (MUXs) 506, 508 is coupled to the odd latch 502 and the even latch 504. An odd multiplexer control logic 510 and an even multiplexer control logic 512 respectively operatively control the odd and even multiplexers (MUXs) 506, 508.

[0028] Receiver apparatus 500 includes a plurality of data paths, each receiving input data DQ and a respective reference level. The input data signal DQ is applied to a plurality of data receiving comparators 520, 522, 524. A reference voltage level, VREF+ above the midpoint reference voltage level VREF is applied to the data receiving comparator 520. The mid-reference voltage level VREF is applied to the data receiving comparator 522. A reference voltage level, VREF− below the midpoint reference voltage level VREF is applied to the data receiving comparator 524. An output of each of the data receiving comparators 520, 522, 524 is applied to both the odd and even multiplexers (MUXs) 506, 508. The odd data output of odd latch 502 is applied to an input of the even multiplexer control logic 512. The even data output of even latch 504 is applied to an input of the odd multiplexer control logic 510. The output of one of the data receiving comparators 520, 522, 524 is selected by the odd and even multiplexer control logic 510, 512 based upon the previous data state and applied by the odd and even multiplexers (MUXs) 506, 508 to the odd latch 502 and the even latch 504.

[0029] A source synchronous clock strobe DQS is applied to a strobe receiving comparator 526 that also receives the midpoint voltage reference level VREF or /DQS. The strobe DQS output of the comparator 526 is applied to a ½ cycle delay 528. The delayed strobe DQS output of the ½ cycle delay 528 is applied to the even multiplexer control logic 512 and the even latch 504. The delayed strobe DQS output of the ½ cycle delay 528 is applied to an inverter 530, inverted and applied to the odd multiplexer control logic 510 and the clock input of the odd latch 502.

[0030] In accordance with features of the preferred embodiment, by having a receiver apparatus 500 with the voltage reference level VREF or VDD/2 and the voltage reference levels above and below VDD/2 (VREF+ and VREF−), through the respective MUX 506, 508, the best receiver output is selected for the setup side, and VDD/2 for the hold side.

[0031] Having reference to FIGS. 5B, and 5C and FIGS. 6A, and 6B, operation of receiver apparatus 500 may be further understood. With each odd and even receiver latch 502, 504, the unique control arrangement of MUX 506 and odd multiplexer control logic 510, and MUX 508 and even multiplexer control logic 512 is used along with knowledge of the previous state by the receiver apparatus 500 to adjust the reference level being used in the current data being latched so that the data eye can be maximized, or stretched. As a result, a portion of the transition time can effectively be reduced allowing the receiver apparatus 500 or data interface to operate at higher speeds.

[0032] Having reference also to FIGS. 2, 3, and 4, the receiver apparatus 500 of the preferred embodiment allows the data eye to be stretched as shown. As shown in FIGS. 2, 3, and 4, the knowledge of the preceding latched data state is used to alter the reference level so that in essence the setup time advantageously is improved or stretched earlier in time. Enabling this function requires more than one receiver for each data latch pin, or the hold time of the preceding bit would be compromised. The cases of a previous “0” or tri-state “0” and a previous “1” or tri-state “1” are the classic form of the improvement that can be gained, as illustrated in FIGS. 2 and 3. The case of a previous mid-point termination is shown in FIG. 4 and the receiver apparatus 500 of the preferred embodiment provides a larger data eye than the prior art data eye shown in FIG. 1A. When the receiver apparatus 500 is used, with a non-altered reference VREF for the leading edge and trailing edge, capturing the first bit when coming out of tri-state provides a comparable gain to the illustrated data eye examples of FIGS. 2 and 3.

[0033] Referring now to FIGS. 5B, and 5C, there are shown simplified TRUTH tables respectively illustrating the operation of the even multiplexer control logic 512 and the odd multiplexer control logic 510 of the data receiver apparatus 500.

[0034] As shown in FIG. 5B, with previous “0” odd data and DQS delayed of “0”, the VREF− reference level is the selected output of the even MUX 508. With previous “0” odd data and DQS delayed of “1”, the VREF mid-reference level is the selected output of the even MUX 508. With previous “1” odd data and DQS delayed of “0”, the VREF+ reference level is the selected output of the even MUX 508. With previous “1” odd data and DQS delayed of “1”, the VREF mid-reference level is the selected output of the even MUX 508.

[0035] As shown in FIG. 5C, with previous “0” even data and inverted DQS (/DQS) delayed of “0”, the VREF− reference level is the selected output of the odd MUX 506. With previous “0” even data and /DQS delayed of “1”, the VREF mid-reference level is the selected output of the odd MUX 506. With previous “1” even data and /DQS delayed of “0”, the VREF+ reference level is the selected output of the odd MUX 506. With previous “1” even data and /DQS delayed of “1”, the VREF mid-reference level is the selected output of the odd MUX 506.

[0036] In accordance with features of the preferred embodiment, receiver apparatus 500 receives a controller input CTL indicating when data is coming from an unused condition, or tri-state, with the level during tri-state being predetermined as high, low, or any mid-point termination value. The controller input CTL is applied to the even MUX control logic 512 shown in FIG. 5A.

[0037] If the received DQ signal is terminated to VDD, as shown in FIG. 1D, the simple tables in FIG. 5B and FIG. 5C apply and the CTL signal forces the even MUX control logic 512 to select the VREF+ path of comparator 520 for the setup portion of the first data bit after the tri-state condition.

[0038] If the received DQ signal is terminated to GND (not shown but similar), the simple tables in FIG. 5B and FIG. 5C apply and the CTL signal forces the even MUX control logic 512 to select the VREF− path of comparator 524 for the setup portion of the first data bit after the tri-state condition.

[0039] If the DQ signal is terminated to VDD/2, as shown in FIG. 1C, the CTL signal is used by the even MUX control logic 512 to select the VDD/2 path of comparator 522 for the first data received after the tri-state condition and the truth table of FIG. 5C is valid for the odd MUX control logic 510 and the truth table of FIG. 6A is valid for the even MUX control logic 512.

[0040] The ½ cycle DQS delay 528 can now be set earlier in the data eye window to center the timing margins. Also note that FIG. 5A is a simplified diagram of receiver apparatus 500 and that DQS delays slightly before and after the latch delays advantageously are used to guarantee a glitch proof MUX transition. It should be understood that although only a scheme with an external reference is shown, this concept can also be applied to interfaces that only have internally generated reference or receiver levels. In this case the receivers would simply be centered for the VDD/2 case and biased low and high for the − and + voltage reference levels, respectively.

[0041] Referring also FIG. 6A, there is shown a more detailed table illustrating the operation of the even multiplexer control logic 512 including the controller input (CTL) of the data receiver apparatus 500. The information CTL from the controller feeds into the even multiplexer control logic 512 since the controller knows exactly when to expect data and when to expect tri-state on the interface. The signal in this example is shown to be a 1 when tri-state is expected on the leading edge of the first bit. There is a fairly wide window for when this signal can be on as shown. The truth table of FIG. 6A shows how that multiplexer control logic 512 operates for all conditions including the tri-state control when asserted active positive.

[0042] FIG. 6B illustrates data latching waveforms for the data receiver apparatus 500 in accordance with the preferred embodiment for double-data-rate two (DDR2) and double-data-rate three (DDR3) synchronous dynamic random access memory (SDRAM) interface operation.

[0043] It should be understood that the present invention is not limited to the illustrated embodiment of the data receiver apparatus 500. For example, data receiver apparatus 500 could be architected so that as opposed to controlling the decision before the data is latched, the reference voltage level could also be determined in a similar fashion after the data is latched. Such arrangement of data receiver apparatus 500 requires multiple latches with the decision logic about which latch to use implemented after captured data. The appropriate data to use would then have to be selected before use later in the data pipe. Such arrangement of data receiver apparatus 500 requires more overhead in terms of transistor count, area, and power, so that the illustrated data receiver apparatus 500 is the preferred, simpler implementation option.

[0044] While the present invention has been described with reference to the details of the embodiments of the invention shown in the drawing, these details are not intended to limit the scope of the invention as claimed in the appended claims.