Abstract:
An improved DDR interface uses single-ended technology and phase-shifts all output data signals and the output source clock signal so that each output signal switches at a different time so that IDDQ spikes caused by I/O switching do not accumulate. A dynamic phase adjustment circuit on the receiver compensates for the phase differences. Clock jitter and skew is reduced and the number of IDDQ pins is reduced to provide a more effective design and high density package.

Description:
BACKGROUND OF THE INVENTION 
   The Double Data Rate (DDR) interface is a very popular source synchronous interface which is widely used in the integrated circuit (IC) industry and in networking products. For a DDR interface, the limiting factor of its operating frequency is clock jitter, clock duty cycle distortion and the skews within the data bus. A standard single-ended DDR interface is depicted in  FIG. 1 . 
   The skew is the variation of the transition point for all signals which are referenced to the same clock signal. The variation can be caused by difference loadings, crosstalk from neighbor signals, ISI (Inter-Symbol Interference), SSO/SSI (Simultaneous Switching Output/Input) noise, difference in propagation path lengths, and/or difference in the rise and fall time of the signal. Clock jitter is a variation in the frequency (or phase) of a clock signal due to instability of the clock source, noises coupled from power supply, and/or crosstalk from other signals. Because the DDR latches the data at both rising and falling edges, the duty cycle distortion could have a directly impact on DDR operation. 
   Based on industry publications and SI analysis by the assignee of the present application, simultaneous switching noise (SSN) due to simultaneous switching of input signals (SSI) and output signals (SSO) has a significant contribution to the clock jitter and data switching uncertainty. The SSI and SSO cause a spike in the quiescent drain current (IDDQ) which in turn causes a spike in the quiescent drain voltage noise (VDDQ_noise). The larger the IDDQ spike the greater the VDDQ_noise, and the larger impact on signal switching uncertainty. 
   In current designs, it is very common to see that the timing uncertainty due to SSO and SSI noise could be +/−250 ps to +/−350 ps separately on a 250 Mhz and a 125 Mhz DDR interface. Combining the timing uncertainty for SSO and SSI together could result in a total timing uncertainty of +/−500 ps to +/−700 ps from SSN and which is the largest portion of DDR timing budget. 
   The VDDQ_noise can be reduced by lowering the resistance and inductance of the IDDQ path. Thus, one way to reduce the SSO/SSI noise is to increase the number of power and ground pins supplying VDDQ current to the I/O buffers, thus reducing the signal pins to power/ground pins ratio. However, due to package size limitation and cost consideration, this is no longer an effective solution for large sized ASICs. 
   The System Packet Interface Level 4, Phase 2 (SPI4.2) Specification utilizes differential signaling and a dynamic clock alignment circuit (DAC) at the receiver. This interface is depicted in  FIG. 2 . The differential signaling could significantly reduce the SSO/SSI noise, and the DAC circuit could compensate some part of fixed data skews within the signal lines, so SPI4.2 can be operated at higher frequency. The main drawback for SPI4.2 is its differential signaling which doubles the total number of pins compared with single-ended signaling. 
     FIG. 2  depicts a transmitter side  2  that utilizes a 2× clock to clock data on the rising edge of the clock, so it can avoid duty cycle distortion requirement on 1× clock which is not shown in this patent. On the receiver  4  side a DAC  6  generates phase-shifted clock signals utilized to sample the data. The clock signal phases are optimized so that data is latched in the center of the data window. The DAC also compensates the fixed skew between received data signals, which could be caused by different trace length and process variation cross the interface. 
   Accordingly, techniques for reducing SSN without increasing total number of pins on an IC are greatly needed in the field. 
   BRIEF SUMMARY OF THE INVENTION 
   One embodiment of the invention is a new enhanced DDR interface which can significantly reduce the SSO and SSI noise in a DDR interface. Since the SSO/SSI noise has a significant impact on DDR interface clock jitter and data switching uncertainty, the enhanced DDR interface increasees the DDR bandwidth and reduce the required number of VDDQ power and ground pins, thus leading to a more effective design and high density package. 
   According to a another embodiment of the invention, a DDR interface utilizes single-ended technology to reduce pin count and reduces the magnitude of IDDQ spikes approximately by a factor of N where N is the number clock phases at the DDR transmitter. 
   According to a another embodiment of the invention, a DDR interface utilizes phase-shifted clock signals to switch single-ended phase-shifted output data signals and a single-ended phase-shifted source clock signal on the transmit side. The magnitudes of the phase shifts between each signal switched could be greater than the settling time of IDDQ spikes caused by switching so that the amplitudes of the spikes don&#39;t add to generate a large spike. If the magnitudes of the phase shifts between each signal switched is less than the settling time of IDDQ spikes caused by switching, the amplitudes of the spikes are still much lower than the case which all signal switch at the same time. 
   According to a another embodiment of the invention, a dynamic clock alignment (DCA) circuit on the receive side of the interface receives a single-ended or differential transmitted source clock signal and generates compensated clock signals which are aligned to sample data from single-ended phase-shifted data signals. Because of the DCA circuit, it is not necessary to have the serpentine clock traces on the PCB board, so the board space can be saved. 
   Other features and advantages of the invention will be apparent in view of the following detailed description and appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram depicting an existing single-ended DDR interface; 
       FIG. 2  is a schematic diagram of a typical SPI 4.2 interface utilizing differential signals; 
       FIG. 3  is a schematic diagram depicting an embodiment of the invention; 
       FIGS. 4A  and B are schematic diagrams of a DDL or PLL suitable for use in an embodiment of the invention. 
       FIGS. 5A  and B are graphs depicting a comparison between the switching voltages of a standard DDR interface and an embodiment of the invention; and 
       FIG. 6  is a timing diagram depicting the IDDQ generated by the embodiment of  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference will now be made in detail to various embodiments of the invention. Examples of these embodiments are illustrated in the accompanying drawings. While the invention will be described in conjunction with these embodiments, it will be understood that it is not intended to limit the invention to any embodiment. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the various embodiments. However, the present invention may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail in order not to unnecessarily obscure the present invention.  FIG. 3  depicts an embodiment of the invention. 
     FIG. 3  depicts a Transmitter (Tx)  10  and Receiver (Rx)  12  coupled N single-ended data lines  14 ( 1 )- 14 (N) and a clock line  16 . On the Tx side  10 , each data line  14  is coupled to a corresponding output driver  20 ( 1 )- 20 (N) and the clock line  16  is coupled to an output clock driver  22 . A 2× clock signal is provided so that data is clocked on rising or falling edge of the 2× clock to transfer data at double the data rate (DDR) which is reference to 1× clock at the output of clock driver  22 . 
   In this embodiment, the 2× clock signal is provided to a phase-shifting block  30  which generates N phase-shifted clock Tx clock signals φ( 1 )-φ(n) each having the same frequency as the 2× clock signal but displaced from that signal by a different multiple of a fixed, unique phase. 
   The two-bits of data provided to each output driver  20 ( n ) per clock cycle are provided to a input series circuit comprising a multiplexer  32 ( n ) having an output coupled to a latch  34 ( n ) having an output coupled to the corresponding output driver  20 ( n ). Each series circuit is coupled to a different one of the phase-shifted clock signals φ(n) and outputs a single-ended phase-shifted data signal onto the coupled single-ended data line. 
   A similar series circuit is coupled to the clock driver  22 . A clock multiplexer  36  is has inputs coupled to fixed 0 and 1 values and an output coupled to clock latch  38  which has an output coupled to the clock driver  22 . The clock multiplexer  36  and clock latch  38  are coupled to the phase-shifted clock signal φ(N/2) and a Tx source clock signal having a phase shifted by a unique multiple of the fixed phase shift is output on the clock line  16 . 
   Turning to the Rx side  12 , each data line  14  is coupled to a corresponding input driver  40 ( 1 )- 40 (N) and the clock line  16  is coupled to an input clock driver  42 . 
   The clock in signal is passed from the input clock driver to an Rx data alignment circuit  44  which receives the transmitted Tx source clock signal and generates a set of 1× compensated Rx clock signals φ( 1 )-φ(N) which compensate for the phase shifts introduced into the phase-shifted data signals at the Tx side. The output of each input driver  40 ( 1 -N) is coupled to a corresponding pairs of HI and LO input latches  46 H( 1 -N) and  46 L( 1 -N). Each pair of output a latches  46 H(n) and  46 L(n) receives a compensated Rx clock signal from the DAC  44  adjusted to sample the data on the phase-shifted data signal. The data outputs of the HI and LO latches for each data line provide the data clocked on the rising and falling edge of the provided clock signal. 
   At the receiver side, each input latch can be clocked by a different compensated Rx clock signal derived from the received Tx source clock signal φ(N/2). This is accomplished by the Rx DAC  44  which receives φ(N/2) and generates compensated Rx clock signals φ( 1 ) to φ(N). The compensated Rx clock signal provided to a particular pair of receive latches  46  is phase-shifted from the Tx clock signal by a phase-shift required to sample the received data signals. 
   Alternatively, a single Rx clock signal can be used to drive multiple input latches to sample different phase-shifted data signals due to trace length variations and the impact of the PLL/DLL on the transmit side. 
   The receiver side of the DLL interface of the described embodiment is similar to the receiver side of the SPI 4.2 interface depicted in  FIG. 2 . In this embodiment training patterns are utilized to select the best clock phases for each input latch prior to normal transmission. In  FIG. 3  the phase-shifting block  44  at the Tx side is utilized to provide multiple phase-shifted clock signals. As is known in the art either a PLL (Phase-Locked Loop) or a DLL (Delay Locked Loop) can be utilized to generate these signals.  FIGS. 4A and 4B  are schematic diagrams of a PLL and a DLL, respectively, that can be utilized in the above-described embodiment. 
   The advantages of the system described with reference to  FIG. 3  will now be described with reference to  FIGS. 5A-B  and  6 . As depicted in  FIG. 3 , data on each output line is clocked by different phase-shifted 2× clock output signal, φ( 1 )-φ(N). Thus, unlike the circuits depicted in  FIGS. 1 and 2  none of the output lines switch at the same time. 
     FIGS. 5A and 5B  depict graphs that illustrate the advantage of not switching all the input lines at the same time.  FIG. 5A  depicts the spike in IDDQ when a single buffer switches. The spike has a width to T_tr which is equal to about 100 to 300 ps and a height equal to XA milliamps.  FIG. 5B  depicts the IDDQ spike when all N buffers switch. The IDDQ spike now has an amplitude of N(XA) milliamps. The contribution to SSN due to all the buffers switching simultaneously is on the order N times that due to a single buffer switching. 
     FIG. 6  depicts the effect of phase-shifting the timing of the switching of various output buffers on the Tx side. In this example, the phase-shift Δt is slightly larger than T_tr. When the first signal, φ 1  is asserted an IDDQ spike of amplitude YA milliamps occurs. However, this spike has damped out during the phase shift interval Δt so that when the second signal, φ 2 , delayed relative to φ 1  by ΔT, is asserted a second spike occurs but the effect is not cumulative since the first spike has damped out. Thus, the phase shifting of the clock signals provided to the output buffers has reduced the amplitude of the IDDQ spike by a factor of N. The magnitude of the cumulative IDDQ spike will be reduced as long as the phase-shift is sufficiently large to shift the peaks of the individual IDDQ spikes to prevent their summation. 
   Accordingly, an embodiment of an improved DDR interface has been described having the following advantages: 1) The peak current from the I/O switching will be significantly reduced; 2) The SSO/SSI noise on VDDQ will be significantly reduced; 3) The jitter and uncertainty caused by SSO/SSI will be significantly reduced; 4) The DDR interface bandwidth can be increased; 5) The required VDDQ power/ground pins can be reduced, so the package size can be reduced; and, 6) Since the VDDQ is the biggest noise source on the PCB board and package, the new enhanced DDR interface will has much smaller noise coupling to other circuits from VDDQ. 
   The invention has now been described with reference to the preferred embodiments. Alternatives and substitutions will now be apparent to persons of skill in the art. For example, different logic circuits could be utilized to switch the data signals onto and off the data lines. Also, in the above described embodiment, each phase-shifted Tx clock signal is utilized to switch data onto a single data line. However, as is known in the art, each phase-shifted Tx clock signal can be utilizied to switch data onto a set of data lines. In this case, the magnitude of the IDDQ spike would be reduced in proportion to the number of phase-shifted Tx clock signals utilized to switch the signals. Accordingly, it is not intended to limit the invention except as provided by the appended claims.