Abstract:
A circuit receives data from a high frequency data line. The circuit determines the data value by employing a decision circuit and an over-sampling circuit. The over-sampling circuit captures the data levels on the data line at spaced apart time intervals. The decision circuit employs the data levels captured by the over-sampling circuit and a previously stored value to determine the data level that should be received from the data line.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to high frequency data transfers. In particular, the present invention relates to a method and apparatus for reliably receiving synchronous data from a high frequency data source. 
     2. Description of the Related Art 
     Data movement within a computer system, for example, can take place in an asynchronous mode or in a synchronous mode. Asynchronous data transfers are accomplished by generating special control signals when data are provided on data lines. For example, in an asynchronous data transfer, a strobe signal may be provided when data are on the data lines, such that a sampling edge of the strobe signal (the edge to which the receiving module is responsive) coincides with steady data levels. Synchronous data transfers are accomplished by providing data to lines, such that steady data levels on the lines coincide with a sampling edge of the clock signal to which the data is synchronized. For example, the data may be synchronized to a system clock. During synchronous data transfers, steady-state or level data values are provided on the data lines coincident with the sampling edges of the system clock. The data are allowed to change (i.e., transition) only between adjacent clock sampling edges. Synchronous data operations generally result in data rates that are generally higher than those resulting from asynchronous data operations, especially with transfers of large amounts of data, because of the one-to-one correspondence between clock cycles and data cycles. 
     Synchronous Dynamic Random Access Memory (hereinafter “SDRAM”) is a generic name for various kinds of Dynamic Random Access Memory (hereinafter “DRAM”) that are synchronized with the system clock. Data operations employing SDRAMs generally comprise burst operations during which a special control signal is followed by a burst of data. 
     A by-product of the higher data rate is the reduction in access time that is allotted for each data read cycle or data write cycle. Data are read from or written to the memory module during a shorter time than that which is available in lower rate systems. For example, employing a 100 Mhz system clock, synchronous data that are read on a single edge of the system clock are at a steady state (not transitioning) for much less than 10 ns during a data cycle. During other times, the data are transitioning from a high level to a low level or from a low level to a high level. The sampling edge of the system clock used to read the synchronous data should generally coincide with steady state values of the data so as to capture valid data. Sampling during transitions of the data will generally result in uncertain data values that should not be used. Because of the very fast sampling rate, misalignment of the sampling edge of the system clock and the steady state data signal can result in sampling of data during clock transitions. 
     Although the clock and the data are synchronized, the sampling edge of the clock signal may occur substantially simultaneously with a data transition such that the clock may gate the state of the data before the transition, gate the state of the data after the transition, or gate an ambiguous state. The sampling edge of the clock signal may align with the data transition because delays in the circuit that generates the sampling clock signal may be different from delays in the circuit that provides the data. Printed circuit board traces have the effect of delaying a signal that is transmitted along the traces. Different length traces provide different delays. The delays are generally not long enough to have a significant effect on lower rate data transfers. Nonetheless, the delays attributed to different length traces may have a significant effect on high rate data transfers. A delay of as little as 2 nanoseconds on a high frequency data line can cause the relative shift of the data such that data transitions occur simultaneously with the sampling edge of the system clock signal. Thus, the outputs of the circuit which gates the data in synchronism with the system clock signal may not present the correct data. Since the outputs of this circuit comprise the data transferred to or from the SDRAM, erroneous data may be transferred. The relative shift of the data transitions in relation to the system clock signal is generally referred to as “data skewing.” 
     Once data skewing becomes severe enough to cause unpredictable data behavior, the condition persists for an extended time interval. Because the data are driven by the same clock signal that is used to control the data sampling, the data transitions are separated by a multiple of clock signal cycles. Thus, once the data transition and a sampling edge of the system clock signal coincide, the next data transition also coincides with the sampling edge of the system clock signal. Therefore, there is a need for a method of reading data from an SDRAM while ensuring that data are correctly received, regardless of data skewing. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a receiving circuit receives synchronous data from data lines that are synchronized with a clock signal. The receiving circuit includes an over-sampling circuit. The over-sampling circuit samples the data during at least three time intervals in response to at least one edge of the clock signal. The samples from the module are provided to a decision circuit. The decision circuit determines the data levels provided on the data line by reference to the samples from the over-sampling circuit and by reference to the previous determinations of the decision circuit. 
     The present invention also provides a method of receiving synchronous data. The method first samples the data level of a data signal line at least three times in response to a sampling edge of a clock signal. The method then determines a least one data value based on the sampled levels and a previously determined data value. 
     In one embodiment the circuit of the invention is used to receive synchronous data from a data line. The data on the data line are synchronized with a clock signal. The circuit includes an over-sampling circuit, which provides data samples to a decision circuit, and which samples the data at least two times in response to at least one edge of the clock signal. The circuit also includes a decision circuit, which determines the data levels provided on the data line by reference to the samples from the over-sampling circuit and by reference to the previous level determined by the decision circuit. 
     The present invention further provides for a method of receiving synchronous data. The method includes a first step where the data level of a data signal line is sampled at least two times in response to a sampling edge of a clock signal. The method then continues with a step during which at least one data value is determined on the basis of sampled levels and a previous determination. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A illustrates the data signal and system clock levels during a properly timed segment of a read operation; 
     FIG. 1B illustrates the data signal and system clock levels when the data signal is skewed relative to the system clock signal; 
     FIG. 2 illustrates a sampling circuit that provides data samples to the decision circuit of the receiving circuit of the present invention; 
     FIGS. 3A-3J illustrate the possible sampling conditions of the sampling circuit of the present invention; 
     FIG. 4 is a state transition diagram of the operation of the decision circuit of the present invention; 
     FIG. 5 illustrates a logic gate implementation of the state transition diagram of FIG. 4; and 
     FIG. 6 illustrates the interconnection of a receiving circuit in accordance with the present invention and an SDRAM bank. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An exemplary method and an exemplary apparatus in accordance with the present invention will now be described with reference to illustrations of an embodiment of a receiving circuit that is used to receive data from an SDRAM. First, a problem that the present invention solves will be discussed. Next, an embodiment of a sampling circuit that is part of the receiving circuit of the present invention will be described. Finally, the structure and operation of a receiving circuit will be described with reference to illustrations of a state machine diagram and a logic diagram. 
     In the discussion below, “1,” and “0” are used to refer to a logical high level and a logical low level of a signal, respectively. Although the illustrated embodiment is a circuit used to provide data to an SDRAM, the disclosure is equally applicable to circuits used to provide data to other elements of a computer system. The signals referred to in the following discussion are assumed to be at any of two logic levels, a high level and a low level. Nevertheless, the discussion below is equally applicable to signals that can have more than two levels. 
     FIG. 1A illustrates a data signal  22  that may be provided, for example, at the output of an SDRAM. The transitions  30 ,  36  of the data signal  22  are at least one system clock cycle apart. Thus, no more than one transition in the data signal may occur per clock cycle. In the illustrated example, the sampling edges of the clock signal are the rising edges  26 ,  32  of the clock signal  24  (transitions from 0 to 1). As may be appreciated, the first sampling edge  26  of the system clock signal  24  coincides with a steady-state level  28  of the data signal  22 . The data signal  22  is at a steady-state level because the level  28  is located between the previous transition to the present level and the next transition from the present level. Thus, the level  28  is unambiguously received as a logic 1. As a second example, a low logic level  34  of the data signal  22  is at a steady-state value when the second rising edge  32  of the clock signal  24  occurs. Thus, the level  28  is unambiguously received as a logic 0. 
     FIG. 1B illustrates a data signal  38  that is skewed in relation to a clock signal  39 . The data signal  38  is skewed because the transition  42  of the data signal occurs substantially simultaneously with the sampling edge  44  of the system clock signal  39 . As may be appreciated, because the data transitions are synchronized to the system clock signal  39 , the next transition  48  of the data signal  38  also occurs substantially simultaneously with the next sampling edge  46  of the system clock signal. Thus, the data that are received from the data line are metastable because the received values depend on the relative timing of the data transitions and the clock transition. For example, the data level read on the first sampling edge  44  of the system clock signal  39  may be a 1 if the sampling edge coincides with the early portion of the data transition  42 . Alternatively, the data level read on the same sampling edge  44  of the clock signal  39  may be a 0 if the sampling edge coincides with the later portion of the data transition  42 . Whether the data level is read as a 1 or a 0 depends on a number of factors, such as the setup and hold times of the circuit gates and also the rise and fall times of the data signals and the clock signals. Thus, the data that transition substantially simultaneously with the sampling edge are unstable. 
     When the transition of the data signal and the sampling edge of the clock signal coincide, the correct data value to be received from the data line generally cannot be determined. For example, if the data signal transitions from a 1 to a 0 and the clock edge is late, a 0 may be received; however, a 1 should have been received because the clock should have arrived during the steady-state 1 level before the transition, and therefore should have gated the logical 1 level. On the other hand, if the clock edge is early, a logical 1 may be received; however, a logical 0 should have been received because the clock edge should have arrived after the transition, and therefore should have gated the logical 0 level. 
     When reading data with a conventional receiving circuit, the relative timing between the clock edge and the direction of data transition, is not known. The only information available is the perceived level on the data signal line when the single sampling edge occurs. Therefore, it is difficult to determine the data value that should correctly be received. 
     The present invention provides an apparatus and a method for correctly determining the value that should be received from the data line by expanding the view of the receiving circuit in order-to capture a broader snapshot of the levels on the data signal. FIG. 2 illustrates an over-sampling circuit  50  used in cooperation with the determination logic circuit (FIG. 4) of the present invention so as to properly receive data from a high frequency data line. 
     The over-sampling circuit  50  takes a snapshot of the data signal over a predetermined time period relative to a sampling edge of the system clock signal. The over-sampling circuit  50  includes three latches or flip-flops  56 ,  64 ,  76  having respective data inputs  58 ,  66 ,  78 . The data inputs  58 ,  66 ,  78  are coupled to a data input line  52 . The clock input  60  of the first latch  56  is coupled to a system clock line  54 . The clock input  68  of the second latch  64  is coupled to the output  73  of a first delay element  72 . The input  71  of the first delay element  72  is coupled to the system clock line  54 . The clock input  80  of the third latch  76  is coupled to the output  77  of a second delay element  74 . The input  75  of the second delay element  74  is coupled to the output  73  of the first delay element  72 . The outputs  62 ,  70 ,  82  of the three latches are respectively coupled to three output signal lines, B 0 , B 1 , B 2 . 
     The over-sampling circuit of FIG. 2 operates to provide three samples of data for every sampling edge of the system clock by generating three spaced apart sampling edges. The first latch  56  stores the data level that was on the data input line  52  when the sampling edge was asserted. The second latch  64  stores the data level that was on the data input line  52  one delay after the sampling edge because the signal at the clock input  68  of the second memory module is delayed once. The third latch contains the data level that was on the data input line  52  two delays after the sampling edge because the clock signal to the clock input of the third latch is delayed twice. Thus, the three signal lines B 0 , B 1 , B 2  from the three latches provide three values making up a single snapshot sample of the data input line  52 . By analyzing the three sample values and the prior output state of the data, the receiving circuit determines the correct next output state of the data so that all input data states are correctly presented at the output of the circuit. 
     FIGS. 3A-3J illustrate the data sampling scenarios that may be provided to the over-sampling circuit of the FIG.  2 . The figures illustrate the data signal levels near a transition point. The data appearing on the signal line represent the values that are clocked into the over-sampling circuit. When a dot in FIGS. 3A-3J appears on the transition from one level to the another, the value received into the memories of the over-sampling circuit may be either a 0 or a 1. Thus, both possibilities are accounted for, as provided by the potential sample values that appear to the right of the corresponding signal level illustration. Note that when a data line is sampled during a transition from a 1 to a 0 or from a 0 to a 1, it is possible for two latches  56 ,  64 ,  76  to store different values for the data level during the transition. Assuming first that all latches  56 ,  64 ,  76  have the same setup and hold times and that the sampling clocks and delayed sampling clocks have approximately the same propagation times to the respective clock inputs of the latches, then the three latches should sample the transition sequentially. In particular, as the data line transitions from a logical 1 to a logical 0, the three latches should latch a  110  or a  100 . However, because of difference in setup and hold times, differences in propagation delays and differences in thresholds of the latches, it is also possible for the middle latch  64  to receive a different value than the values received by either the first latch  56  or the last latch  76 . For example, if the middle latch has a longer setup time, it may continue to detect a 1-to-0 transition as a 1 although the first latch  56  has already detected the level as a 0. Similarly, if the middle latch  64  has a short setup time compared to the last latch  76 , the middle latch  64  may detect the level as a 0 although the last latch  76  continues to detect the level as a 1. 
     It should also be noted that when the sample clock is occurring at substantially the same time as a transition in the data level, it is not readily apparent whether the sample clock is intended to sample the data level that was present before the transition or to sample the data level that is present after the transition. As discussed below, the one aspect of the present invention is to use the data history to determine whether the sampled data should be considered to be a 1 or a 0. By using the history as well as the samples, this aspect of the present invention assures that a particular data level that ends or begins near a transition in the sample clock is not missed. When all three values of a sample are of a single level, such as the samples that may be provided by the sampling conditions of FIGS. 3A,  3 E,  3 F and  3 J, the determined level is the same as that of the three sample values regardless of the history of the circuit. The history becomes important when the values of a sample contain different levels, such as the samples that may be provided by the sampling conditions of FIGS. 3B,  3 C,  3 D,  3 G,  3 H, and  3 I. The different levels in a single sample, indicate that the data is changing states during the sampling period. One possibility is to cause the output level of the circuit to be the same as the data level measured by the last value in the sample. Another possibility is to cause the output level to be the same as the data level measured by the first value in the sample. Knowing the history of the circuit, the receiving circuit can make the correct determination as to which sample value to use. If the data level of the first value in the sample is the same as the prior output level and the data level of the last value in the sample is opposite the prior output level, then it is presumed that the data level from which the data signal is transitioning has already been detected and that the data level to which the data signal is transitioning is the data value for the current sample. On the other hand, if the data level of the first value in the sample is opposite the prior output level and the data level of the last value in the sample is the same as the prior output level, then it is presumed that the data level from which the data signal is transitioning has not been detected and is therefore taken as the data value for the current sample. 
     The logic that analyzes the received samples is part of a state machine that utilizes the three values in a sample and the current output of the state machine to determine the next state of the state machine. The state machine is developed by observing that a data line can have a maximum of one transition per clock cycle. Therefore, when a sample from the over-sampling circuit contains a value that is different from the previously determined value, a single transition in the data must have occurred after determining the previous value and before receiving the present sample. The receiving circuit thereby provides an output value that is opposite the previous output value when any of the three samples is opposite of the previously determined value. When none of the samples of the over-sampling circuit has a value that is opposite the previous output value, indicating that the data did not transition during the clock cycle, the determined value provided by the receiving circuit is the same as the previously determined value, as discussed above. 
     A special case may be provided for samples that are unlikely to be received by the over-sampling circuit but should nonetheless be accounted for. The special case arises when a sample of the over-sampling circuit contains a first value, B 0 , and a last value, B 2 , that are of the same logic level, and a middle sample, B 1 , of the opposite level. Because the data are synchronized with the system clock, a sample having a first value and a last value of the same logical level, with the opposite value in between, should not occur; however, as discussed above, differences in setup and hold times, propagation delay and thresholds can cause the middle sample to be different than either the first sample or the last sample. As set forth below, the different middle sample does not affect the integrity of the data sampling. 
     FIG. 4 is an illustration of a state table  85  that implements the decision logic outlined above. When the previously determined value is a 0, the state machine  85  is at a first state  84 . When the previously determined value is a 1, the state machine  85  is at a second state  86 . The state machine  85  transitions from the first state  84  to the second state  86  when any input sample contains a 1. The state machine  85  transitions from the second state  86  to the first state  84  when any input sample contains a 0. The state machine  85  remain in the first state  84  when the input sample is 0,0,0. The state machine  85  remains in the second state  86  when the input sample is 1,1,1. 
     FIG. 5 is a logic diagram of a decision circuit  90  that is configured to implement the state table  85  of FIG.  4 . The decision circuit  90  includes a first three input NAND gate  104  having inputs  106 ,  107 ,  108  connected to receive the values B 0 , B 1 , and B 2 , respectively, from each sample of the over-sampling circuit. An output  110  of the NAND gate  104  is a signal ANY_ZERO which is high (i.e., a logical 1) when the level on any of the input lines  106 ,  107 ,  108  is a zero. Thus, the NAND gate  104  serves as a zero detector. 
     A two-input AND gate  116  has a first input  112  coupled to the output  110  of the NAND gate  104 . A second input  114  of the AND gate  116  is coupled to a latch  152  (FIG. 6) that stores the previously determined value of the receiving circuit as a value S i−1 . 
     An output  118  of the AND gate  116  is coupled to a first input  102  of a first two-input NOR gate  98 . The output  100  of the first NOR gate  98  is provided as the new output value of the receiving circuit, designated as S i . A second input  96  of the first NOR gate  98  is coupled to an output  94  of a second three-input NOR gate  88 . The second NOR gate  88  has three inputs  91 ,  92 ,  93  that are coupled to the first value, B 0 , the second value, B 1 , and the last value, B 2 , respectively, from each sample of the over-sampling circuit. The output  94  of the second NOR gate  88  is an ANY_ONE signal which is active low (i.e., logical zero) when any of the samples B 0 , B 2 , B 2  is a logical one. (The line across the top of the ANY_ONE signal indicates that it is an active low signal). 
     If all of the sample inputs are low, the ANY ONE signal on the output  94  of the second NOR gate  88  is high (i.e., logical  1 ), causing the input  96  of the first NOR gate  98  to be high. Therefore, the output  100  of the first NOR gate  98  is forced low to cause S i  to be a 0, thus implementing the transition of the state table responsive to the 0,0,0 sample inputs. 
     If all the sample inputs are high, the ANY_ZERO signal on the output  110  of the NAND gate  104  is low, the ANY_ONE signal on the output  94  of the second NOR gate  88  is low, and the output  118  of the AND gate  116  is forced low. Therefore, both inputs of the first NOR gate  98  are low, and the signal Si on the output  100  of the first NOR gate  98  is forced high; thus, implementing the transitions of the state table responsive to the 1,1,1 sample inputs. 
     If any other combination of ones and zeros occurs (i.e., 001, 010, 011, 100, 101, 110), then the ANY_ONE signal on the output  94  of the second NOR gate  88  is low, causing the input  96  of the first NOR gate  98  to be low. The ANY_ZERO signal on the output  110  of the NAND gate  104  is high. The output  118  of the AND gate  116  and the input  102  of the first NOR gate  98  depend on the value of S i−1  (i.e., the previously latched output value). In particular, if S i−1  is a 1, then the output  118  of the AND gate  116  and the input  102  of the first NOR gate  98  are high, forcing the output  100  of the first NOR gate  98  to be low. If S i−1  is a 0, then the output  118  of the AND gate  116  and the input  102  of the first NOR gate  98  are forced low, causing the output  100  of the first NOR gate  98  to be high. It can be seen that any combination of mixed ones and zeroes causes the next output state to be the opposite as the previous output state. 
     FIG. 6 illustrates a receiving circuit  131  arranged to receive data from a data line  137  and to provide the data to an SDRAM bank  120 . The circuit of FIG. 6 includes the over-sampling circuit  50 , the decision circuit  90 , an SDRAM bank  120 , and a flip-flop or latch  152 . The data input  138  of the over-sampling circuit  50  is coupled to the data line  137 . The clock input  136  of the over-sampling circuit  50  is coupled to the system clock signal line  134 . The sample outputs  140 ,  142 ,  144  of the over-sampling circuit  50  are coupled to the data inputs  160 ,  162 ,  164  of the decision circuit  90 , respectively. The output  166  of the decision circuit  90  is provided as the output of the receiving circuit  131 . The data input  122  of the SDRAM bank  120  is coupled to the next state output (S i )  166  of the decision circuit  90 . The clock input  128  of the SDRAM bank  120  is coupled to the system clock signal line  134 . The previous state input (S i−1 )  156  of the decision circuit  90  is coupled to the Q output  154  of the flip-flop  152 . The data input  150  of the flip-flop  152  is coupled to the Si output  166  of the decision circuit  90 . The clock input  148  of the flip-flop  152  is coupled to the system clock signal line  134 . Although shown for only one bit of data, one skilled in the art will appreciate that the sample circuit and decision circuit are repeated for each data signal. 
     The circuit of the embodiment disclosed above employs an over-sampling circuit that provides three values in each sample. In other embodiments the circuit can be extended to employ an over-sampling circuit that provides a greater number of samples by adding delay elements and sample latches to the arrangement of FIG.  2 . Similar state rules, used to construct the state machine of the illustrated embodiment, can be used to construct a state machine for any size input sample. 
     Although the receiving circuit described above employs the current sample and a previously determined value to determine the next value, other determination schemes may be employed with similar effectiveness. For example, the over-sampling circuit can store values for two system clock cycles such that sample values for two data values are available to the decision circuit. The decision circuit can then determine the value for the data corresponding to the previous clock cycle based on the current sample, previous sample, and the previously determined value. 
     Although the invention has been described in terms of certain preferred embodiments, other embodiments that are apparent to those of ordinary skill in the art, including embodiments which do not provide all of the features and advantages set forth herein, are also within the scope of this invention. Accordingly, the scope of the invention is intended to be defined by the claims that follow.