Abstract:
The propagation of a feedback signal, such as a DQS signal generated in response to a read request in a Double Data Rate (DDR) memory system, into a digital host system, such as an ASIC, is controlled by using delay tracking to compensate for variable I/O delay. The memory system includes a controller and an interface, both on the ASIC, and memory units coupled to the controller through the interface, all configured on a printed circuit board (PCB). The interface uses the read request signal, sent by the controller to initiate read operations, to generate a read-enable signal, which is transmitted to a trace on the PCB one-half cycle of the system clock before DQS is expected to reach the interface. The trace tracks the total delay encountered by the system clock and DQS between the interface unit and memory units, and is routed back to the interface unit, where read-enable is used to generate an enable signal that allows DQS to propagate into the ASIC only when DQS is a valid digital signal.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates generally to the field of digital interface design and, more particularly, to a system for controlling and timing the propagation of a digital signal.  
           [0003]    2. Description of the Related Art  
           [0004]    The design of interfaces plays a significant role in the implementation of many digital systems. One example of a digital system is a Double Data Rate Synchronous Dynamic Random Access Memory (DDR SDRAM) system. FIG. 1 illustrates a common implementation of a DDR SDRAM system configured on a printed circuit board and comprising a DDR SDRAM Controller coupled to a DDR SDRAM unit through an interface unit (Memory I/O), wherein the Memory I/O is coupled to the DDR SDRAM unit through a set of pad circuits. In a DDR SDRAM, read-data is accompanied by a corresponding trigger signal, commonly referred to as a DQS signal. The DQS signal is commonly used by a host system, which may include an Application Specific Integrated Circuit (ASIC) coupled to the DDR SDRAM through an interface unit such as the Memory I/O, to sample and latch the read-data. The DQS signal does not always assume a distinctly defined low state or high state, considered valid states by the ASIC, during operations that are affected by the DQS signal, but frequently assumes a high impedance state, or stays at a mid-supply level, considered invalid states by the ASIC during the aforementioned operations. The ASIC uses the DQS signal to capture the read-data. The ASIC also typically needs to ascertain when the DQS signal resides in a valid state.  
           [0005]    In current low-speed designs the ASIC may use internal timers to determine when the DQS signal is valid. Variations in delay due to process, voltage and temperature make this method very difficult to use for high-speed designs. Current solutions for high-speed systems focus on the design of special pad cells that are used in coupling the Interface unit to the DDR SDRAM. These special pad cells contain digital and analog (mixed-signal) circuit elements used in preventing the DQS signal from propagating inside the ASIC when the DQS signal resides in an invalid state. Such pad cells may require extensive design times and often result in larger than desired cell areas. Most of the time they are also slower than standard pad cells, thus degrading the overall performance of the ASIC in such instances. Therefore, there exists a need for a system and method for controlling the propagation of a signal such as the DQS signal, and like signals, which provide a simple and inexpensive solution while tracking all delay variations that the circuit experiences during operation, without requiring the design of special pad cells.  
         SUMMARY OF THE INVENTION  
         [0006]    In one set of embodiments the invention comprises a system and method that provides an improved digital interface for controlling and timing the propagation of digital signals. In one embodiment, the system comprises a printed circuit board. A memory controller, a memory unit, and an interface unit may be configured on the printed circuit board. The memory controller may be coupled to the memory unit through the interface unit. In one embodiment, the interface unit is embedded on the same integrated circuit as the memory controller.  
           [0007]    The interface unit may be configured to receive an input clock signal and a read signal from the memory controller and a trigger signal from the memory unit. The interface unit may further be configured to generate an enable signal based on the read signal and to conditionally propagate the trigger signal based on the enable signal. In one embodiment, the interface unit is further configured to provide an output clock signal to the memory unit based on the input clock signal. The output clock signal may be routed from the interface unit to the memory unit through a first trace route on the printed circuit board.  
           [0008]    In one embodiment the memory unit is configured to receive an encoded version of the read signal and to generate the trigger signal in response to the encoded version of the read signal. The trigger signal may be routed from the memory unit to the interface unit through a second trace route. The interface unit may further be configured to generate the enable signal by performing one or more of the following:  
           [0009]    (a) delaying the read signal by a predetermined latency;  
           [0010]    (b) sending the delayed read signal through an external pathway;  
           [0011]    (c) receiving the delayed read signal from the external pathway; and  
           [0012]    (d) operating on the received delayed read signal to obtain the enable signal.  
           [0013]    In one embodiment the external pathway is on the printed circuit board. The delay of the external pathway matches the delay incurred between the interface unit sending an output clock signal pulse and the interface unit receiving a first trigger signal pulse prior to the interface unit sending a next output clock signal pulse. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The foregoing, as well as other objects, features, and advantages of this invention may be more completely understood by reference to the following detailed description when read together with the accompanying drawings in which:  
         [0015]    [0015]FIG. 1 illustrates a DDR SDRAM system configured on a printed circuit board in accordance with prior art;  
         [0016]    [0016]FIG. 2 illustrates a behavior of a DQS signal generated by a DDR SDRAM unit in accordance with prior art;  
         [0017]    [0017]FIG. 3 illustrates a timing relationship between a read signal, a DQS signal, a delayed version of the DQS signal, and a data bus signal during a DDR SDRAM read operation in accordance with prior art;  
         [0018]    [0018]FIG. 4 illustrates an improved DDR SDRAM system with an improved interface unit, implemented in accordance with one set of embodiments of the present invention;  
         [0019]    [0019]FIG. 5 a  illustrates one embodiment of a DQS signal qualifying circuit that supports read operations for a burst length of four;  
         [0020]    [0020]FIG. 5 b  illustrates one embodiment of a DQS signal qualifying circuit that supports read operations for multiple burst lengths;  
         [0021]    [0021]FIG. 6 illustrates a timing relationship between input and output signals of an embodiment of a DQS qualifying circuit; and  
         [0022]    [0022]FIG. 7 illustrates part of a method for controlling the propagation of a digital signal using delay tracking, as pertaining to an interface unit configured on a printed circuit board. 
     
    
       [0023]    While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. Note, the headings are for organizational purposes only and are not meant to be used to limit or interpret the description or claims. Furthermore, note that the word “may” is used throughout this application in a permissive sense (i.e., having the potential to, being able to), not a mandatory sense (i.e., must).” The term “include”, and derivations thereof, mean “including, but not limited to”. The term “connected” means “directly or indirectly connected”, and the term “coupled” means “directly or indirectly connected”.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    As used herein, a “trigger” signal is defined as a signal that is used to initiate an event or a sequence of events in a digital system. A trigger signal is said to be in a “triggering state” at a time when it initiates a desired event, or sequence of events. A periodic trigger signal may commonly be referred to as a “clock”. In a “synchronous” digital system, generally a clock, commonly referred to as a “system clock”, may be used for initiating most events, or sequences of events. An example of a triggering state may be a rising edge of a clock in a synchronous digital system. When an event, or a sequence of events, is said to be initiated “in response to” receiving a stimulus signal, it may be implied that the event, or the sequence of events, is initiated as a result of a combination of a trigger signal, used in triggering the event or sequence of events, being in a triggering state at a time when the stimulus signal is asserted. In one set of embodiments, the sending of a pulse through an output port may indicate a point in time at which a leading edge of the pulse occurs at the output port, and the receiving of a pulse through an input port may indicate a point in time at which a leading edge of the pulse occurs at the input port.  
         [0025]    A set of flip-flops is said to be connected in a series when an output of a first flip-flop of the series is coupled to an input of a second flip-flop of the series, and an output of the second flip-flop of the series is coupled to an input of a third flip-flip flop of the series, and so forth until a last flip-flop of the series is reached. Any such series may be comprised of at least one flip-flop. A flip-flop may have a “preset” or “set” input which, when activated, presets an output of the flip-flop to a logic high level. Furthermore, a flip-flop may have a “reset” input which, when activated, resets an output of the flip-flop to a logic low level.  
         [0026]    When discussing signals, a “mid-supply level” may refer to a state that represents a level nominally equidistant from a logic high level and logic low level, as illustrated in FIG. 2. A mid-supply level  206  may be set at a halfway distance between logic high  202  and logic low  204 . When a signal is said to be residing in a “high impedance state” its value may be indeterminate, and thus unpredictable. A high impedance state  208  is also shown in FIG. 2. In some embodiments, “encoding” a single signal may mean the generation of a set of one or more signals for the purpose of representing the signal to be encoded, and where the set of signals may be propagated in lieu of the single signal to be encoded. A signal is said to be “qualified” when it meets the validity requirements as set forth by a host system. A host system may be, or may include an integrated circuit such as an ASIC. A signal that may assume a mid-supply level  206  or a high impedance state  208  in a digital host system during operations that are affected by the signal is generally considered not to meet the validity requirements of the digital host system. If a trigger signal routed to a digital host system exhibits the behavior illustrated in FIG. 2 during operations affected by the trigger signal, the trigger signal may be considered as one not meeting the validity requirements set forth by the digital host system. A given timing relationship between signals may be deemed “substantially invariant” with respect to a nominal timing relationship between the signals, insofar as a behavior (pertaining to a system using the signals) implied or predicted by the given timing relationship does not deviate from a behavior implied or predicted by the nominal timing relationship.  
         [0027]    The term “bus” is used to refer to a plurality of signals or conductors that may be used to transfer one or more of various types of information, such as data, addresses, control, or status. “Burst length” is used to refer to a number that indicates how many consecutive data bus transfers may be performed in response to a single bus transfer request sent by a host system. For example, if a burst length is four and a memory read operation is performed, there may be four consecutive sets of data transferred on the data bus in response to a single read signal pulse sent by a host system. The size of a set of data for a single transfer over a data bus is typically commensurate with the size of the data bus. Common burst lengths for a DDR SDRAM may include the values of two, four or eight.  
         [0028]    According to prior art, a read operation for a DDR SDRAM may be performed as follows. A host system, which may be an ASIC that may include a DDR SDRAM controller and an interface unit, sends out a single Read request. The DDR SDRAM receives the Read request through a set of control signals. A predetermined number of clock cycles after having received the Read request, the DDR SDRAM sends a predetermined number of consecutive DQS signal pulses back to the host system. The predetermined number of clock cycles may be referred to as “CAS latency”, where CAS stands for “Column Address Strobe”. The number of consecutive DQS signal pulses may be commensurate with one half the value of a predetermined burst length for the read operation. The DDR SDRAM also sends a predetermined number of sets of data to the host system, where the predetermined number may be commensurate with the value of the predetermined burst length for the read operation. A timing relationship between a Read request (READ)  406 , a DQS signal (DQS_SIG)  402 , the DQS signal delayed by a quarter clock cycle (DDQS_SIG)  408  and data signal (DATA_SIG)  404  for the read operation is shown in FIG. 3, wherein for the purposes of illustration the burst length and the CAS latency have both been assigned a value of two.  
         [0029]    [0029]FIG. 4 illustrates part of a DDR SDRAM system implemented in accordance with one set of embodiments of the present invention. In this set of embodiments a DDR SDRAM Controller (DDRC)  302 , an Interface unit (IU)  304 , and a DDR SDRAM unit (DDRU)  306  are configured on a printed circuit board (PCB)  300 , and DDRC  302  is coupled to DDRU  306  through IU  304 . A Read signal  320  generated by DDRC  302  is connected to IU  304 , and a DQS signal  330  generated by DDRU  306  is also connected to IU  304 . A Memory Data Bus (MDB)  360  carrying Memory Data (MDATA)  336  from DDRU  306  is also coupled to IU  304 . A clock signal (TCLK)  322  provided by DDRC  302  is connected to IU  304 .  
         [0030]    DDRC  302  may be a part of a Host ASIC (HASIC)  350 . IU  304  may also be embedded on HASIC  350 . In one embodiment, IU  304  contains a Read Enable Generate Logic block (REGL)  310 , a Control Signal Encode block (CSE)  370 , a Clock Control Logic block (CCL)  314 , a First-In First-Out buffer (FIFO)  312 , a DQS Signal Qualifying Circuit (DSQC)  316  and a Data Delay Logic (DDL)  318  block. Read signal  320  may be connected to CSE  370  to generate an encoded version of Read signal  320  (ENC_READ)  372 , which may be directly routed to DDRU  306  from IU  304 . Furthermore, Read signal  320  may be connected to and used by REGL  310  to generate a Read Enable Output signal (READEN_OUT)  324 . In one embodiment, TCLK  322  is connected to and used by CCL  314  to generate an output clock signal (TCLK_OUT)  326  that is used as a clock by DDRU  306 . TCLK_OUT  326  is connected to DDRU  306  via a first trace  354  on PCB  300  and DQS  330  is connected to IU  304  via a second trace  352  on PCB  300 . DDL  318  may operate on MDATA  336  resulting in DDL  318  generating a delayed version of MDATA  336  (DMDATA)  338 .  
         [0031]    In one set of embodiments a third trace  356  is laid down on PCB  300  running parallel and near to first trace  354  and a fourth trace  358  is laid down on PCB  300  running parallel and near to second trace  352 . In particular, third trace  356  may run from IU  304  to a point in close proximity to DDRU  306  without actually being connected to DDRU  306 . Furthermore, fourth trace  358  may run from a point in close proximity to DDRU  306  without actually being connected to DDRU  306 , to IU  304 . Third trace  356  and fourth trace  358  may be coupled through a capacitive load (CAPL)  332 . CAPL  332  may match a pin loading incurred by TCLK_OUT  326  at DDRU  306 . READEN_OUT  324  leaves IU  304  via third trace  356  and a Read Enable Input signal (READEN_IN)  328  is returned to IU  304  via fourth trace  358 . Any delay experienced by TCLK_OUT  326  traveling on first trace  354  from IU  304  to DDRU  306  is substantially matched by a delay experienced by READEN_OUT  324  traveling on third trace  356  from IU  304  to CAPL  332 . Similarly, any delay experienced by DQS  330  traveling on second trace  352  from DDRU  306  to IU  304  is substantially matched by a delay experienced by READEN_IN  328  traveling on fourth trace  358  from CAPL  332  to IU  304 . The matching delays provide for a timing relationship between READEN_IN  328  and DQS  330  to remain substantially unchanged, regardless of variations in delay due to process, voltage and temperature. The timing relationship is shown in FIG. 6, illustrated by a READEN_IN  328  pulse and a first of one or more DQS  330  pulses. As shown in FIG. 6, a READEN_IN  328  pulse may precede a first of one or more DQS  330  pulses by one-half of a clock cycle, wherein the clock cycle is a cycle of TCLK_OUT  326 .  
         [0032]    DSQC  316  may use READEN_IN  328  and DQS  330  to generate a qualified DQS signal (QDQS)  334 . In one embodiment, FIFO  312  uses QDQS  334  to sample and latch DMDATA  338 . As a result of DDL  318  having operated on MDATA  336 , a timing relationship between DMDATA  338  and QDQS  334  closely matches the nominal timing relationship illustrated in FIG. 3 between DATA_SIG  404  and DDQS_SIG  408 , respectively. The nominal timing relationship may be required in order to successfully latch DMDATA  338 . FIFO  312  may be coupled to DDRC  302  via a Read-Data Valid (RDV)  340  signal and a Read-Data Bus (RDB)  344  carrying Read-Data (RDATA)  342  to DDRC  302 . DDRC  302  may use RDV  340  to qualify RDATA for HASIC  350 .  
         [0033]    One implementation of DSQC  316  for supporting read operations with a burst length of four is illustrated in FIG. 5 a . In this implementation, READ_EN  328  is used for generating an enable signal (ENB)  522  that is coupled to one input of a NAND gate (NAND)  504  with DQS  330  coupled to another input of NAND  504 , where the output of NAND  504  is QDQS  334 . A two-stage shift register is implemented by connecting two rising-edge triggered flip-flops (DFF)  506  and DFF  508  in a series, with an input of DFF  506  tied to a logic low level and READEN_IN  328  coupled to an active low preset input of DFF  506  and an active low preset input of DFF  508  through an inverter (UV)  510 , where READEN_IN  328  is connected to the input of INV  510  and the output of INV  510  is connected to the active low preset input of DFF  506  and to the active low preset input of DFF  508 . An output of DFF  508  is connected to an input of an OR gate (OR)  502  and READEN_IN  328  is connected to another input of OR  502 , where the output of OR  502  is ENB  522 . DQS  330  is connected to the input of an inverter (INV)  512  with the output of INV  512  connected to a clock input of DFF  506  and to a clock input of DFF  508 , resulting in both DFF  506  and DFF  508  being triggered by a trailing edge of a DQS  330  pulse.  
         [0034]    A timing relationship between DQS  330 , READEN_IN  328  and ENB  522  is shown in FIG. 6. As illustrated, a READEN_IN  328  pulse may arrive a half clock cycle before a first DQS  330  pulse. ENB  522  may remain asserted for the duration of all DQS  330  pulses. ENB  522  may not be asserted when DQS  330  resides in an invalid state. As a result, QDQS  334  may not reside in an invalid state and may otherwise mirror DQS  330 .  
         [0035]    More generally, for supporting a burst length of value 2*N, where N is a positive integer, N flip-flops may be connected in a series, creating an N-stage shift register. Furthermore, in support of active high preset inputs, IV  510  may be omitted and READEN_IN  328  may be directly connected to an active high preset input of each flip-flop of the series. FIG. 5 b  illustrates one implementation of DSQC  316  that supports any burst length of value 2*N less than or equal to 2*M, where N and M are both positive integers. In this implementation, M flip-flops, (FF)  562 , . . . FF  564 , FF  566 , are connected in a series, creating an M-stage shift register, where FF  566  is the last flip-flop of the series, and a preload circuit  570  is configured and coupled to a preset input and a reset input of each flip-flop of the series, where READEN_IN  528  is coupled to an input of the preload circuit. Upon receiving a READEN_IN  528  pulse, the preload circuit may preset the last N flip-flops of the series, and reset the remaining flip-flops of the series. Similarly to the implementation shown in FIG. 5 a , DQS  530  is coupled to a clock input of each flip-flop through an inverter (INV)  550 , where the DQS  530  is connected to the input of INV  550  and the output of INV  550  is connected to the clock input of each flip-flop. READEN_IN  528  is coupled to an input of an OR gate (OR)  552  and an output of FF  566  is coupled to another input of OR  552 , where the output of OR  552  is a resulting enable signal ENB  580  that is coupled to an input of a NAND gate (NAND)  554 . DQS  530  is connected to another input of NAND  554  producing a qualified DQS signal (QDQS)  534  as the output of NAND  554 . A timing relationship between DQS  530 , READEN_IN  528  and ENB  580  substantially matches the timing relationship between DQS  330 , READEN_IN  328  and ENB  522 , respectively, as illustrated in FIG. 6.  
         [0036]    A flowchart of a method for controlling the propagation of a digital signal using delay tracking, as pertaining to an interface unit, is shown in FIG. 7. The method is discussed for a simple system comprised of a controller coupled to a controlled unit through the interface unit, the system configured on a printed circuit board. The method comprises two sets of events ( 700  and  750 ), which may take place concurrently.  
         [0037]    The set of events  750  describes the providing of an output clock by the interface unit, based on an input clock received by the interface unit. The interface unit receives the input clock ( 720 ) and generates the output clock by operating on the input clock ( 722 ). The output clock is then provided as an output of the interface unit ( 724 ). In one embodiment, generation of the output clock  722  may comprise operations that result in the output clock being a synchronized version of the input clock, such that the output clock and a set of output signals provided by the interface unit are all synchronized. The synchronization may be achieved using a separate clock signal that may have twice the frequency of the input clock and may also be an input to the interface unit. The input clock may be provided to the interface unit by the controller.  
         [0038]    The set of events  700  describes the generating of an enable signal that may be used to qualify a trigger signal, and thus control its propagation. The interface unit receives a request signal ( 702 ) and generates a first control signal by operating on the request signal ( 706 ). In one embodiment the request signal is provided to the interface unit by the controller. The first control signal is sent to an external pathway ( 708 ) through what may be a first control signal output port of the interface unit. The external pathway may be routed from the output port to what may be a first control signal input port of the interface unit, where the first control signal is received by the interface unit ( 710 ), after the first control signal has traversed the external pathway. The interface unit also receives a second control signal ( 712 ). A delay may be incurred by the first control signal while traversing the external pathway from the output port to the input port. The delay may match a time period that is defined as starting with a sending of a pulse of the output clock by the interface unit, and ending with a receiving of a pulse of the second control signal by the interface unit, where the pulse of the second control signal is received prior to the interface unit sending a next pulse of the output clock.  
         [0039]    In  708 , a pulse of the first control signal is sent substantially coincident with either a sending of a pulse of the output clock, or a point in time that occurs one-half cycle of the output clock following a sending of a pulse of the output clock. In one embodiment, the second control signal is generated by the controlled unit, which sends a series of one or more pulses of the second control signal in response to the controlled unit having received either a pulse of the request signal or a pulse of an encoded version of the request signal, where the series of one or more pulses reach the interface unit in  712 . In this embodiment, a pulse of the first control signal in  710  reaches the interface unit one-half cycle of the output clock before a first pulse of the series of one or more pulses of the second control signal reaches the interface unit.  
         [0040]    The interface unit uses the second control signal to generate an enable signal from the first control signal ( 714 ), and uses the enable signal to conditionally propagate the second control signal ( 716 ).  
         [0041]    Although the embodiments above have been described in considerable detail, other versions are possible. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications. Note the section headings used herein are for organizational purposes only and are not meant to limit the description provided herein or the claims attached hereto.