PATENT ABSTRACT
In a synchronized memory system comprising a memory controller externally coupled to a synchronous memory, a read valid loop back signal is introduced for the memory controller to track the delays of signals exchanged between the memory controller and the synchronous memory, so that the uncertainty introduced by I/O pads and PCB traces used to facilitate the coupling of the memory controller with the sychronous memory is no longer the limiting factor for the speed of the memory controller. An asynchronous FIFO buffer is used to latch read data returned by the synchronous memory based on the read valid loop back signal.

PATENT DESCRIPTION
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to synchronized memory technologies, and particularly to memory controllers for synchronous memories. 
     2. Background of the Invention 
     As synchronized memory technology progresses, there is an increasing need for developing synchronous memory controllers that can support the high clock speeds required as the state of the art for memory devices advances. Due to the improvements in processing technologies used to fabricate memory controllers, memory controller logic can be designed to run at such high clock rates. However, since typically a memory controller is externally coupled to a synchronous memory, signals exchanged between the memory controller and the synchronous memory may be delayed due to input/output (“I/O”) pads and printed circuit board (“PCB”) traces which facilitate the coupling between the memory controller and the synchronous memory. 
     The problem is more serious with higher clock frequencies or lower clock cycles. For example, the internal clock cycle of a high-speed memory has gone down to about 5 nanoseconds, but the delay of a signal due to impedance associated with an I/O pad and a PCB trace can be as long as about 10 nanoseconds, which is two times the value of the clock cycle. Delays like this may cause the memory controller to be out of sync with the synchronous memory. When synchronization is lost, wrong data will be latched by the memory controller in an attempt to read from the synchronous memory. 
     Since the delays are caused by impedance associated with the I/O pads and PCB traces, the I/O pads and PCB traces associated with a synchronized memory system must be designed and made carefully to meet the timing requirements. However, this goal is difficult to meet consistently because the I/O pads and PCB traces transmitting the signals have impedance characteristics that vary depending on the fabrication process, the voltage of the clock signal, and the operating temperature of the memory controller. These variations introduce clock uncertainty, thus reducing the actual memory clock frequency that the memory controller can use. Clock uncertainty, in turn, makes it difficult for such memory controllers to operate consistently and/or properly at high clock rates in the real world with synchronous memory. 
     Therefore, there is a need for a high-speed memory controller that can operate at its highest selected internal frequency or clock frequency when coupled to a synchronous memory, and still remain relatively immune from impedance variations caused by variations in fabrication process, voltage of clock signal, and operating temperature. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and system that eliminate or significantly reduce the effect of signal delays caused by I/O pads and PCB traces during the operation of a synchronized memory controller, thus allowing the memory controller to run at the highest internal clock frequency. The present invention advantageously provides a high-speed memory controller that can operate at high clock rates and remain relatively immune from impedance variations caused by fabrication processes, voltage of the clock signal, and/or operating temperature. 
     In one embodiment of the present invention, a memory controller is externally coupled to a synchronous memory. The memory controller generates a master clock signal to control the operation of the memory controller and the synchronous memory. A latch clock signal is generated by routing the master clock signal off-chip and then back to the memory controller through an I/O pad, at least one PCB trace, and another I/O pad. The memory controller also generates a read valid signal. The read valid signal is then routed off-chip and back to the memory controller as a read valid loop back signal. The read valid signal is routed off-chip via an I/O pad, at least one PCB trace, and another I/O pad. The memory controller latches read data from the synchronous memory based on the latch clock signal and the read valid loop back signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a synchronous memory system in accordance with an embodiment of the present invention. 
     FIG. 2 is a timing diagram associated with the memory system as illustrated in FIG.  1 . 
     FIG. 3 is a flowchart diagram illustrating the operation of a memory controller in a synchronous memory system in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In one embodiment of the present invention, a memory controller is externally coupled to a synchronous memory. The synchronous memory returns read data a certain number of clock cycles after receiving a read command from the memory controller. The memory controller includes an asynchronous First-In-First-Out (“FIFO”) buffer, which latches the read data from the synchronous memory in response to two signals, a latch clock signal and a read valid loop back signal. The read valid loop back signal is asserted when read data for a given read command is anticipated. Both the latch clock signal and the read valid loop back signal originate from the memory controller and are routed off-chip in such a manner so that they encounter a similar delay characteristic as the read data from the memory. Therefore, any variations, due to fabrication process, voltage of clock signal and/or operating temperature, that may affect the I/O pads and PCB traces used by the read command and read data also affects the I/O pads and PCB traces used by the off-chip routing of the latch clock signal and read valid loop back signal. This permits the read valid loop back signal, the latch clock signal, and the read data to be synchronized regardless of variations in process, voltage, and/or temperature. Therefore, only valid read data signals are latched into the asynchronous FIFO buffer. 
     FIG. 1 is a block diagram of a system  100  in accordance with one embodiment of the present invention. As shown In FIG. 1, the system  100  comprises a memory controller  101  externally coupled to a synchionous memory  102 , such as, for example, a synchronous dynamic random access memory (“SDRAM”) or a synchronous graphics random access memory (“SGRAM”). The system  100  also includes PCB traces  103 A,  103 B,  103 C,  103 D,  103 E,  103 F. The memory controller  101  is externally coupled to the synchronous memory  102  by, for example, PCB traces  103 A,  103 B, and  103 F. The memory controller  101  includes a phase lock loop (PLL)  110 , a memory control logic  120  coupled to the PLL  110 , a data buffer  121  associated with the memory control logic  120 , an asynchronous first-in-first-out buffer (“Async_FIFO”)  130  coupled to the PLL  110  and to the memory control logic  120 , and six I/O pads  140 A,  140 B,  140 C,  140 D,  140 E, and  140 F. The I/O pad  140 A is connected to the PCB trace  103 A and coupled to the Async_FIFO  130 . The I/O pad  140 B is connected to the PCB trace  103 B and coupled to the memory control logic  120 . The I/O pad  140 F is connected to the PCB trace  103 F and coupled to the PLL  110 . As shown in FIG. 1, the system  100  also comprises a PCB trace  103 E which connects the PCB trace  103 F to the I/O pad  140 E. The system  100  also comprises two PCB traces  103 C and  103 D which are connected with each other and which together connect I/O pad  140 C with I/O pad  140 D. 
     FIG. 2 is a timing diagram associated with the system  100 , in accordance with an embodiment of the present invention. FIG. 2 shows a master memory clock (“MCLK”) signal  210  generated by the PLL  110 , a valid memory clock signal (“VALID_MCLK”)  220  generated by the memory control logic  120 , and a read command (“CTR_MCLK”)  230  also generated by the memory control logic  120 . FIG. 2 also shows a memory clock (“MEM_CLOCK”) signal  211  received by the synchronous memory  102 , a read valid signal (“READ_VALID”)  221  which is at least a delayed version of VALID_MCLK  220  after passing the I/O pad  140 C and the PCB trace  103 C, and a read command signal (MEM_CTRL)  231  as received by the synchronous memory  102 . FIG. 2 also shows a memory data signal (“MEM_DATA”)  240  returned by the synchronous memory  102  in response to receiving the MEM_CTRL  231 . FIG. 2 further shows a return clock signal (“RCLK”)  212  which is the loop back of MEM_CLOCK  211 , a read valid loop back signal (“READ_VALID_I”)  222  which is a loop back of READ_VALID  221 , and a read data signal (“RDAT”)  241  received by the Async_FIFO  130 . The MCLK signal  210  includes cycles C 1 , C 2 , C 3 , C 4 , and C 5 , as shown in FIG.  2 . The MEM_CLOCK signal  211  includes cycles  C 1   ,  C 2   ,  C 3   ,  C 4   , and  C 5   , as shown in FIG.  2 . The RCLK signal  212  includes cycles C 1 , C 2 , C 3 , C 4 , and C 5 , as shown in FIG.  2 . 
     Now referring to both FIG.  1  and FIG. 2, the function of the PLL  110  is to generate MCLK  210 , and send this signal to the memory control logic  120 , the Async_FIFO  130 . All of the communications between the memory control logic  120  and the Async_FIFO  130  are with reference to MCLK  210 . The PLL  110  also sends MCLK  210  to the synchronous memory  102  for the purpose of synchronizing the communications between the memory controller  101  and the synchronous memory  102 . However, since the synchronous memory  102  is externally coupled to the PLL  110  through the PCB trace  103 F and the I/O pad  140 F, MCLK  210  becomes MEM_CLOCK  211  when it arrives at the synchronous memory  102 . MEM_CLOCK  211  is at least a delayed version of MCLK  210  due to the impedance associated with the I/O pad  140 F and the PCB trace  103 F. This is illustrated in FIG. 2 where MEM_CLOCK  211  is shown to be delayed from MCLK  210  by a period of time T 1 . This delay period of time T 1  is unpredictable because it depends on the geometric features of the I/O pad  140 F and the PCB trace  103 F, which are different from system to system due to variations in fabrication processes. T 1  is also dependent on the voltage of the clock signal MCLK  211 , which varies because of instabilities in any power source used by the memory system  100 . T 1  is also dependent on the operating temperature of the memory system, which varies depending on, for example, the environment in which the memory system  100  is being operated. 
     Part of the functions of the memory control logic  120  is to issue read commands per requests from a user (not shown) of the system  100 . Still referring to FIG.  1  and FIG. 2, the memory control logic  120  issues a read command CTRL_MCLK  230  at cycle C 1  of MCLK  210 . The read command CTRL_MCLK  230  needs to go through the I/O pad  140 B and the PCB trace  103 B in order to reach the synchronous memory  102 , and there, it becomes signal MEM_CTRL  231 . MEM_CTRL  231  is delayed from CTRL_MCLK  230  due to impedance associated with the I/O pad  140 B and the PCB trace  103 B. Since the impedance associated with the I/O pad  140 B and the PCB trace  103 B, and that associated with the I/O pad  140 F and the PCB trace  103 F, are subject to the same variations in fabrication processes, voltage of clock signal and operating temperature, the I/O pads  140 B and the PCB traces  103 B can be designed in reference to the design of the I/O pad  140 F and the PCB trace  103 F so that MEM_CTRL  231  is delayed from CTRL_MCLK by the same time period T 1  as MEM_CLOCK  211  is delayed from MCLK  210 . Therefore the MEM_CTRL  231  is received by the synchronous memory  102  at cycle  C 1    of MEM_CLOCK  211 . 
     Still referring to FIG.  1  and FIG. 2, in one embodiment of the present invention, in response to receiving the MEM_CTRL  231  at cycle  C 1    of MEM_CLOCK  211 , the synchronous memory  102  returns read data (“MEM_DATA”)  240  at cycle  C 3    of MEM_CLOCK  211 . Since the read data has to go through the PCB trace  103 A and the I/O pad  140 A in order to reach the Async_FIFO  130 , the read data RDAT  241  received by the Async_FIFO  130  is delayed from MEM_DATA  240  by a period of time T 2 , due to the impedance associated with the PCB trace  103 A and the I/O pad  140 A. 
     A loop back of the MEM_CLOCK  211 , the RCLK  212  is used as a latch clock signal by the Async_FIFO  130  to latch read data. The RCLK  212  is created by looping back the MEM_CLOCK  211  through the PCB trace  103 E and the I/O pad  140 E. This is intended so that, due to the impedance associated with the PCB trace  103 E and the I/O pad  140 E, the RCLK signal  212  is delayed from MEM_CLOCK  211  just as RDAT  241  is delayed from MEM_DAT  240 . Since the impedance associated with the I/O pad  140 E and the PCB trace  103 E, and that associated with the I/O pad  140 A and the PCB trace  103 A, are subject to the same variations in fabrication processes, voltage of clock signal and operating temperature, the I/O pad  140 E and the PCB trace  103 E can be designed in reference to the design of the I/O pad  140 A and the PCB traces  103 A so that RDAT  241  and RCLK  212  are synchronized. 
     In anticipation of receiving the read data, the memory control logic  120  also issues a read valid signal VALID_MCLK  220  for each read command. The read valid signal is intended for the Async_FIFO  130  to use as a write enable when latching the read data from the synchronous memory  102 . In order to synchronize the read valid signal with the read data RDAT  241  and the latch clock signal RCLK  212 , the read valid signal is routed off-chip and looped back to the memory controller  101 , through the I/O pad  140 C, the PCB trace  103 C, the PCB trace  103 D and the I/O pad  140 D. The read valid loop back signal READ_VALID_I  222  is then used as a write enable signal for the Async_FIFO  130  to latch read data from the synchronous memory  102 . Since any variations due to fabrication processes, the voltage of the clock signal or the operation temperature that may affect the I/O pads and PCB traces used by RCLK  212  also affects the I/O pads and PCB traces used by READ_VALID_I  222 , the I/O pad  140 C, the PCB trace  103 C, the PCB trace  103 D and the I/O pad  140 D can be designed so as to create a delay characteristic for the READ_VALID_I  222  that is similar to that of RCLK  212 , i.e., the READ_VALID_I  222  is delayed from VALID_MCLK  220  by a same time period as RCLK  212  is delayed from MCLK  210 , regardless of the aforementioned variations. In one embodiment of the present invention, the I/O pad  140 C, the PCB traces  103 C and  103 D, and the I/O pad  140 B are designed in reference to the design of the I/O pad  140 F, the PCB traces  103 F and  103 E, and the I/O pad  140 E, so that the READ_VALID signal  221 , is delayed by a period of time T 1  from the VALID_MCLK  220 , and the loop back of READ_VALID  221 , the READ_VALID_I signal  222 , is delayed from READ_VALID  221  by a period of time T 2  due to the impedance associated with the PCB trace  103 D and the I/O pad  140 D, as shown in FIG.  2 . Similarly, as recited above, MEM_CLOCK  211  is delayed from MCLK  210  by a period of T 1  due to impedance associated with the I/O pad  140 F and the PCB trace  103 F, and RCLK  212  is delayed from MEM_CLOCK  211  by a period of T 2  due to impedance associated with the PCB trace  103 E and the I/O pad  140 E. 
     In one embodiment of the present invention, the Async_FIFO  130  is a conventional asynchronous first-in-first-out buffer comprising two ports for communicating with two different agents with two different clocks. The memory control logic  120  as one agent communicates with one port of the Async_FIFO  130  using the MCLK signal  210 . The synchronous memory  102  as another agent communicates with another port of the Async_FIFO  130  using the RCLK signal. This port of the Async_FIFO  130  that communicates with the synchronous memory  102  is designed to latch read data RDAT  241  at the rising edge of RCLK  212  when the read valid loop back signal READ_VALID_I  222  is asserted. Since the I/O pads and the PCB traces used to route these signals are now subject to the same variations in fabrication processes, the voltage of the clock signal, and the operating temperature, the latch clock signal RCLK  212 , the write enable READ_VALID_I  222 , and the read data RDAT  241  are synchronized regardless of aforementioned variations. Therefore this embodiment of the present invention permits the right read data to be captured by the Async_FIFO  130  regardless of the variations due to fabrication processes, voltage of clock signal and operating temperature. 
     Once the read data RDAT is captured by the Async_FIFO  130 , it is stored in the Async_FIFO  130  and then sent to the data buffer  121  associated with the memory control logic  120  on a first-in-first-out basis and under the control of MCLK  210 . In an alternative embodiment, the data buffer  121  is not used, and the read data is provided to the user of the system  100  from the Async_FIFO  130  by the memory control logic  120 . 
     FIG. 3 is a flowchart diagram illustrating a read operation of the memory controller  101  in response to receiving a user request  310  to read data from the synchronous memory  102  in the memory system  100  in accordance with one embodiment of the present invention. The memory controller  101  issues  320  a read command CTRL_MCLK  230  corresponding to the user request and the read command is sent to the synchronous memory  102 . In anticipation of receiving read data returned by the synchronous memory  102 , the memory controller  101  issues  330  a write enable signal VALID_MCLK  220 . The write enable signal VALID_MCLK  220  is routed  340  off-chip and back to the memory controller  101 . The memory controller  101  then checks  350  if the loop back of the write enable signal, READ_VALID_I  222 , is asserted. If it is asserted, the memory controller  101  latches  360  the read data from the synchronous memory  102  at the rising edge of the latch clock signal RCLK  212 . The read data is then provided  350  to the requesting user. 
     While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and components disclosed herein and that various modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the method and apparatus of the present invention disclosed herein without departing from the spirit and scope of the invention as defined in the appended claims.