Abstract:
A selectable function is provided that permits the impedance of an output driver or an addressable memory device to be configured without adding extra signal connections. The output driver impedance control function of the invention is achieved through the use of the data bus of a memory array for control. The data lines thus serve two purposes one for normal use and the other for control of the impedance. In the invention, the output impedance of each DRAM in a subassembly array that drives a common data bus is individually separately adjusted.

Description:
[0001]     This Application is a Continuation in Part Application of parent application, Ser. No. 10/072,346 Filed Feb. 6, 20002. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to the providing of a control function for output drivers of addressable random access memory devices that provides a capability for changing the impedance of an addressable memory output drive circuit without disturbing the contents of the memory device.  
       BACKGROUND  
       [0003]     In the art, dynamic random access memory arrays, DRAMS, are assembled in arrangements of devices involving data lines and control lines. However, in many constructions both types of lines do not have to connect to each device. As the art is progressing a need is arising for an ability to be able to adjust the output impedance of entire arrays and such a capability would lead to having each device being separately adjustable which in turn would require that the control through each type of line would be desirable at each device.  
       SUMMARY OF THE INVENTION  
       [0004]     The invention is a selectable function that permits the impedance of an output driver or an addressable memory device to be configured without adding extra signal connections. The output driver impedance control function of the invention is achieved through the use of the data bus of a memory array for control. The data lines thus serve two purposes, one for normal use and the other for control of the impedance. In the invention, the output impedance of each DRAM in a subassembly array that drives a common data bus is individually separately adjusted. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  is a perspective illustration of the arrangement of standard in the art Synchronous Dynamic Double Data Rate (SDRAM-DDR) memory array in a typical in the art computer system.  
         [0006]      FIGS. 2-5  illustrate the application of the principles of the invention to the control of the impedance of a standard component of SDRAM assemblies, the“off chip”driver(OCD), wherein:  
         [0007]      FIG. 2  illustrates a typical DRAM data path through the assembly for writing data.  
         [0008]      FIG. 3  illustrates one arrangement of adjustment additions to a typical writing data path such as is shown in  FIG. 2  in order to implement the principles of the invention.  
         [0009]      FIG. 4  is a timing chart illustrating the effect of the adjustment additions of  FIG. 3 .  
         [0010]      FIG. 5  illustrates an alternate arrangement of adjustment additions to a typical data path such as is shown in  FIG. 2  in implementing the principles of the invention.  
     
    
     DESCRIPTION OF THE INVENTION  
       [0011]     As the performance of Dynamic Random Access Memories (DRAMS) in data processing systems moves to ever higher frequencies, precise control of the data input and output in the memory system assembly becomes crucial to ensure that there is reliable transfer into and out of each individual one of the assembly of DRAMS that make up the memory assembly. Included in that precise control is the ability to adjust the impedance of the drivers that move the data in the array. The drivers are separate units, known in the art as “off chip” drivers (OCD)s and receivers (OCR)s.  
         [0012]     To calibrate the drive strength and impedance of an OCD, DC current measurements can be taken while the OCD is driving a known logical state load and the impedance is adjusted until the required I-V characteristic is obtained. To accomplish such an operation however, the memory controller must be able to establish a desired logical state for the OCD load and then communicate adjustment instructions to the DRAM. The situation is illustrated in connection with  FIGS. 2-7  which illustrate the application of the principles of the invention to the control of the impedance of an OCD wherein in  FIG. 2  there is illustrated a typical DRAM data path for describing the essential operations in the writing of data. The particular DRAM arrangement in  FIGS. 1 and 2  have an example four independent data array banks with the read/write data bus communication channel for the data labelled RWD. The data on the RWD is multiplexed into the arrays.  
         [0013]     In  FIG. 2  during a write command, the RW switch places the DRAM in a state to receive and store data. Data is input to the DRAM through the DQ Off-Chip receivers at a location labelled (OCR&#39;s and DQ&#39;s SYNC) and may be synchronized through a data strobe labelled DQS. In the event that the particular architecture is of the type known in the art as the “prefetch” type where several bits of serial data are latched in parallel on consecutive clock cycles, such data may be reordered if necessary in a multiplexer labelled (WRITE MUX). In either case the data is then driven into a bidirectional bus labelled (RWD) and is finally stored in the memory array under circuitry standard in the art for column control and for column decoding, not shown.  
         [0014]     In  FIG. 3  the invention is shown through a depiction of the write data path of a DRAM that contains the features of, and operates essentially the same, as the write data path illustrated in connection with  FIG. 2 . In  FIG. 3  in addition there is illustrated within the dotted bordered section, the features used in providing calibration and control of the impedance of off-chip drivers.  
         [0015]     Referring to  FIG. 3 , in the invention an additional control signal is provided, labelled ADJUST. The ADJUST control signal or command is generated by the DRAM control circuits in response to a mode register set command from the memory controller. During the time when the ADJUST command is active, the RWD bus is disconnected from the data array banks and the write command to the column is suppressed. In other words any data in the memory array will remain undisturbed because the memory array is disabled from accepting and storing data.  
         [0016]     Therefore, with the ADJUST command active, data can be written onto the RWD bus as with a normal write command, but the data cannot be stored in the memory array. It will be apparent that if at the time it is desired to provide impedance calibration, there is no data in the memory array that it is desirable to remain undisturbed, then any arrangements to inhibit storage would not be required.  
         [0017]     In accordance with the invention, under the ADJUST command, there is enablement of added control circuitry, labelled OCD IMPEDANCE CONTROL, that can receive programming instructions from any data on the RWD bus. The OCD IMPEDANCE CONTROL is clocked using the command control signal together with a delayed version of that signal. The OCD IMPEDANCE CONTROL element performs the functions of interpretation of the programming instructions and the generation of vector signals which drive the OCDs and set them to the desired pull up and pull down levels.  
         [0018]     An example set of control settings are tabulated in Table 1.  
                                             TABLE 1                           DQ inputs               DQ &lt;2&gt;   DQ &lt;1&gt;   DQ &lt;0&gt;   Command                                X   0   0   Do Nothing       0   0   1   Increase pulldown impedance       0   1   0   Decrease pulldown impedance       0   1   1   Reset pulldown to default impedance       1   0   1   Increase pullup impedance       1   1   0   Decrease pullup impedance       1   1   1   Reset pullup to default impedance                  
 
         [0019]     In accordance with the invention, with the ADJUST signal activated, a normal write command becomes useable to program the OCD impedance through data received on the DQ inputs.  
         [0020]     Referring to  FIG. 4 a  timing chart is provided that shows example timings for the impedance calibration operation. In the chart of  FIG. 4  the write command signal is labelled PCAS and the column command is labelled CCAS. In the timing chart of  FIG. 4  the standard in the art write data burst architecture of four bits. Only the first bit of a burst from a subset of n DQs is used for programming information. Alternatively, consecutive bits in a burst could contain programming information. Further alternatively, the 2nd, 3rd or 4th bit of a burst could contain the programming information. In the event that just one bit of a burst contains the programming information, the command should send some value for the full burst.  
         [0021]     An example protocol for achieving the impedance adjustment as described in connection with  FIG. 3  would be as follows. 
        The extended mode register set (EXTMRS) activates the ADJUST mode.     The ADJUST mode signal places the RWDMUX in a high impedance mode and disables the write command to the column.     The ADJUST mode signal prepares the OCD impedance control circuit to receive adjustment instructions.     A single write command captures the DQs and drives them onto the RWDs.     The first bit of the burst on DQ &lt;0:n&gt;contains the impedance adjustment command. An example command tabulation is shown in TABLE 1.        
 
         [0027]     An alternate option is to write impedance vectors directly to each OCD circuit utilizing the RWD bus to transfer the data to all OCDs and storing the value in a latch at each OCD. This would require that the clocking and mode signals PCAS and ADJUST be distributed to each OCD circuit. Since the existing RWD bus is available to transfer the data to all OCDs the vector bus from the OCD impedance would no longer be needed, thereby saving wiring space.  
         [0028]     In  FIG. 5  there is illustrated an alternate arrangement of adjustment additions to one DQ circuit in a typical data path such as is shown in  FIG. 2  in implementing the principles of the invention. Referring to  FIG. 5 , the arrangement does not involve the RWD bus at all and further allows each OCD to be programmed independently. Two programming mode signals are involved, one labelled ADJUST-PU for adjusting the OCD pullup and another labelled ADJUST-PD for adjusting OCD pulldown. Each can be activated at different times by, a standard in the art, mode register set command. When either mode is active, the write operation to the array is suppressed.  
         [0029]     During a write command to the DRAM, the serial data is received by the off-chip receiver labeled OCR at each DQ and is stored in parallel at an element labeled DQ WRITE LATCH. The serial burst length would be four bits. For comparison, in a usual write command the data would be written in parallel over the RWD bus and stored in the memory array however, in this situation the ADJUST-PU or ADJUST-PD mode prevents it. Instead, in this situation, the parallel data is stored directly into the latches located near the OCD. This data contains the value as illustrated through TABLE 1 of the desired impedance for either the pullup or pulldown which is then decoded in selecting the desired OCD impedance.  
         [0030]     Therefore with one of the ADJUST-PU or ADJUST-PD signals activated a normal write command can be used to program the OCD impedance with the impedance values provided in a serial burst fashion over the DQ inputs. It should be noted that each OCD receives the impedance values from a unique DQ so that independent programming of different OCDs is enabled. It should also be further noted that there is thus no restriction to a four bit burst length.  
         [0031]     An example protocol for achieving the impedance adjustment as described in connection with  FIG. 5  would be as follows. 
        The extended mode register set (EXTMRS) activates the ADJUST-PU or ADJUST PD mode.     The ADJUST-PU or ADJUST-PD mode signal places the RWDMUX in a high impedance mode and disables the write command to the column     A four bit burst is written to each DQ write latch as in a normal write command.     During a DQS to WRTCLK synchronization the four bit burst is transferred to the pullup or pulldown impedance latch and decoder.     The extended mode register set deactivates the ADJUST-PU or ADJUST-PD mode signal.     The memory controller performs the impedance measurement.     The procedure is repeated until the measurement is complete.        
 
         [0039]     What has been described is a function for setting the strength or impedance of output data driver circuits in an addressable memory system. The function can be realized without the addition of external data, addresses, or control signals to the memory device.