Abstract:
A memory reading apparatus transfers digital data from a memory array that is independent of external clocking rate, where the data transmission time is not limited by the external clock period, and the internal timing of controls permits flexible column selection and no conflicts in the timing between external clock signals and internal bit line sensing ready signal. The memory read apparatus has a data read path circuit and a memory read control apparatus. The data read path circuit is in communication with the memory to acquire the selected data read from the memory, synchronize the selected data, and transfer the selected data from the memory. The memory read control apparatus is in communications with the data read path circuit for selecting the data to be read from the memory, for providing self-feedback signals for synchronizing the selected data for transfer from the memory.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to an electronic memory devices. More particularly, this invention relates to circuits for the extraction or reading of digital data from an electronic memory. 
   2. Description of Related Art 
   In present electronic memory devices an address is decoded into row addresses and column addresses. The row addresses activates word lines of a row within an array of memory cells. All the memory cells of the row are activated and the digital data is transferred through bit line connection to sense amplifiers for recovery. The column address are used to activate bit line switches for selecting which column is to transfer its recovered data to a data line sense amplifier for further conditioning and amplification. The output of the data line sense amplifier is applied to a data line latch for synchronization with an external clock. The output of the data line latch is transferred to a data output latch for transfer through an output driver circuit to external circuitry. 
   This read path is structured as a three layer pipeline. The first stage is from the word line access to the bit line switch selection. The second stage is the data line sense amplifier to the data line sense amplifier latch and the third stage is the data output latch. 
   The latency of an access of data is determined by the time from the presentation of an address to the presence of the data at the output of the driver circuit. The structure of the second pipeline stage allows for shortening of the stage to improve the data access. If the pipeline stages (especially the second pipeline stage) are not shortened then the minimum latency is determined by the long cycle applications where data for different word lines are accessed sequentially. However, if the pipeline stages are shortened then the data transmission time form the bit line sense to the data output latch at the third pipe line stage is limited by the maximum external clock rate. 
   Refer now to  FIG. 1  for a more detailed discussion of a read data path of the prior art. Memory cells  5  are arranged in rows and columns to form the sub-arrays  10   a , . . . ,  10   n . An address is decoded to form the word line addresses  15  and the bit line addresses  45  for selecting the desired rows and columns of the memory sub-array  10   a , . . . ,  10   n . Each of the memory cells  5  of a selected word line  15  is activated and the digital data is transferred to the bit lines (BL 00 ,  BL 00   , . . . , BLmn,  BLmn ). The bit line sense amplifiers  22   a , . . . ,  22   n  acquire, amplify, and condition the digital data. The bit line switches  32   a , . . . ,  32   n  are connected to the terminal ends of the bit lines BL 00 ,  BL 00   , . . . , BLmn,  BLmn  to receive the digital data from the bit line sense amplifiers  22   a , . . . ,  22   n . Each of the bit line switches  32   a , . . . ,  32   n  are formed of a pair of metal oxide semiconductor (MOS) transistors (M 1  and M 2 ). The gates of the MOS transistors (M 1  and M 2 ) are connected to receive the bit line selection signals BS  55  from the column decoder  50 . The column decoder is connected to the column decode control circuit  40 , which receives a bit line sense amplifier ready signal  35  indicating the digital data present on the bit lines BL 00 ,  BL 00   , . . . , BLmn,  BLmn  has been sense, amplified, and conditioned for transfer from the memory array  25 . The column address  45  is decoded and one of the desired bit line switch  32   a , . . . ,  32   n  is activated to transfer the digital data from the memory array  25  through the bit line switches  32   a , . . . ,  32   n  to the data line sense amplifier  60 . The data line sense amplifier  60  further amplifies and conditions the digital data. 
   The output of the data line sense amplifier  60  is connected to the input of the data line sense amplifier latch  65 . The data lines sense amplifier  65  is a data storage element used to synchronize the digital data with an external clock for transfer to external circuitry. 
   The output of the data line sense amplifier  60  is transferred to the input of the data output latch  70 . The data output latch  70  is a second data storage element used to retain the digital data during transfer of the digital data through an off chip driver  75  to a data output terminal DQ  80  and to external circuitry. 
   The bit line switches  32   a , . . . ,  32   n  form the boundary  30  of the first pipeline stage. The data line sense amplifier latch  65  forms the boundary of the second pipeline stage and the data output latch forms the boundary of the third pipeline stage. As noted above, the second pipeline stage can be shortened to minimize the latency of the first access of the digital data from the memory. Thus the performance of the memory system is limited by this first access. If the pipeline transmission time is reduced, then the performance of the memory system is determined by the maximum clock frequency that determines the minimum transmission time from the bit line sense amplifiers  22   a , . . . ,  22   n  to the output terminal DQ  80 . 
   “A 9 Ns 16 Mb CMOS SRAM with Offset Reduced Current Sense Amplifier.” Seno, et al., Digest of Technical Papers: 40th ISSCC IEEE International Solid-State Circuits Conference, 1993, pp.: 248-249, 297 describes a 4-Mb×4 SRAM (static random access memory) with a current-mode nonequalized read data path. The read data path has an offset-reduced stabilized-feedback current sense amplifier and a quadrant-organization architecture. 
   U.S. Pat. No. 5,959,900 (Matsubara) illustrates a synchronous semiconductor memory having a register with an input gate and an output gate, for holding read-out data between the input gate and the output gate. An input gate control circuit controls an open/close of the input gate with a output switch feedback signal in the form of a one-shot pulse generated by an output gate control circuit for controlling an open/close of the output gate. The open/close, in synchronism with an output gate switch signal, so that only after the data held in the register has been transferred to an external of the register, the next data to be successively transferred from the read/write bus to the register is actually latched in the register. 
   U.S. Pat. No. 6,452,865 (Wolford) provides a single common symmetrical double data rate (DDR) synchronous random access memory (SDRAM) read data path structure and corresponding storage addressing scheme. The read data path structure implements both an N-bit interface and an (N/2)-bit interface to the DDR memory. The read data path structure uses a feedback loop of a lower data path to a higher data path in conjunction with the translation of the physical addressing of the data stored into a memory. The feedback loop and address translation mechanism is enabled for (N/2)-bit mode and disabled for N-bit mode. 
   U.S. Pat. No. 6,539,454 (Mes) describes an asynchronously pipelined SDRAM. The asynchronously pipelined SDRAM has separate pipeline stages that are controlled by asynchronous signals to synchronize data at each stage, an asynchronous signal is used to latch data at every stage. The asynchronous control signals are generated within the chip and are optimized to the different latency stages. The data is synchronized to the clock at the end of the read data path before being read out of the chip. 
   SUMMARY OF THE INVENTION 
   An object of this invention is to provide a data reading apparatus for transferring digital data from a memory array that is independent of external clocking rate. 
   Another object of this invention is to provide a data reading apparatus for transferring digital data from a memory array where the data transmission time is not limited by the external clock period. 
   Another object of this invention is to provide a data reading apparatus of transferring digital data from a memory array such that internal timing of controls permits flexible column selection and no conflicts in the timing between external clock signals and internal bit line sensing ready signals. 
   To accomplish at least one of these objects, a memory read apparatus within a memory system is in communication with an array of memory cells for transferring selected data read from the memory. The memory read apparatus has a data read path circuit and a memory read control apparatus. The data read path circuit is in communication with the memory to acquire the selected data read from the memory, synchronize the selected data, and transfer the selected data from the memory. The memory read control apparatus is in communications with the data read path circuit for selecting the data to be read from the memory, for providing self-feedback signals for synchronizing the selected data for transfer from the memory. 
   The data read path includes a plurality of bit line switches in communication with bit line sense amplifiers within the array of memory cells for selectively transferring the data from selected memory cells. A data line sense amplifier in communication with the plurality of bit line switches to receive the data from the selected memory cell. A data line sense amplifier latch is in communication with data line sense amplifier to acquire the data for synchronization and a data output latch is in communication with the data line sense amplifier latch to synchronously transfer the data from the memory. 
   The memory read control apparatus has a data output latch control circuit receiving an external timing signal to provide a timing signal to the data output latch for synchronization of the transferring of the selected data from the memory. A sense amplifier latch control circuit is in communication with the data output latch control circuit to receive a sense amplifier latch clear signal to provide a sense amplifier latch control signal to the data line sense amplifier latch to synchronize the selected data read from the memory comprising. A sense amplifier control circuit is in communication with the data line sense amplifier to provide a data line sense amplifier enabling signal to the data line sense amplifier and with the sense amplifier control circuit to provide a sense amplifier enable signal to the sense amplifier latch control circuit and receive the sense amplifier latch signal from the sense amplifier latch control circuit to indicate that the data line sense amplifier is to be disabled. 
   The memory read path apparatus further has a column control circuit in communication with the sense amplifier control circuit to receive a read synchronization signal. The column control circuit is further in communication with the memory to receive a bit line sense ready signal to generate a bit line switch enable signal, and with a column address decoder with the memory to provide a bit line switch activation signal for selecting a desired data for transfer from the memory. 
   The memory array may be such memories as a pseudo-static random access memory, static random access memory, read only memory, or dynamic random access memory. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a memory device illustrating the data read path circuitry of the prior art. 
       FIG. 2  is a schematic diagram of a memory device illustrating the data read path circuitry of this invention. 
       FIG. 3  is a schematic diagram of a memory device illustrating the data read path circuitry of this invention highlighting the self-feedback control paths. 
       FIG. 4  is a timing diagram of the operation of the memory device with the data read path circuitry of this invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The data read path circuitry within a memory system of this invention controls and synchronizes a memory read operation for transferring selected data read from the memory. The data read path circuitry receives an external timing or clock signal and generates a data output latch timing signal from the external timing signal. The data output latch timing signal is transferred to a data output latch for synchronization of the transferring of the selected data from the memory. The external timing or clock signal is used to generate a sense amplifier latch clear signal that is then combined with a senses amplifier enable signal to produce the sense amplifier latch control signal. The sense amplifier latch control signal is then transferred to a sense amplifier latch of the memory to gate the selected data read from the memory. 
   A column decode circuit receive a column address, decodes the address to generate the bit switch activation signals to activate bit switches of each column of the array of memory cells to select one of the outputs of the bit line sense amplifiers to a data line sense amplifier. The bit switch enable signal is used to generate a data line sense amplifier enable signal. The sense amplifier latch control signal is used to stop the data line sense amplifier enable signal to deactivate the data line sense amplifier. 
   A read synchronization signal generated from a combination of the bit line switch enable signal and the sense amplifier latch control signal. A bit line sensing ready signal is received from the array of memory cells when the bit lines have been retrieved by the sense amplifiers. The read synchronization signal and bit line sensing ready signal are combined to generate one of the bit switch activation signals for selecting a desired data for transfer from the memory. 
   Refer now to  FIG. 2  for a more detailed discussion of a read data path of a memory device of this invention. In a structure similar to those of  FIG. 1 , the memory cells  105  are arranged in row and columns to form the sub-arrays  110   a , . . . ,  110   n . An address is decoded to form the word line addresses  115  and the bit line addresses  145  for selecting the desired rows and columns of the memory sub-array  110   a , . . . ,  110   n . Each of the memory cells  105  of a selected word line  115  is activated and the digital data is transferred to the bit lines (BL 00 ,  BL 00   , . . . , BLmn,  BLmn ). The bit line sense amplifiers  122   a , . . . ,  122   n  acquire, amplify, and condition the digital data. The bit line switches  132   a , . . . ,  132   n  are connected to the terminal ends of the bit lines BL 00 ,  BL 00   , . . . , BLmn,  BLmn  to receive the digital data from the bit line sense amplifiers  122   a , . . . ,  122   n . Each of the bit line switches  132   a , . . . ,  132   n  are formed of a pair of metal oxide semiconductor (MOS) transistors (M 1  and M 2 ). The gates of the MOS transistors (M 1  and M 2 ) are connected to receive the bit line selection signals BS  155  from the column decoder  150 . The column decoder is connected to the column control circuit  140 , which receives a bit line sensing ready signal  135  indicating the digital data present on the bit lines BL 00 ,  BL 00   , . . . , BLmn,  BLmn  has been sensed, amplified, and conditioned for transfer from the memory array  125 . The column address  145  is decoded and the desired bit line switch  132   a , . . . ,  132   n  is activated to transfer the digital data from the memory array  125  through the bit line switches  132   a , . . . ,  132   n  to the data line sense amplifier  160 . The data line sense amplifier  160  further amplifies and conditions the digital data. 
   The output of the data line sense amplifier  160  is connected to the input of the data line sense amplifier latch  165 . The data lines sense amplifier latch  165  is a data storage element used to synchronize the digital data with an external clock for transfer to external circuitry. 
   The output of the data line sense amplifier latch  165  is transferred to the input of the data output latch  170 . The data output latch  170  is a second data storage element used to retain the digital data during transfer of the digital data through an off chip driver  175  to a data output terminal DQ  180  and to external circuitry. 
   The bit line switches  132   a , . . . ,  132   n  form the boundary  130  of the first pipeline stage. The data line sense amplifier latch  165  forms the boundary of the second pipeline stage and the data output latch forms the boundary of the third pipeline stage. The external clock  185  is applied to a data output control circuit  190  to generate the data output latch timing signal  195  to control the activation of the data output latch  170  for transfer of the data to the off chip driver  175  to the data output terminal DQ  180 . The data output control circuit  190  further generates a data line sense amplifier latch clear signal  200 . The data line sense amplifier latch clear signal  200  and a data line sense amplifier enable signal  220  are combined in the sense amplifier latch control circuit  205  to generate the data line sense amplifier latch control signal  210 . 
   The data line sense amplifier latch control signal  210  is transferred to the data line sense amplifier control circuit  215 . The data line sense amplifier latch control signal  210  is combined with the bit line switch activation signals  155  to generate the data line sense amplifier enable signal  220 . The data line sense amplifier latch control signal  210  is further combined with the bit line switch enable signal  230  to generate the read synchronization signal  225  that is applied to the column control circuit  140 . 
   The bit line sensing ready signal  135  is an input to the column control circuit  140  and is combined with the read synchronization signal  225  to generate the bit line switch enable signal  230 . The bit line switch enable signal is combined with the bit line addresses  145  to activate the appropriate bit switch activation signal  155  at its appropriate time. 
     FIG. 3  shows the data read path structure of the memory device of this invention and highlights the self-feedback control paths of the pipeline of the data read path. The first feedback control path  250  synchronizes the bit line switch enable signals  155  such that they are activated based ultimately on the external clock  185  and the bit line sensing ready signal  135 . The second feedback control path  255  provides the timing for the data line sense amplifier enable signal  220  and the data line sense amplifier latch control signal  210 . The basic control path (control path  3 )  260  provides the timing for the third pipeline stage. When the current data present DT 2  at the data line sense amplifier latch  165  is latched to the data output latch  170 , the data line sense amplifier latch  165  can be release and made ready for the next data DT 1  from the data line sense amplifier  160 . 
   Refer now to  FIG. 4  for a discussion of the function of the first feedback control path  250 , second feedback control path  255 , and the basic control path  260 . The bit line sensing ready signal  135  is brought to an active state at a time after one of the transitions of the external clock  185 , as determined by the latency of the access of the memory array. The bit switch enable signal  230  is activated based on the transition of the bit line sensing ready signal  135  which in turn acts to activate one of the selected the bit line switch activation signals  155  to turn on one of the bit line switches  132   a , . . . ,  132   n  of  FIGS. 2-3 . The first feedback control path  250  of  FIG. 3  provides the control of the active interval time for the bit line switches  32   a , . . . ,  32   n  to transfer the selected digital data to the data line sense amplifier  160  of  FIGS. 2-3 . The first segment (*a) of the feedback control path  250  activates the data line sense amplifier enable signal  220  for the transfer of the selected data signals to the data line sense amplifier  160 . The duration of the first segment (*a) tracks the time taken for the data line sense amplifier  160  to develop the digital data DT 1  at its output. This determines the amount of time that the data line sense amplifier enable signal  220  is active after the selected bit line switches  132   a , . . . ,  132   n  is activated to effectively connect the selected bit line BL 00 ,  BL 00   , . . . , BLmn,  BLmn  to the data line sense amplifier  160 . The second segment (*b) and the third segment (*c) determines the time at which the read synchronization signal  225  is activated and from the read synchronization signal  225 , the bit line switch enable signal  230  and thus the bit line switches  132   a , . . . ,  132   n  are deactivated as quickly as possible. 
   The second feedback control path  255  begins with the data line sense amplifier enable signal  220  determine the time at which the data line sense amplifier latch control signal  210  is activated to capture the data into the data line sense amplifier latch  165  of  FIGS. 2-3  during the first segment (*d). In the prior art, the data line sense amplifier  60  of  FIG. 1  was deactivated or reset by the external clock  85 . This forces the data at the output of the data line sense amplifier latch  65  to be held until it the data output latch  70  is activated or set. 
   The data line sense amplifier latch control signal  210  determines the time at which the data line sense amplifier enable signal  220  is deactivated to disable the data lines sense amplifier  160  during the second segment (*e). This in turn determines the time at which the next read cycle is initiated. In the third segment (*f), the read synchronization signal  225  is set to a level which allows the bit line switch enable signal  230  to be activated and the next bit line switch  132   a , . . . ,  132   n  to be activated again. 
   The third pipeline includes the segments *g and *h to control the capturing of the digital data DT 2  in the data line sense amplifier latch  165  and the reset or release of the data line sense amplifier latch  165 . In the segment *g, the external clock triggers the data line sense amplifier latch clear signal  200 . 
   The data output latch timing signal  195  is adjusted by the data output control circuit  190  to account for the memory latency. The mode register code  191  provides a user defined code that adjust the latency cycles of the data output latch timing signal  195 . 
   The data line sense amplifier latch clear signal  200  then determines the time for the deactivation or reset of the data line sense amplifier latch  165 . This permits the acquisition of the next data DT 2  by the data line sense amplifier latch  165 . 
   The self-feedback structure of the read data path control circuitry of this invention provides an internal timing margin that is independent of the frequency of the external clock  185 . Further, the structure provides a relatively simple solution that is easily implemented in an integrated circuit. This provides more flexibility relative to that of the prior art where the data transit time for each stage is limited by the period of the external clock  185 . 
   This structure is suitable for static random access memory, read only memory, or dynamic random access memory. However, this structure is particularly suitable for pseudo-static random access memory because it lacks a clear column read command at each read cycle. This prevents any problems of boundary alignment of the data access where overlap of the external clock  185  and the bit line sensing ready signal  135 . 
   While this invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.