Abstract:
Circuitry, including a non-volatile dynamic random access memory cell, a sense amplifier and a data latch affords non-destructive accessing and comparison of the data stored within the volatile and non-volatile portion of the memory cell. In certain applications, it is desirable to restore the volatile data to the volatile portion of the memory cell, and the circuitry also provides a path for such restoration.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Related, co-pending application of particular interest to this invention is application U.S. Ser. No. 654,332, filed Sept. 24, 1984, on behalf of Patrick Chuang, Robert Yau and Ron Maltiel entitled &#34;An Improved Non-Volatile Dynamic RAM Cell&#34;, assigned to the assignee of the instant application. 
     FIELD OF THE INVENTION 
     This invention relates to electronic memory circuits, and more particularly, to method and circuitry providing non-destructive access to both volatile and non-volatile data. 
     BACKGROUND OF THE INVENTION 
     A method and device which provides a non-volatile memory cell capable both of volatile dynamic random access memory (DRAM) storage and non-volatile electrically erasable programmable read-only memory (EEPROM) storage is described in the related co-pending application U.S. Ser. No. 654,332, filed Sept. 24, 1984, on behalf of Patrick Chuang, Robert Yau and Ron Maltiel entitled &#34;An Improved Non-Volatile Dynamic RAM Cell&#34;, and that description is hereby incorporated by reference in the instant application. The memory cell disclosed in the related, co-pending application is a non-volatile dynamic RAM (NVDRAM) which operates as an ordinary dynamic RAM cell in one mode of operation, and as a non-volatile ROM cell in another mode of operation. 
     A shadow memory array composed of a number of NVDRAM cells which can store and compare volatile and non-volatile data is useful in pattern recognition and instrument calibration. However, in the prior art, when recalling the data stored in the non-volatile portion, from the so-called &#34;shadow memory&#34;, the data stored in the volatile memory is lost. 
     SUMMARY OF THE INVENTION 
     In that the NVDRAM disclosed in the related, co-pending application is capable of storing both the volatile, as well as the non-volatile data, the instant invention provides non-destructive access to both the volatile and non-volatile data. Circuitry is provided which includes the NVDRAM cell disclosed in the related, co-pending application, a sense amplifier, and a data latch. The data from the volatile portion of the NVDRAM cell is temporarily stored in the data latch and then the data from the non-volatile portion of the NVDRAM cell is sensed by the amplifier. These two data may then be compared. In certain applications, it is desirable to restore the volatile data to the volatile portion of the NVDRAM, and the circuitry of the present invention provides a path for such restoration. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the dual storage device of the present invention employing an NVDRAM, a sense amplifier and a latch; and 
     FIG. 2 is a timing diagram of the dual storage device of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1, the input/output, or bit line, of a NVDRAM cell 10 constructed according to the teachings of the related, co-pending application, is shown connected to a first terminal of a bidirectional sense amplifier 12 and, via a transfer transistor 14, to the input terminal to a data latch 16. 
     An output terminal of sense amplifier 12 is connected to a first terminal of a second transfer transistor 18 and a second terminal of transistor 18 is connected to an output terminal of data latch 16. The NVDRAM cell 10 has a set of control lines which permit selection of the mode of operation of the cell; this selection is described in detail in the related, co-pending application and incorporated herein by reference. Connection of the NVDRAM 10 to the various voltage sources, and the conducting plate and word control lines is not explicitly shown in FIG. 1; these connections being described in detail in the related, co-pending application and incorporated herein by reference. The operation and construction of bidirectional sense amplifier 12 and data latch 16 are conventional and well-understood by persons of ordinary skill in the art. Accordingly, they will not be described herein. 
     Sense amplifier 12 receives a pair of timing signals, SENSE and SENSE, which, when activated, cause the data at the first terminal of the sense amplifier 12 to be compared to a reference voltage and a signal will subsequently be generated at a second terminal of sense amplifier 12 which corresponds to the complement of the data applied at the first terminal. An active timing signal, TRANSFER, applied to the gate of transfer transistor 14 causes the signal applied at a first terminal of transistor 14, connected to the sense amplifier 12, to be passed to a second terminal of transistor 14, connected in turn to the input terminal of data latch 16. Conversely, an active TRANSFER signal applied to the gate of transfer transistor 14 causes a signal applied at the second terminal of transistor 14 to be passed to the first terminal. Second transfer transistor 18 also receives the TRANSFER signal at a gate which causes passage of signals between the first and second terminals of transistor 18 in the manner described in connection with transistor 14. Data latch 16 receives an ENABLE signal which, when active, causes the latch to retain the data applied to the input terminal until the ENABLE signal is deactivated. The signal at the output terminal of data latch 16 follows the signal applied at the input terminal at all times other than when the ENABLE signal is active. Accordingly, a signal generated on the bit line of NVDRAM cell 10 during a READ operation can be applied to the first terminal of the sense amplifier 12 upon application of the SENSE and SENSE signals to sense amplifier 12. Application of the TRANSFER signal to transistors 14 and 18, will cause the data read from NVDRAM cell 10 to be transferred from sense amplifier 12 to the data latch 16. 
     As described in the related, co-pending application, the NVDRAM cell 10 is responsive to a set of control signals which cause it to perform a variety of operations: a RECALL operation, a STORE operation, a dynamic RAM (DRAM) READ operation, a DRAM WRITE operation, and a REFRESH operation. The RECALL operation transfers the data stored within the non-volatile portion of cell 10 to the volatile portion for subsequent generation on the bit line of cell 10. Accordingly, depending on the sequential application of certain of these control signals, the contents of the volatile or the non-volatile portion of NVDRAM cell 10 will be generated at the bit line terminal of cell 10. 
     Illustrative of one application of the instant invention, the output terminals of sense amplifier 12 and data latch 16 are connected to a positive input, and a negative input, respectively, of a comparator 20, via signal lines 22 and 24, respectively. Upon application of a COMPARE signal to the comparator 20, a signal will be generated at an output terminal of comparator 20 indicative of whether the signals at the positive and negative terminals are equal or not. As will be explained below, these signals at the comparator input terminals will be the contents of the non-volatile and volatile portion, respectively, of the NVDRAM cell 10. In this manner, the circuit illustrated in the block diagram of FIG. 1 can be used to temporarily store and compare the contents of the non-volatile and volatile data stored within a NVDRAM cell. 
     The operation of the circuit illustrated in FIG. 1 can best be understood with reference to the timing diagram of FIG. 2. Shown in FIG. 2 are the signals to be applied, in the sequence shown, to the various elements of the circuit illustrated in Fig. 1. While the signals in FIG. 2 are shown active HIGH, the signals may be, in fact, an active LOW, and the following explanation of the circuit elements illustrated in FIG. 1 will nonetheless be equally applicable. The control signal applied to the NVDRAM cell 10 which causes a DRAM READ or DRAM WRITE operation to be performed therein is shown in FIG. 2 labelled 50 thereon. As shown, the DRAM READ operation control signal is activated during the time interval beginning at T1 and continuing to T4. In this way, the volatile data is read in the manner described in the related, co-pending application. The sense signals applied to the sense amplifier 12, SENSE and SENSE, are shown in FIG. 2 labelled 52 and 54, respectively, and as shown are activated beginning at T2, following T1, and deactivated prior to T4 at T3. In this manner, the volatile data is transferred from the NVDRAM cell to the sense amplifier 12 during the interval T1 to T4. 
     Following T4, at T5, an active ENABLE signal 56 is applied to data latch 16 and an active TRANSFER signal 58 is applied to transfer transistors 14 and 18 beginning at T5 and continuing to T6. This causes the volatile data stored within the sense amplifier 12 during the interval T1 to T4, prior to T5, to be transferred to the data latch 16, via transistors 14 and 18, for temporary storage, during the interval ending at T6. Latch ENABLE signal 56 is kept active until at least T13 so that the volatile data will be retained until that time for comparison. 
     Beginning at T7, following T6, the RECALL operation control signal 60 is applied to the NVDRAM cell 10, continuing to T8, thereby causing the non-volatile data stored within the cell to be transferred to the volatile portion thereof prior to T8, as described in the related, co-pending application. Following T8, at T9, an active DRAM READ operation control signal 50 is applied to NVDRAM cell 10 which causes the non-volatile data to be generated at the bit line of the cell. The DRAM READ signal 50 is deactivated at T12, following T9. 
     The sense signals SENSE and SENSE, 52 and 54, respectively, are again activated at T10, following T9, and deactivated at T11. In this manner, the non-volatile data is transferred from the NVDRAM cell 10 to the sense amplifier 12 during the interval T9 to T12, as explained above. 
     An active COMPARE signal 62 is then applied to the comparator 20 beginning at T13, following T12, and continuing to T14, during which time a signal will be generated at the output of comparator 20 indicative of whether the non-volative and the volatile data accessed from the NVDRAM cell 10 during the intervals T1 to T4 and T7 to T10, respectively, was equal. At T15, following T14, the active ENABLE signal 56 applied to data latch 16 can be deactivated, unless it is needed, as will now be described. 
     Since the volatile data stored in the NVDRAM cell 10 is overwritten by the non-volatile data during the RECALL operation, as described in the related, co-pending application, it may be desired to restore the original non-volatile data stored within the data latch 16 to the volatile portion of NVDRAM cell 10. The circuit illustrated in FIG. 1 can be adapted to perform this replacement as follows: Application of any active TRANSFER signal 58 to transistors 14 and 18 at time T15 will cause the volatile data signal stored within latch 16 to be transferred to the two corresponding terminals of the sense amplifier 12 via transfer transistors 14 and 16, respectively. Upon application of an active DRAM WRITE operation control signal 50 applied to NVDRAM cell 10 at time T16, following T15, and the subsequent application to sense amplifier 12 of the SENSE and SENSE 52 and 54 signals during the interval T17 to T18, following T16, will cause the volatile data signal to be placed onto signal line 22 and it can be conducted therefrom to the volatile portion of the NVDRAM cell 10 via the bit line in a DRAM WRITE operation, as described more fully in the related, co-pending application. DRAM WRITE signal 50, TRANSFER signal 58, and LATCH ENABLE signal 56, are set inactive at times T19, T20 and T21, respectively, following completion of the DRAM WRITE operation at T18. 
     The foregoing is a detailed description of a preferred embodiment of the invention. It is understood by persons of ordinary skill in the art that alternative embodiments of the invention are within the scope and spirit of the invention. For example, the method taught by the invention can be utilized whenever it is desired to read non-volatile data from a shadow memory, such as a static non-volatile E 2  PROM, without destroying the volatile data.