Abstract:
Circuits and methods are disclosed for a row activation control logic for memory arrays. This invention utilizes a row activation control circuit and a NOR gate, in conjunction with a previously disclosed timing reference circuit, to allow the shortening of the row precharge time, yet insuring that the bitline is getting charged well enough without causing the chip to read wrong data at the next row activation. The circuits and methods disclosed can be applied to different types of dynamic random access memories.

Description:
RELATED PATENT APPLICATION 
     ETRON96-005, t RAS  PROTECTION CIRCUIT title filing date Jun. 10, 1996, Ser. No. 08/661,228, assigned to a common assignee. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to timing circuits for semiconductor memory arrays, in particular to row activation and precharge command timing circuits as used in dynamic random access memory (DRAM) arrays. 
     2. Description of the Prior Art 
     The memory cycle of semiconductor memory arrays is divided into a ROW ACTIVATION (active period) and ROW PRECHARGE period (inactive period). During row activation the memory cells are accessed and during row precharge the bitlines and inverse bitlines, which were at a high or low voltage depending on the state of the cell, must be precharged to an initial voltage to be ready for the next active period. Included in the row precharge time is a safety margin, mainly to insure that precharging is completed. If not completed, it will cause the chip to get the wrong data at the next row activation. Efforts have been made to reduce the row precharge time while at the same time guaranteeing the completion of precharge. 
     A number of U.S. Patents deal with precharge timing in memory arrays. U.S. Pat. No. 4,998,222 (Sussman) discloses the use of an internally gated RAS signal to allow for the elimination of the need to skew the beginning of Precharge due to process variation. U.S. Pat. No. 5,185,719 (Dong et al.) discloses extending RAS reset/precharge using an on-chip generated control signal. U.S. Pat. No. 5,522,064 (Aldereguia et al.) describes a memory controller having timing registers to allow for the dynamic setting of memory timings for a plurality of memory chips that differ in size and speed. U.S. Pat. No. 5,563,831 (Ting) reveals a timing reference circuit which maintains a higher voltage at the gates of the isolation MOSFET&#39;s in the memory array during the precharge period and returning the voltage to a lower level before the end of the inactive period. This patent is assigned to the same assignee as the current invention. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide circuits and methods to allow row activation in a semiconductor memory array during row precharge and without interrupting row precharge. 
     Another object of the present invention is to allow flexibility in timing of the row activation command after the precharge command. 
     A further object is to make the present invention applicable to different types of random access memories. 
     These objects have been achieved by combining a previously disclosed timing reference circuit with a row activation control circuit and a NOR gate. The addition of the row activation control circuit and the NOR gate insures that the bitline is getting precharged fully when the row precharge time is shortened to insure that the chip is reading out the correct data at the next row activation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a high level block diagram of the present invention. 
     FIG. 2 is a circuit diagram of the present invention. 
     FIG. 3 is a block diagram showing the timing reference circuit and high voltage generator circuit of the prior art. 
     FIG. 4 is a schematic diagram of the timing reference circuit of the prior art. 
     FIG. 5 is a circuit diagram of the row activation control circuit of the present invention. 
     FIG. 6 is a state diagram of FIG. 3, relating inputs to output. 
     FIG. 7a is a view of the input and output signals of the high level block diagram as shown in FIG. 1, when τ is smaller than row precharge. 
     FIG. 7b is a view of the input and output signals of the high level block diagram as shown in FIG. 1, when τ is larger than row precharge. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Memory arrays, such as dynamic random access memory or DRAM arrays, are divided into an active, or row activation, period and an inactive, or row precharge, period. Information is either read from the memory cell or stored in the memory cell during the active period. During the inactive period bitlines and inverse bitlines must be precharged to an initial voltage level in preparation for the next memory cycle (the active period). 
     Referring now to FIG. 1 we show the embodiment of the present invention. FIG. 1 is a high level block diagram of the row activation control logic, consisting of row activation control circuit 20 and timing reference 30. Row activation control circuit 20 has a first input, Registered Row Activation (RRA) 10, a second input, TR precharge  11, and an output, Row Activation (RA) 12. Output 12 (RA) provides a logical one voltage level and a logical zero voltage level. Input 10 (RRA) provides a logical one voltage level during the active period of a memory cycle and a logical zero voltage level during the inactive period of the memory cycle. Input 11, TR precharge , also provides a logical one and zero voltage level. Output 12 is input 13 to timing reference 30. 
     Referring now to FIG. 2, a detailed block diagram of the row activation control logic is shown. Timing reference 30 consists of timing reference circuit 31 and NOR gate 32. The function and purpose of timing reference circuit 31 (consisting of Charging Circuit 56, Reference Capacitor 57, Level Shifting Circuit 58, and High Voltage Generator Circuit 54) is described in detail in U.S. Pat. No. 5,563,831 (Ting) &#34;TIMING REFERENCE CIRCUIT FOR BITLINE PRECHARGE IN MEMORY ARRAYS&#34; and is incorporated herein by reference. FIG. 4, block diagram, and FIG. 6, schematic diagram, of the referenced patent are shown here again as FIG. 3 and FIG. 4 respectively. 
     Part of timing reference 30 is NOR gate 32, with a first input connected to input 13 of timing reference 30. The second input is connected to output 53 (V o ) of the referenced Level Shifting Circuit 58. The output of NOR gate 32 is connected to input 11 (TR precharge ) of row activation control circuit 20. The second input 51, V a , of Charging Circuit 56 is connected to input 13 of timing reference 30. 
     Referring now to FIG. 5, we show a circuit diagram of row activation control circuit 20. The first input of first NAND gate 21 is connected to input 10 (RRA) of row activation control circuit 20, the second input is connected to the output of the second NAND gate 22. The output of the first NAND gate 21 is connected to the input of the second inverter 24. The first input of the second NAND gate 22 is connected to the output of the first NAND gate 21. The second input of the second NAND gate 22 is connected to the output of the first inverter 23. The input of the first inverter 23 is connected to the second input 11 (TR precharge ) of row activation control circuit 20. The output of second inverter 24 is connected to output 12 (RA) of row activation control circuit 20. 
     The state diagram for row activation control circuit 20 is depicted in FIG. 6. The number within the circle designates the &#34;state&#34; of the row activation control circuit; the first number above the circle indicates the logic level of first input RRA, and the second number indicates the logic level of second input TR precharge . The bottom number is the logic level of output RA. There is a state 2 where the output RA=0, designated as 2-0, and there is a state 2 where the output RA=1, designated as 2-1. It is noted that 2-0 cannot be reached directly from state 3, and conversely 2-1 cannot be reached directly from state 0. This characteristic is important for the functioning of the row activation control logic and is demonstrated in FIG. 7a and 7b. 
     Curves 1a, 2a, 3a, and 4a of FIG. 7a demonstrate the condition where the command ROW PRECHARGE, Curve 1a, is longer than the bitline precharge time τ. RRA equal to logical voltage 1 corresponds to the active part of the cycle and RRA equal to logical 0 corresponds to the inactive part of the cycle, Curve 2a. RA, Curve 4a, is being held at logical voltage 1 because during most of RRA active, state 2-1 is forcing RA to a &#34;1&#34;. When RRA drops, so does RA and the charging of capacitor 57 starts. When charged, after time τ, TR precharge  becomes active, Curve 3a, because both RA and output 53, FIG. 2, are at logical &#34;0&#34;. 
     Curves 1b, 2b, 3b, and 4b of FIG. 7b demonstrate the condition where the command ROW PRECHARGE, Curve 1b, is shorter than the bitline precharge time τ. This corresponds to RRA being at logical 0 for a shorter time, Curve 2b. For the same reasons as explained above, RA (Curve 4b) is being held at logical voltage 1. When RRA drops, so does RA and the logic circuit enters state 0. When RRA becomes active during the charging of capacitor 57, state 2-0 is entered, and RA stays at &#34;0&#34;. On the completion of charging, output 53 (V o ) goes to logical zero voltage level. In conjunction with RA=0, TR precharge  goes to logical &#34;1&#34; (Curve 3b). RA then switches to &#34;1&#34; (Curve 4b) and in turn resets TR precharge  (Curve 3b). 
     What this invention accomplishes is that RRA can go to logical &#34;1&#34;, while capacitor 57 is charging, without causing this capacitor to be discharged prematurely. Since the duration of the inactive period (RRA at logical &#34;0&#34;) does not affect row precharge (capacitor charging), the row activation command can be issued (i.e. RRA=1) before row precharge is finished. This feature allows flexibility in timing of the row activation command after the precharge command. 
     The condition under which the chip can go into row activation is not only when receiving the row activation command, but also when the internal bitline precharge is completed, as controlled by the timing reference 30. Therefore the transition point between row precharge (t rp ) and row activation (t rod  or t RAS ) is controlled by the timing reference, where τ is the optimum bitline precharge time. The invention allows the row activation command to be issued as early as possible i.e. at the optimal time. 
     Since timing reference 30 tracks the bitline precharge time, when TR precharge  goes to &#34;1&#34; the real bitline signals are precharged completely and the row activation is allowed to execute. Tracking is assured because both the bitline and the reference capacitor 57 have matched capacitances and are fabricated by the same process. Process variations, therefore, affect both capacitances the same way. 
     The circuits and method described apply to any of the dynamic random access memories such as, but not limited to, synchronous dynamic random access memory arrays, synchronous graphics random access memory arrays, extended data out dynamic random access memory arrays, and fast page mode dynamic random access memory arrays. 
     While the 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.