SEMICONDUCTOR MEMORY WITH STATIC COLUMN DECODE AND PAGE MODE ADDRESSING CAPABILITY

Abstract: A semiconductor memory includes a memory array (10) that is operable to be addressed in either the page mode or the static column decode mode. A column address transparent latch (20) is provided which is controlled to either directly input a column address to a column decoder (26) or to latch the address in response to the generation of the column address strobe. A sequence detect circuit (30) detects the sequence to the row address strobe and the column address strobe to determine whether the page mode or the static column decode mode is generated. The sequence detect cir­ cuit (30) generates a Y-enable signal in a circuit (31) for control of the latch (20).

Description: The present invention pertains in general to semiconductor memories and, more particularly, to a semiconductor memory selectively accessible in the page mode or in the static column decode mode and to a method of operating such a memory device. BACKGROUND OF THE INVENTION Advances in semiconductor technology have yielded denser and faster circuits. Operation speed or response time determines the minimum time in which information can be either accessed from the storage locations or information written thereto. There are a number of factors that determine the speed with which information can be accessed in a memory array. These are such things as intermediate latches for column and row addresses, generation of timing signals, buffer delays, etc. Some of these factors are a function of design such as, for example, the sequence of timing signals utilized to access the memory array which require a predetermined number of logical steps before access allowed. Other factors that affect response time are such things as circuit parasitics. In accessing a memory array, a column address and a row address are input to the device and normally strobed into column and row latches by a column address strobe ( CAS ) and a row address strobe ( RAS ), respectively. RAS and CAS in a typical memory would allow access to one location in the memory. An additional location would require this cycle to be repeated with its inherent delays. There are a number of modes that have heretofore been utilized to increase access time for a particular application. One technique is "paging" and the other technique is "static column decode". In the page mode, a row address is strobed into the row address latch to select a row in the memory array. Thereafter, it is only necessary to generate CAS followed by an address to access a particular column. Sequential CAS signals followed by a different address will allow each column in a particular row to be accessed without requiring the generation of RAS for each memory location accessed in a given row. This eliminates the required delay between the generation of RAS and CAS , thus increasing access speed. In the static column decode mode, no column address latch is provided. Rather, only a row address latch is provided for latching the row address with the column address being fed directly to the column decoder. After the selection of a given row by latching the row address into the row address latch, it is only necessary to then asynchronously input column addresses. As soon as the column address is input, the column decoder decodes it and activates one of the column lines. Therefore, only the inherent delays of the column decoders and the inherent access time for each of the memory cells determines the delay between generation of the column address and output of valid data. There is no delay incurred as a result of the column address latch or the timing signals required to effectively latch data therein. The use of page mode requires both a RAS input and a CAS input and an internal latch whereas the static column decode mode requires only a RAS input. The two are therefore not compatible in present circuits. Therefore, a need exists for a memory having an integrated page mode and static column decode mode on the same device without increasing the number of pins over that required for the page mode device. From "IEEE JOURNAL OF SOLID STATE CIRCUITS", vol. SC-18, no. 5, October 1983, New York, USA pages 470-478, FUJISHIMA et al.: "A 256K Dynamic RAM with Page-Nibble Mode" it is known that the performance of a memory device may be improved by having a single device selectively operate in different modes, namely in a page mode and in a nibble mode. Nothing is disclosed about combining page mode and static column decode mode. In "ELECTRO", May 1984, New York, USA, pages 17/1-1 - 17/1-9, FALLIN: "Optimising System Performance using High Density CHMOS Dynamic RAMs" a transparent column address latch is disclosed for use in a memory device accessible in the static column decode mode, but there is no disclosure of a device selectively accessible in the page mode or in the static column decode mode. SUMMARY OF THE INVENTION The present invention disclosed and claimed herein comprises a semiconductor memory selectively accessible the page mode or the static column decode mode. The memory includes an array of memory elements arranged in rows and columns; row access means for receiving an external row address and accessing a row of said memory elements defined by said row address in response to receiving a row address strobe signal, said row access means including a row address latch for maintaining access to said row after said row address is removed; column access means for receiving an external column address and accessing a column of said memory elements defined by said column address and operable to selectively function alternatively in the page mode or in the static column decode mode; said column access means including a transparent column address latch for receiving said external column address and operating in said page mode to latch and store said received column address for output therefrom in response to receiving a column address strobe signal, said column address latch of said column access means maintaining access to said column after said column address is removed; said transparent column address latch operating in said; static column decode mode to be transparent to said column address to access said column of memory elements in response to receiving said external column address and maintaining access only in the presence of said column address; mode means for receiving external signals and selecting the one of said page or static column decode modes in which said column access means functions, said external signals as received by said mode means comprising said row and column address strobe signals; and said mode means comprising means for detecting the sequence of said row and column address strobe signals, a first sequence selecting said page mode and a second reverse sequence selecting said static column decode mode. The method of operating a semiconductor device is defined in claim 6. For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying Drawings in which: FIGURE 1 illustrates a schematic block diagram of the memory of the present invention for operating in either the page mode or the static column decode mode; FIGURE 2 illustrates a timing diagram for the page mode of the operation; FIGURE 3 illustrates a timing diagram for the static column decode mode of operation; FIGURE 4 illustrates a schematic block diagram of the RAS/CAS sequential detection circuit; FIGURE 5 illustrates a logic diagram of the Y-enable circuit; and FIGURE 6 illustrates a logic diagram for the transparent column latch. Referring now to FIGURE 1, there is illustrated a schematic block diagram of a semiconductor memory operable to access a memory array 10 in both the page mode and the SCD mode. The memory array 10 is a conventional array of memory elements arranged in rows and columns. This can either be a Read Only Memory (ROM) or a Random Access Memory (RAM). One example of a RAM array is illustrated in U.S. Patent No. 3,940,747, issued February 24, 1976 to Kitagawa and U.S. Patent No. 4,081,701, issued to White et al. on March 20, 1978, both assigned to Texas Instruments Incorporated: Memory elements in the array 10 are accessed by activating one of a plurality of word lines 12 for selecting the row and one of a plurality of Y-decode lines (not shown) for selecting the desired column or columns. An address is input on address lines 14 to an address bus 16. The address bus 16 is input to a row address latch 18 and also into a column address transparent latch 20. The address on the lines 14 is. multiplexed to first input the row address into the row address latch 18 and then the column address into the column address latch 20. The row address latch 18 is then output on a bus 22 to a row decode circuit 24, the output of which comprises the word lines 12. In a similar manner, the column address latch 20 latches the column address on the input of a column decoder 26 through a latched address bus 28. As will be described hereinbelow, the column address latch 20 has two modes of operation. In the first mode, it operates as a latch for latching the data in response to receiving a column address strobe and, in the second mode, connects the address bus 16 and the latched address bus 28 together to become "transparent". The first mode allows paging and the second mode allows operation in the static column decode mode "SCD". The column address latch 20 is controlled by a RAS / CAS sequence detect circuit 30 which receives the RAS and CAS signals and determines whether RAS precedes CAS or CAS precedes RAS . The detector 30 outputs a signal RBC which is high when RAS occurs first and low when CAS occurs first. RBC is input to a Y-enable circuit 31 which controls the operation of the column address label 20 to determining the operating mode thereof. The memory is controlled by various timing signals which are generated by a clock generator and control circuit 32 that receives the RAS and the CAS signals and generates various timing signals. These timing signals can be either delayed representations of CAS or RAS or can be logic functions of the various timing signals generated that generally control access to the array 10. An input/output control circuit (1/0) 34 is provided for interfacing with input data and output data and is controlled by the READ/WRITE signal (R/W) to control whether data is input to the memory array 10 or is output therefrom, the memory array 10 being a RAM. In operation, the sequence detect circuit 30 is designed to generate RBC as a function of the sequence of RAS and CAS . If RAS precedes CAS , the circuit operates in the page mode and RBC is high. The Y-enable signal is generated in response to receiving successive CAS signals to latch in new addresses. However, if CAS precedes RAS , the circuit operates in the SCD mode and the column address latch 20 is placed in a transparent configuration to allow successive column addresses to be asynchronously input to the column decoder 26. Referring now to FIGURE 2, there is illustrated a timing diagram for operation in the page mode. In the page mode, RAS is first generated by changing the RAS signal from a logic high to a logic Low, as represented by a transition 36. This transition 36 causes generation of a transition 38 from a low to a high state, as indicated by a cause and effect arrow 40, the transition 38 comprising a signal R12. The signal R12 is a delayed inverted RAS signal and is operable to control the row address latch 18 to latch an address therein. The address latched in the row address latch 18 is represented by an address X₀. In addition, R12 at transistion 38 causes the Y-enable signal ( YAN ) to go from a high to a low to connect the column address to the column decoder 26. After generation of the RAS signal, the CAS signal is generated by changing the logic state of CAS from high to low, as represented by a transition 42. The transition 42 causes generation of a transition 43 on a signal C2 which is delayed and inverted CAS signal. The C2 signal causes YAN to go high and latch the address in the latch 20. Thereafter, CAS is raised high and then low, as indicated by a transition 44 and YAN is again pulled low or toggled by CAS to latch a column address Y₁ in the column address latch 20. Thereafter, subsequent column addresses Y₂ - Y n are latched into the column address latch 20 to address the remaining column positions in row X₀. At the end of the page, CAS is again raised high, as indicated by a transition 46, and then RAS is raised high, as indicated by a transition 48. when RAS precedes CAS , RBC denoting RAS before CAS is raised from a low to a high, as indicated by a transition 50. As will be described hereinbelow, this signal in part determines the operation of the YAN signal. A Write signal ( W ) is also generated with the state thereof determining whether the memory is in the Read mode or the Write mode. In the Read mode, W is raised high and, in the Write mode, W is pulled low. Although not shown, conventional timing requires that the data be written into the access memory locations after a predetermined amount of delay to allow settling of the decoders and associated accessing circuitry. Referring now to FIGURE 3, there is illustrated a timing diagram for the SCD mode. In this mode, CAS is first pulled low, as indicated by a transition 52. Thereafter, RAS is pulled low, as indicated by a transition 54 which also causes R12 to be pulled high, as indicated by a transition 56. The transition 56 causes the address X₀ on the address bus 16 to be latched into the row address latch 18 for selection of one of the word lines 12. After a predetermined amount of delay, YAN is pulled low by R12 and transition 56, as represented by a transition 58 and remains low. During this time, addresses Y₀ - Y n are then asynchronously input on the address bus 16. The column address latch 20 in this mode is transparent such that the column decoder 26 is directly connected through the bus 28 to the address bus 16. Therefore, the access delay is a function only of the internal delays of the transparent latch 20, the column decoder 26 and the memory array 10. There are no timing delays resulting from the requirement to generate CAS , wait a predetermined amount of time and then place the address on the bus, as was necessary in the page mode of FIGURE 2. In the SCD mode, RBC is maintained low since CAS preceded RAS . Therefore, both the page mode and the SCD mode are resident in a single device with mode selection determined by the sequence in which RAS and CAS . are generated. Referring now to FIGURE 4, there is illustrated an expanded block diagram of the sequence detect circuit 30 of FIGURE 1. CAS and R12 are input to two inputs of a three input NAND gate 60, the output of which is connected to one input of a two input NAND gate 62. The output of NAND gate 62 is input through two series connected inverters 64 and 66 to output RBC. The output of NAND gate 62 is also input to one input of a two input NAND gate 68, the other input of which is connected to R12. The output of NAND gate 68 is connected to the other input of NAND gate 62 and also one input of NAND gate 70. The output of NAND gate 62 is also input to one input of a NOR gate 72, the output of which is connected to the third input of NAND gate 60. A NOR gate 74 has one input thereof connected to a signal R11 and the output connected to both the other input of the NOR gate 72 and one input of a NOR gate 76. The signal R11 is a delayed version of RAS and inverted with respect to R12. The output of NOR gate 76 is connected to the other input of NOR gate 74. The output of NOR gate 70 is connected to the gates of a p-channel transistor 78 and an n-channel transistor 80. The n-channel transistor 80 has the source thereof connected to ground and the drain thereof connected to a node 82 which is connected to the other input of the OR gate 76. An n-channel transistor 84 is connected between node 82 and ground with the gate thereof connected to R11 . Node 82 is connected to ground through two series n-channel transistors 86 and 88, the gate of transistor 88 being connected to V CC and the gate of transistor 86 being connected to CAS . Transistor 78 has the drain thereof connected to node 82 and the source thereof connected to the drain of a p-channel transistor 90. Transistor 90 has the gate thereof connected to CAS and the source thereof connected to the drain of the p-channel transistor 92. Transistor 92 has the gate thereof connected to R11 and the source thereof connected to V CC . In operation, the circuit of FIGURE 4 functions in one mode when RAS occurs before CAS and in another mode when CAS occurs before RAS . In the first mode, which is the page mode, RBC goes from a low to a high. In the second mode, RBC is maintained low as a result of CAS occurring before RAS . Initially, before either RAS or CAS changes, RBC is low as a result of the output of NAND gate 62 being low. This is a result of a high on both inputs thereof. Therefore, the output of NAND gate 60 is high. The output of NAND gate 60 is maintained high since R12, the inverse of RAS , is low. Until R12 changes, the output of NAND gate 60 will be maintained high. The NAND gate 68 has a low on both inputs thereof since R12 is high before RAS changes. This maintains a high on the other input on the NAND gate 62. In addition, it places a high on one input of NAND gate 70. The NOR gate 74 has a high on the R11 input, thus resulting in a low on the output thereof. This low is input to both NOR gate 72 and NOR gate 76. NOR gate 72 also has a low received from the output of the NAND gate 62 resulting in a high on the output thereof for input to NAND gate 60. CAS is high, resulting in transistor 86 being turned on and pulling node 82 to ground, thus placing a low on the other input of NOR gate 76. This results in a high on the output thereof and a subsequent high on the other input of NOR gate 74. In order to change the output state of NAND gate 62, all three inputs of NAND gate 60 must be raised high. When R12 goes high before CAS goes low, this results in three high inputs to NAND gate 60, resulting in the output thereof going low. The low on the output of NAND gate 60 results in a high on the output of NAND gate 62, resulting in RBC going high. The high on the output of NAND gate 62 is input to NAND gate 68 in addition to the other input going high as a result of R12 going high. The output of NAND gate 68 goes low as a result of the low input from the output of NAND gate 62, thus latching the high on the output thereof. The low output from NAND gate 68 is also input to NAND gate 70, thus changing the output thereof from a low to a high and turning on transistor 80 to maintain node 82 low. In addition, R11 goes low, placing a low on one input of NOR gate 74. However, the output of NOR gate 74 does not change. Therefore, R12 causes RBC to go high. The NOR gate 72 also changes state from a high to a low as a result of the output of NAND gate 62 going high. This changes the output of the NAND gate back to a high, thus latching the high on the output of NAND gate 62. when CAS goes low, transistor 86 is turned off and transistor 90 turned on. Since R11 is also low, transistor 92 is on and transistor 84 off. However, NAND gate 70 keeps transistor 80 on and node 82 low. When R12 returns low, the output of NAND gate 68 returns to high, resulting in a high on both inputs of NAND gate 62 and a low on the output thereof. This effectively resets RBC low. When CAS changes before RAS , NAND gate 60 has the output thereof maintained high, thus preventing NAND gate 62 from changing states on the output thereof. This maintains RBC low. In addition, node 82 goes high, thus changing the output state of NOR gate 76 to a low. When RAS is generated, R11 goes low, resulting in two lows on. the input of NOR gate 74 and a high on the output thereof. This high is input to NOR gate 72, resulting in a low on the output thereof. This maintains NAND gate 60 in an inhibited state until RAS changes to a high again. Therefore, generation of CAS followed by RAS latches RBC low until RAS goes high again. Referring now to FIGURE 5, there is illustrated a schematic diagram of the Y-enable circuit 31 of FIGURE 1. CAS is input to one input of an OR gate 96, the other input of which is connected to RAS through a delay 98 to form a delay. This is similar to A11 . The output of OR gate 96 is input to an inverter 100 through a delay 102. The output of inverter 100 is input to one input of a NAND gate 104. The other input of the NAND gate 104 is connected to RBC. The output of NAND gate 104 is input to one input of a three input NAND gate 106, the other inputs of which are connected to a late write signal (LW) and to R12. The output of NAND gate 106 is connected to a node 108 through two series inverters 110 and 112. Node 108 comprises the inverted Y-enable signal YAN . Node 108 is also connected to the input of an inverter 114, the output of which comprises the Y-enable signal YAN. In operation, the initial state of NAND gate 104 on the output thereof is high since RBC is initially low and the output of inverter 100 is initially low. The resultant circuit of OR gate 96, delay 102 and inverter 100 is an inverted and delayed CAS when RAS is low, which is referred to as signal "C2". LW is normally high except during the Write cycle. R12 is initially low, thus maintaining the output of NAND gate 106 high, thus maintaining YAN high and YAN low. when RAS goes low, R12 goes high after a predetermined delay. This toggles NAND gate 106 low and YAN loin. This is represented by a transition 116 in FIGURE 2. YAN is maintained low until either LW goes low or NAND gate 104 goes low. NAND gate 104 goes low when both RBC goes high and C2 goes high. This occurs in the page mode only. Therefore, in the SCD mode, YAN is maintained low since RBC is always low, as represented by the transition 58 in FIGURE 3. YAN will remain low until RAS goes high and R12 goes low. Referring now to FIGURE 6, there is illustrated a schematic diagram of the column address transparent latch 20 for the addresses AYO-AY7 where only the portion of the latching circuit associated with bit AYO is. illustrated. Bit AYO is input to a NOR gate 120, the other input of which is connected to YAN . The output of NOR gate 120 is connected to a node 122 through an inverter 124. An n-channel pass transistor 126 is connected between the node 122 and a node 128 with the gate thereof connected to YAN. A p-channel pass transistor 130 is connected between the nodes 122 and 128 with the gate thereof connected to YAN . Two series connected inverters 132 and 134 are connected between the node 128 and a node 136. An n-channel pass transistor 138 is connected between the node 128 and 136 with the gate thereof connected to YAN and a p-channel transistor 140 is connected in parallel with transistor 138 between nodes 128 and 136 with the gate thereof connected to YAN Node 136 is connected to the output of the column address latch 20 for the bit AYO through two series connected inverters 142 and 144. In operation, YAN is initially high, thus turning off pass transistors 126 and 130 and turning on transistors 138 and 140. This results in the state on the input of inverter 132 being latched on node 136 and that state being latched to the output AYO from address latch 20. When YAN goes low, transistors 126 and 130 are turned on and transistors 138 and 140 turned off and the logic state of AYO placed on the node 128. In this mode, the latch is "transparent". When YAN returns high in the page mode, the logic state of bit AYO is latched with the inverters 132 and 134 and the pass transistors 138 and 140. In summary, there has been provided a semiconductor memory that is capable of operating in either the page mode or the static column decode mode. This mode is selected by the sequence in which RAS and CAS are generated. A circuit is provided for detecting the sequence of RAS and CAS and this circuit operates a transparent column address latch. The transparent column address latch is operable to continually connect the column address to the column address decoder or to latch the information therein in response to toggling of CAS for operation in the page mode. Although the preferred embodiment has been described in detail, it should be understood that various changes, substitutions and alterations can be made therein without departing from the scope of the invention as defined by the appended claims.

Claims: A semiconductor memory selectively accessible in a page mode or a static column decode mode comprising: an array (10) of memory elements arranged in rows end columns; row access means (16, 18, 22, 24) for receiving an external row address (14) and accessing a row (12) of said memory elements defined by said row address in response to receiving a row address strobe signal ( RAS ), said row access means including a row address latch (18) for maintaining access to said row after said row address is removed; column access means (16, 20, 26, 28) for receiving an external column address (14) and accessing a column of said memory elements defined by said column address and operable to selectively function alternatively in the page mode or in the static column decode mode; said column access means (16, 20, 26, 28) including a transparent column address latch (20) for receiving said external column address and operating in said page mode to latch and store said received column address for output therefrom in response to receiving a column address strobe signal ( CAS ), said column address latch (20) of said column access means (16, 20, 26, 28) maintaining access to said column after said column address is removed; said transparent column address latch (20) operating in said static column decode mode to be transparent to said column address to access said column of memory elements in response to receiving said external column address and maintaining access only in the presence of said column address; mode means (30, 31) for receiving external signals and selecting the one of said page or static column decode modes in which said column access means (16, 20, 26, 28) functions, said external signals as received by said mode means (30, 31) comprising said row and column address strobe signals; and said mode means (30, 31) comprising means (30) for detecting the sequence of said row and column address strobe signals, a first sequence selecting said page mode and a second reverse sequence selecting said static column decode mode. A semiconductor memory as set forth in claim 1, wherein said page mode is selected when said row address strobe signal precedes said column address strobe signal and said static column decode mode is selected when said column address strobe signal precedes said row address strobe signal. A semiconductor memory as set forth in either of claims 1 or 2, wherein said sequence detecting means (30) of said mode means (30, 31) generates a sequence signal which is at a first state when a first sequence is detected and at a second state when a second and opposite sequence is detected; and said mode means (30, 31) further including enable means (31) connected to said sequence detecting means (30) to receive said sequence signal therefrom for controlling said transparent column address latch (20) to operate in said page mode when said sequence signal from said sequence detecting means (30) is in said first state and in said static column decode mode when said sequence signal from said sequence detecting means (30) is in said second state. A semiconductor memory as set forth in any of claims 1 to 3, wherein said column access means further includes: column decode means (26) for receiving said column address output by said transparent column address latch (20)and decoding said column address and accessing a selected one of said columns of memory elements as defined by said column address. A semiconductor memory as set forth in any of claims 1 to 4, wherein said row access means (16, 18, 22, 24) further includes a row decoder (24) for decoding an externally generated row address to select one of said rows of said memory elements to be accessed; and said row address latch (18) being connected to said row decoder (24) for receiving and storing said row address for output to said row decoder (24) in response to receiving a row address strobe signal ( RAS ). A method for operating a semiconductor memory selectively in a page mode or a static column decode mode, comprising: disposing memory elements in an array (10) of rows and columns, selection of one of the memory elements requiring selection of the associated row and column; latching an externally generated row address into a latch (18) for output therefrom in response to receiving the address and a row address strobe signal (RAS); decoding the latched row address and selecting the row defined by the row address; disposing a transparent latch (20) and a decoder (26) between a column address input and the array for receiving an externally generated column address and a column address strobe signal ( CAS ) and selecting one of the columns in the array; and controlling the transparent latch (20) to operate in said page mode or said second static column decode mode by detecting the sequence of generation of the row address strobe signal and the column address strobe signal, a first sequence selecting said page mode and a second reverse sequence selecting said static column decode mode; operation in the page mode being effected by controlling the transparent latch (20) to store the column address for decoding thereof in response to receiving a column address strobe signal, access being maintained by said transparent latch (20) after the column address is removed such that continual reception of the column address is not required; and operation in the static column decode mode being effected by controlling the transparent latch (20) to become transparent to said column address such that continual reception of the column address is required to maintain access to the selected column in the array. A method as set forth in claim 6, wherein in controlling the transparent latch (20) the page mode is selected when the row address strobe signal is detected as preceding the column address strobe signal, and the static column decode mode is selected when the column address strobe signal is detected as preceding the row address strobe signal.

Inventors: SHAH, ASHWIN H., WANG, CHU-PING, WOMACK, RICHARD H.
Applicants: TEXAS INSTRUMENTS INCORPORATED
Application year: 1986
Main classification: G11C8
IPC codes: G11C11, G11C8, G11C7