FAST FLUSH FOR A FIRST-IN FIRST-OUT MEMORY

Abstract: In a memory circuit including a write bit-line (32, 36) for writing data into a memory cell (10), and a read bit-line (34, 38) for reading data from the cell, a transistor (50, 52) is included, connected with the write bit-line and the read bit-line, so that when a fast flush signal is applied by an AND gate (44) to the gate of that transistor, direct connection is made between the write bit-line and read bit-line, so that data is written into the cell, but can be read simultaneously from the read bit-line, reducing the fall-through delay.

Description: THIS INVENTION relates to memory cell circuits and, more particularly, to a memory cell circuit capable of minimizing fall-through delay. In a typical dual-port memory cell for a first-in first-out memory circuit, the cell is provided with a pair of write bit-lines through which data, in the form of a 1 or a 0, may be entered into the cell. The cell is likewise connected to a pair of read bit-lines through which one can read the contents of the cell. In such a first-in first-out memory circuit, data entered at the inputs appears at the outputs in the same order, the data "falling through" to the output queue with a small delay. While such typical design is rapid and effective in use, it is generally desirable to reduce the overall delay time for providing to the output queue the data written into the memory. According to the invention, there is provided a memory circuit having reduced fall-through delay of data in a memory cell, comprising a write bit-line connected to the memory cell by a first MOS pass transistor, the write bit-line and first MOS pass transistor serving to communicate data to the cell, and a read bit-line connected to the memory cell by a second MOS pass transistor, the read bit-line and second MOS pass transistor serving to communicate data from the cell, the first MOS pass transistor being controlled by voltage on a write row line, the second MOS pass transistor being controlled by voltage on a read row line, which circuit is characterised by: a third MOS pass transistor directly connecting the write bit-line with the read bit-line independent of the memory cell when the write row line and read row line are sensed by an AND gate to be concurrently activated, an output of the AND gate being coupled to a gate of the third MOS pass transistor for controlling the conductivity of the third MOS pass transistor, so that data carried by the write bit-line can be communicated directly to the read bit-line through said third MOS pass transistor. EP-A-0 149 049 and EP-A-0 052 669 describe sensing the coincidence of addresses for reading and writing from a same memory cell and, in response to this sensed coincidence, effectively by passing the memory cell so that the data to be written into the cell is presented to the read port of the cell. In order that the invention may be more readily understood, an embodiment thereof will now be described, by way of example, with reference to the accompanying drawing, the single Figure of which is a schematic diagram of a circuit embodying the invention. Referring to the drawing, a memory cell 10 is shown in the dotted box, which cell 10 is part of a memory array. The memory cell 10 indicates a voltage supply terminal 12, a resistor 14 connected to the voltage supply terminal, and an N-channel MOS transistor 16 having its drain connected to the resistor 14. In parallel with the resistor 14 and transistor 16 are another resistor 18 connected to the voltage supply terminal 12, and another N-channel MOS transistor 20 having its drain connected to the resistor 18. The sources of the transistors 16, 20 are connected to a ground terminal 22. The drain of transistor 16 is connected to the gate of transistor 20, while the drain of transistor 20 is connected to the gate of transistor 16. N-channel MOS transistors 24, 26 have their sources connected to the drain of the transistor 16, while N-channel MOS transistors 28, 30 have their sources connected to the drain of the transistor 20. The drain of the transistor 24 connects to the external write bit-line 32, while the drain of the transistor 26 connects to the external read bit-line 34. Similarly, the drain of the transistor 28 connects to another external write bit-line 36, while the drain of the transistor 30 connects to another external read bit-line 38. The gates of the transistors 24, 28 connect to the external write row line 40, while the gates of the transistors 26, 30 connect to the external read row line 42. The logic states of the write row line 40 and read row line 42 are input into an AND gate 44. The output signal of the AND gate 44 is input to the gates of N-channel MOS transistors 50, 52, the transistor 50 providing connection between the write bit-line 32 and read bit-line 34 in the on condition of the transistor 50 with the transistor 52 providing connection between the write bit-line 36 and read bit-line 38 in the on condition of the transistor 52. Data is communicated from the data input line 54 through a buffer 56 to the write bit-line 32 and through an inverter 58 to the write bit-line 36. Thus, whatever the state of the input signal, the write bit-line 32 and write bit-line 36 will carry opposite values (D and D ). Assuming, for example, that a 1 is provided as the input signal on line 54 to be written into the memory cell 10, the write bit-line 32 will carry a high signal and the write bit-line 36 will carry a low signal. The write row line 40 is activated to turn transistors 24, 28 on, so that the drain of transistor 16 goes high, turning the gate of transistor 20 on and bringing the drain of transistor 20 and the gate of transistor 16 low, so that transistor 16 is in the off state, and transistor 20 is in the on state. Assuming that such data is needed to be read from the memory cell 10, the read row line 42 is activated, turning on transistors 26, 30, and the cell's data is placed on the read bit lines 34, 38, the data being applied to a sense amplifier 60 which communicates with line 74 and from which the ultimate output is taken. In the present circuit, upon the write row line 40 and read row line 42 being simultaneously addressed, the signal from the AND gate 44 is high. The transistors 50, 52 are turned on, providing direct communication between the write bit-line 32 and read bit-line 34, and also between the write bit-line 36 and read bit-line 38. Thus, in this situation, data is written into the memory cell 10 as was previously described. However, because of the direct communication between the write and read lines as described above, the data being supplied by the write bit-lines 32, 36 can be read faster than in the prior art circuit, since the data is transferred directly from the write bit-lines 32, 36 through transistors 50, 52 to the read bit-lines 34, 38, rather than having to wait for data to be written into the cell 10 through transistors 24, 28 and read back out to the read bit-lines through transistors 26, 30. It will be understood that the presently described invention is not to be limited to first-in first-out memory cells only, but can be applied to any dual-port memory cell.

Claims: A memory circuit having reduced fall-through delay of data in a memory cell (10), comprising a write bit-line (32) connected to the memory cell by a first MOS pass transistor (24), the write bit-line and first MOS pass transistor serving to communicate data to the cell, and a read bit-line (34) connected to the memory cell by a second MOS pass transistor (26), the read bit-line and second MOS pass transistor serving to communicate data from the cell, the first MOS pass transistor being controlled by voltage on a write row line (40), the second MOS pass transistor being controlled by voltage on a read row line (42), which circuit is characterised by: a third MOS pass transistor (50) directly connecting the write bit-line with the read bit-line independently of the memory cell when the write row line and read row line are sensed by an AND gate (44) to be concurrently activated, an output of the AND gate being coupled to a gate of the third MOS pass transistor for controlling the conductivity of the third MOS pass transistor, so that data carried by the write bit-line can be communicated directly to the read bit-line through said third MOS pass transistor. A circuit according to claim 1, wherein the third MOS pass transistor (5) has a drain connected to the write bit-line (32), a source connected to the read bit-line (34), and a gate connected to an output of the AND gate (44), inputs to the AND gate being connected to the write row line (40) and the read row line (42). A circuit according to claim 1 or 2, comprising a sense amplifier (60) connected with the read bit-line for receiving data therefrom.

Inventors: AUVINEN, STUART T., HOBERMAN, BARRY A.
Applicants: ADVANCED MICRO DEVICES, INC.
Application year: 1988
Main classification: G11C19
IPC codes: G06F5, G11C8, G11C11