Fast Flush for a First-in First-Out Memory

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.

The present invention allows for rapid access to the data on memory cell write bit-lines by the read bit-lines of that memory cell in the case where the memory cell is being accessed by appropriate row and column pointers.

Broadly stated, in accordance with the invention, a circuit is provided for minimizing the fall-through delay of data in a first-in first-out memory circuit. The circuit includes one or more write bit-lines connected with the memory cell for communicating data to the cell, one or more read bit-lines connected with the memory cell for communicating data from the cell, and means selectively connecting the write bit-line(s) with the read bit-line(s) independent of the cell so that data carried by the write bit-line(s) can be communicated to the read bit-line(s) independent of the memory cell

Other objects of the invention will become apparent from a study of the following specification and drawing, which is a schematic view of the circuit incorporating 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 includes 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 f 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 transistor 50 on condition, with the transistor 52 providing connection between the write bit-line 36 and read bit-line 38 in the transistor 52 on condition.

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, and the data applied to a sense amplifier 60, communicating 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 lines as described above, the data being supplied by the write bit-lines 32, 36 can be read faster than in the prior art description, since the data is transferred directly from the write bit-lines 32, 36 through transistors 50, 52 to the read bit-lines 34, 38.

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.

A preferred embodiment has been illustrated, of which modifications and adaptations within the scope of the invention will occur to those skilled in the art. The invention is limited only by the scope of the following claims.