Patent GB-993678-A (GB)
Title: A memory cell for a content addressable memory
IPC: G11C11/38
Filed: 1963-04-30
Abstract: 993,678. Tunnel diode bi-stable circuits; gating circuits. THOMPSON RAMO WOOLDRIDGE Inc. April 30, 1963 [April 30, 1962], No. 17054/63. Heading H3T. [Also in Division G4] A bi-stable negative resistance device has diodes connected one to each terminal such that an interrogating signal applied to one diode will provide an output signal at one of the terminals when the device is in one state and applied to the other diode will provide an output signal at the same terminal when it is in the other state. Fig. 1a shows a tunnel diode 11 arranged to have two stable states. Switching from the high resistance " 1 " state to the low resistance " 0 " state is effected by applying a pulse comprising a positive portion followed by a negative portion to the word line 20 concurrently with a pulse at point 45 derived from an AND gate 43 operated by the simultaneous presence of a data write signal from a bi-stable flip-flop 42 and a " write " control pulse from a bi-stable flip-flop 47. The first (positive) portion of the pulse from the word line is ineffective but the second (negative) portion coacts with the positive pulse at 45 to switch the tunnel diode to its low resistance state. The high resistance state is restored by applying a pulse to the word line only, the positive going portion effecting the switching. Nondestructive read out of the condition of the tunnel diode may be effected, either by applying a pulse to terminal 30 to determine whether diode 22 is conducting or applying a pulse to terminal 40 to determine whether diode 34 is conducting or by applying a read pulse to the word line 20 constituted by the negative going portion of the " WRITE " pulse. Thus, for example, the negative pulse on line 20 will pass through diode 22 if the tunnel diode 10 is in its low voltage condition and these pulses if concurrent with a read control pulse from the bi-stable flip-flop 48 will open an AND gate 46 to operate a data read bi-stable flip-flop 44. Fig. 2 shows a " contents addressable" storage matrix in which a circuit such as is shown in Fig. 1a is inserted at each intersection of the word rows A, B, C, D with the bit column 1, 2 and 3. A word is written into the matrix from the write register 64 on the simultaneous occurrence of a write signal from a bistable flip-flop 47 and a pulse on the required word line applied through a wave shaper 62 which produces the bipolar or monopolar pulses required for the corresponding line 20 of Fig. 1a. Read-out is effected by applying the monopolar pulse from the row shaper of the desired word row concurrently with a read-out pulse from a bi-stable flip-flop 48 so that the word is transferred through the AND gates 46 to the read register 66. If it is desired to determine which rows stores a particular word or how many store this word the desired word is set up in a register 68 and the appropriate non-destructive read-out pulses are applied to the matrix. When the words match that in the register a zero output is produced and an amplitude detector comprising " sum " circuits 74 and amplitude detector 76 produce an output on the simultaneous occurrence of a clock pulse on line t1 and a zero word input signal. The output from these detectors may be applied to a summing amplifier which will indicate the number of stored words which match the interrogating word. It is stated that the " read " register 66 and the interrogating register 68 may be one and the same. Reading out from the successive rows of information which include the basic information stored in the interrogation register may be controlled by the steering circuit shown in Fig. 3 in which successively reads out only from those rows which have been shown by the matching process already described to carry the desired words. These rows apply a mark to one input of a corresponding AND gate 86 so that when a "read" pulse is applied from the bi-stable flip-flop 48 to the OR gate 88 and the other input of the AND gate 86 is energized an output signal is produced. This output signal conditions a corresponding bi-stable flip-flop 92 to provide an input for the corresponding " AND " gate 84 so that this input to the AND gate is energized when the desired word is stored in the corresponding row. The AND gates 84 also have a clock input and, in the case of all but the top row carrying the word, an input which is normally zero from all the higher rows. Thus, for example, if the row carries the desired word the flip-flop 92 will be set to allow the first clock pulse to pass through gate 84 to the wave shape circuit 62 which produces the read out pulse but the flip-flop 92 will disable all the other gates 84. This clock pulse will operate the flip-flop 92 to energize the corresponding input to all the other AND gates 84 so that the next clock pulse will then pass through the next lower gate 84 which is marked. Accordingly reading pulses are applied to in succession only to those rows which store the desired word. The steering circuit may also be used to select the first vacant row by setting the interrogating register to 000 and energizing the " write " flip-flop 47. The OR gate 98 prevents a new word being written into more than one vacant word row. It is stated that one bit may alternatively be used to indicate whether or not a location is vacant and that where it is desired to write information into a specific vacant location it would be convenient to store an address code as part of the word in each location. Specification 979,473 is referred to.