Patent GB-1536013-A (GB)
Title: Data storage memories
IPC: G11C11/414
Filed: 1976-02-20
Abstract: 1536013 Semiconductor memory INTERNATIONAL BUSINESS MACHINES CORP 20 Feb 1976 [15 March 1975] 06710/76 Heading H3T A data storage cell comprising a pair of cross coupled bipolar transistors TX1, TX2 connected to a pair of load elements TX3, TX4, with the node between B, C, connected via a Schottky diode SX1, SX2 to one of a pair of bit lines B0, B1 and the non- cross-coupled electrodes connected to a wordline WL is operated in read and write operating cycles, each consisting of selection, recovery and restore phases or as a standby state. The word and bit line potentials are controlled so that in a standby condition the bit lines are held at a common standby potential and no current flows into the cell from the bit lines, in the selection phase of a read cycle a potential difference indicating the state of the cell is produced between the bit lines, in the recovery phase the circuit nodes B, C are charged until one of them reaches a predetermined potential, and in the restore phase the bit lines are forced to re-assume the common standby potential. Preferably in the write cycle the potential of a selected one of the bit lines is held substantially constant throughout the selection and recovery phases and during the initial part of the restore phase. The load elements may be bipolar transistors. The recovery and restore portion of the circuit is shown above the storage cell while the read-write portion lies below it. Potentials V N  and V S  are fixed. In the standby condition the word line W L  is at -2À2 volts, transistors T3, T4 conduct a low current of 10 micro-amps to B0, B I and the voltage level of these bit lines is limited by diodes S1, S2 to that on the restore line V REST , -2À8V. No current flows in SX1, SX2 therefore. The voltage on V REC  prevents currents flowing from transistors T1, T2 to the bit lines. In the read cycle, the current flows are as indicated by thickened lines in Fig. 2 (not shown, being otherwise identical to Fig. 1). In the read phase the word line WL is made more negative, -3À2V. Consequently, the Schottky diode SX1 on the side of the cell in which conduction is taking place, say in TX1, conducts and the bit line B0 goes negative. At the same time the restore line V REST  is made more positive and bit line B 1 goes positive likewise. The difference is derived by the differential amplifier T8, T9, the appropriate cell having been selected out of the matrix by application of a negative voltage to B5L thus turning transistor T5 on. The recovery phase commences by removing the selection voltage, making the voltage V REC  positive to turn off the diodes S5, S6, and restoring the word line WL to its normal level. (Fig. 4, not shown, indicates current paths.) The four transistors T1, T3 and T2, T4 conduct and supply charge to the bit lines and the nodes B, C likewise charge but to a lower voltage due to the drop across the Schottky diodes SX1, SX2. Simultaneously the capacitors C0, C1 are charged until one bit line, B1 if TX1 is conducting, reaches the voltage V sense less the drop in Schottky diode S8. In the restore phase, the bit lines have charged to different voltages and the restore line V REST  is returned to its normal, more negative, potential turning on the diodes S1, S2 and this equalizes the bit line potentials to the standby voltage. The write cycle has the same recovery and restore phases as the read cycle, the write phase commences by the word line WL going negative to select the cell. The signal W0 (or W1) is made more positive and this turns transistor T6 (or T7) assuming that TX2 (TX1) of the cell is to be made conductive and the inverse signal W0 turns transistor T12 (T11) on. The other signal W1, W1 is left unchanged so keeping T7 and T1 off. The bit line B1 is pulled down by transistor T12 preventing the standby current from T4 flowing into the cell. An additional current supplied by T6 to bit line B0 aids the standby current from T3 and causes the node B to rise in potential until TX2 starts to conduct. Regenerative action causes the cell to change state completing the write phase.