Electronic neuromorphic system, synaptic circuit with resistive switching memory and method of performing spike-timing dependent plasticity

1. A synaptic circuit performing spike-timing dependent plasticity (STDP) interposed between a pre-synaptic neuron and a post-synapse neuron and comprising:

a memristor having a variable resistance value (R) and being configured to receive a first signal (V TE ) from the pre-synaptic neuron;

transistor structure connected in series with the memristor and being configured for receiving a second signal (V CG ) from the pre-synaptic neuron and being configured for providing an output signal (I BE ) to the post-synaptic neuron,

wherein the transistor structure is further configured to receive a retroaction signal (V FG ) generated from the post-synaptic neuron, and

the memristor is configured to modify the resistance value (R) based on a delay (Δt) between two at least partially overlapped input pulses, a spike event (F E1 ) of the first signal (V TE ) and a pulse (V FGMAX ) of the retroaction signal (V FG ), in order to induct a potentiated state (STP) upon a decreasing the resistance value (R) and a depressed state (STD) upon increasing the resistance value (R) at the memristor, and

wherein the transistor structure comprises a first transistor and a second transistor that are connected in parallel and are interposed between a first inner node and a second inner node of the transistor structure, the first transistor having a control terminal coupled to the pre-synaptic neuron for receiving the second signal (V CG ) and the second transistor having a control terminal connected to the post-synaptic neuron for receiving the retroaction signal (V FG ), and

wherein the memristor is a two terminals memristor interposed between the first input terminal and the first inner node.

2. The synaptic circuit of claim 1 , wherein the first transistor and the second transistor are n-MOS transistors.

3. The synaptic circuit of claim 1 , wherein the memristor is a bipolar memristor.

4. The synaptic circuit of claim 1 , wherein the resistance value (R) is configured to decrease by inducting the potentiated state (LTP) at the memristor when the pulse (V FGMAX ) of the retroaction signal (V FG ) is successive to the spike event (F E1 ) of the first signal (V TE ).

5. The synaptic circuit of claim 1 , wherein the resistance value (R) is configured to increase by inducting the depressed state (LTD) at the memristor when the pulse (V FGMAX ) of the retroaction signal (V FG ) precedes the spike event (F E1 ) of the first signal (V TE ).

6. The synaptic circuit of claim 1 , wherein the pre-synaptic neuron and the post-synaptic neuron are each constructed and arranged to integrate a signal and to induce a fire event.

7. The synaptic circuit of claim 6 , wherein the post-synaptic neuron comprises an integration unit having an operational amplifier connected in series by an internal terminal with a fire unit comprising a comparator stage, the integration unit being configured to integrate an output signal (I BE ) issued from the transistor structure and to generate an internal voltage (V INT ) at the internal terminal the fire unit being configured to generate:

a further first signal (V TE ) by a first output terminal,

further second signal (V CG ) by a second output terminal and

the retroaction signal (V FG ) by the retroaction terminal.

8. The synaptic circuit of claim 7 , wherein the post-synaptic neuron is configured to provide a reset state by imposing a constant predetermined voltage at the internal terminal when the internal voltage (V INT ) reaches a threshold value (V TH ).

9. The synaptic circuit of claim 1 , wherein the two at least partially overlapped input pulses, the pulse of the first signal (V TE ) and the pulse (V FGMAX ) of the retroaction signal (V FG ), have opposite exponential shapes and the first signal (V TE ) further having spike events (F E1 ) at the end of each pulse.

10. The synaptic circuit of claim 1 , wherein the two at least partially overlapped input pulses have square shapes, the pulses of the first signal (V TE ) and the pulse (V FGMAX ) of the retroaction signal (V FG ), the first signal (V TE ) further having a spike event (f e1 ) at the end of each pulse.

11. An electronic neuromorphic system comprising a multi-layer structure wherein an intermediate layer (L x ) comprises a first set (N) of pre-synaptic neurons and a subsequent layer (L X+1 ) comprises a second set (M) of post-synaptic neurons, synaptic circuits performing spike-timing dependent plasticity (STDP) being interposed between the pre-synaptic neurons of the first set (N) and the post-synaptic neurons of the second set (M), each synaptic circuit comprising:

a memristor having a variable resistance value (R) and being configured to receive a first signal (V TE ) from the pre-synaptic neuron;

transistor structure connected in series with the memristor and being configured for receiving a second signal (V CG ) from the pre-synaptic neuron and being configured for providing an output signal (I BE ) to the post-synaptic neuron,

wherein the transistor structure is further configured to receive a retroaction signal (V FG ) generated from the post-synaptic neuron, and

the memristor is configured to modify the resistance value (R) based on a delay (Δt) between two at least partially overlapped input pulses, a spike event (F E1 ) of the first signal (V TE ) and a pulse (V FGMAX ) of the retroaction signal (V FG ), in order to induct a potentiated state (STP) upon a decreasing the resistance value (R) and a depressed state (STD) upon increasing the resistance value (R) at the memristor, and

wherein the transistor structure comprises a first transistor and a second transistor that are connected in parallel and are interposed between a first inner node and a second inner node, the first transistor has a control terminal coupled to the pre-synaptic neuron for receiving the second signal (V CG ) and the second transistor has a control terminal connected to the post-synaptic neuron for receiving the retroaction signal (V FG ), and

wherein the memristor is a two-terminals memristor interposed between the first input terminal and the first inner node of the transistor structure.

12. The electronic neuromorphic system of claim 11 , wherein the synaptic circuits are arranged in an array (NxM) comprising N-lines (L 1 -L N ) and M-columns (C 1 -C M ), and wherein each line comprises a pre-synaptic neuron of the first set (N) of the intermediate layer (L x ) and each column comprises a post-synaptic neuron of the second set (M) of the subsequent layer (L X+1 ).

13. The electronic neuromorphic system of claim 12 , wherein each pre-synaptic neuron of the intermediate layer (L x ) comprises a first output terminal configured to provide the first signal (V TE ) to each of the M synaptic circuits arranged at the same line, and configured to provide the second signal (V CG ) by a second output terminal of the M synaptic circuits of such line.

14. The electronic neuromorphic system of claim 11 , wherein each post-synaptic neuron of the subsequent layer (L X+1 ) comprises an input terminal configured to receive the output signal (I BE ) from each transistor structure of the N synaptic circuits arranged at the same column, and configured to provide the retroaction signal (V FG ) to each retroaction terminal of the N synaptic circuits of such column.

15. A method of performing spike timing dependent plasticity (STDP) by a synaptic circuit comprising a memristor having a variable resistance value (R), the memristor being interposed between a pre-synaptic neuron and a post-synaptic neuron, the method comprising:

receiving a first signal (V TE ) from the pre-synaptic neuron;

receiving a retroaction signal (V FG ) from the post-synaptic neuron, and modifying the resistance value (R) based on a delay (Δt) between two at least partially overlapped input pulses, a spike event (F E1 ) of the first signal (V TE ) and a pulse (V FGMAX ) of the retroaction signal (V FG ), by inducting a potentiated state (STP) upon a decreasing the resistance value (R) and a depressed state (STD) upon increasing the resistance value (R) at the memristor ( 10 );

providing the synaptic circuit comprising a two terminals memristor;

connecting a transistor structure in series with the memristor;

receiving at the memristor the first signal (V TE );

receiving at the transistor structure a second signal (V CG ) from the pre-synaptic neuron;

providing an output signal (I BE ) to the post-synaptic neuron by the intermediate unit;

receiving the retroaction signal (V FG ) at the transistor structure;

providing the transistor structure by comprising a first transistor and a second transistor;

connecting the second transistor in parallel with the first transistor interposed between a first inner node and a second inner node;

connecting the memristor to the first inner node; and

connecting the post-synaptic neuron to the second node by an output terminal.

16. The method of claim 15 , further comprising:

providing the synaptic circuit comprising a bipolar memristor.

17. The method of claim 15 , further comprising:

decreasing the resistance value (R) when the pulse (V FGMAX ) of the retroaction signal (V FG ) is successive to the spike event (F E1 ) of the first signal (V TE ) issued from the pre-synaptic neuron by providing the potentiated state (LTP) at the memristor.

18. The method of claim 15 , further comprising:

increasing the resistance value (R) when the pulse (V FGMAX ) of the retroaction signal (V FG ) precedes the spike event (F E1 ) of the first signal (V TE ) issued from the pre-synaptic neuron by providing the depressed state (LTD) at the memristor.