Memory device performing multiplication using logical states of memory cells
Systems, methods, and apparatus related to memory devices that perform multiplication using logical states of memory cells. In one approach, a memory cell array has memory cells programmed to store weights for performing the multiplication. Voltages are applied to the memory cells. Each voltage represents one or more input bits to be multiplied by one of the weights. Output currents from the memory cells are accumulated in a common bitline. A sum of the output currents is digitized to provide a digital result. The digital results from several bitlines can be shifted based on bit significance and added to provide a final accumulation result from the multiplication.
1. An apparatus comprising:
a memory cell array comprising memory cells, wherein each memory cell is programmable to store a respective bit for a multi-bit weight, and each memory cell has a position in the array corresponding to a significance of its stored respective bit;
voltage drivers configured to apply respective voltages to the memory cells, wherein each respective voltage represents a one-bit input, and wherein the voltages are applied so that the memory cells do not threshold;
a first line coupled to first memory cells storing a respective bit of a first significance, wherein the first line is configured to sum first output currents from each of the first memory cells;
a second line coupled to second memory cells storing a respective bit of a second significance, wherein the second significance is less than the first significance, and the second line is configured to sum second output currents from each of the second memory cells; and
a logic circuit configured to provide an accumulation result based on the summed first output currents and the summed second output currents.
2. The apparatus of claim 1 , wherein each memory cell is programmable to vary a conductance or resistance of the memory cell, and an extent of the output current from the memory cell corresponds to the conductance or resistance.
3. The apparatus of claim 1 , wherein the memory cells are organized in horizontal tiers of cells, the tiers are stacked vertically above a semiconductor substrate, and first memory cells in a first tier are selected and coupled to the line for performing the multiplication.
4. The apparatus of claim 3 , wherein the memory cells are resistive random access memory (RRAM) cells with a local selector, RRAM cells without a local selector, NAND flash memory cells, or NOR flash memory cells.
5. The apparatus of claim 3 , wherein the memory cells are programmable by varying charge stored in a floating gate or a charge trap of each memory cell.
6. The apparatus of claim 3 , wherein the memory cells are programmable by varying a resistance of each memory cell.
7. The apparatus of claim 1 , further comprising:
at least one digitizer configured to provide a first result based on the summed first output currents, and provide a second result based on the summed second output currents;
at least one shifter configured to at least shift the first result; and
at least one adder configured to provide the accumulation result by at least adding the shifted first result to the second result.
8. The apparatus of claim 7 , wherein the first result is left shifted by one bit.
9. The apparatus of claim 1 , wherein the accumulation result is from multiplication of a column of multi-bit weights by a column of input bits.
10. The apparatus of claim 1 , wherein the memory cells are phase-change memory cells, NAND flash memory cells, or NOR flash memory cells.
11. An apparatus comprising:
a memory cell array comprising memory cells, wherein each memory cell is programmable to store a respective bit for a multi-bit weight, and each memory cell has a position in the array corresponding to a significance of its stored respective bit; and
a multiplier-accumulator unit configured to perform a multiplication of the multi-bit weight by a multi-bit input and provide an accumulation result, wherein:
the memory cells are kept in a sub-threshold mode during the multiplication;
each bit of the multi-bit input is applied to the multiplier-accumulator unit at a respective time instance to obtain a respective result, wherein the respective result is based on summing output currents from the memory cells; and
the accumulation result is based on the respective results obtained from applying the bits of the multi-bit input.
12. The apparatus of claim 11 , wherein summing the output currents comprises summing currents on lines for columns of memory cells in the memory cell array to provide column results for the applied bit.
13. The apparatus of claim 12 , wherein the multiplier-accumulator unit comprises shifters and adders configured to combine the column results to provide the respective result for each applied bit.
14. The apparatus of claim 11 , wherein:
each respective result is shifted left according to a position of the applied bit of the multi-bit input to provide shifted results; and
the shifted results are added to provide the accumulation result.
15. An apparatus comprising:
a memory cell array comprising memory cells, wherein each memory cell is programmable to store a respective bit for a multi-bit weight, and each memory cell has a position in the array corresponding to a significance of its stored respective bit;
voltage drivers configured to apply voltage pulses to the memory cells, wherein a respective width of each pulse represents a multi-bit input;
a first line coupled to first memory cells storing a bit of a first significance, wherein the first line is configured to collect first output currents from each of the first memory cells for accumulating a first charge;
a second line coupled to second memory cells storing a bit of a second significance, wherein the second significance is less than the first significance, and the second line is configured to collect second output currents from each of the second memory cells for accumulating a second charge; and
a logic circuit configured to output a result based on the accumulated first charge and the accumulated second charge.
16. The apparatus of claim 15 , wherein the memory cells are kept in a sub-threshold mode while the voltage pulses are applied.
17. The apparatus of claim 15 , further comprising a first capacitor coupled to the first line to accumulate the first charge, and a second capacitor coupled to the second line to accumulate the second charge.
18. The apparatus of claim 15 , further comprising at least one digitizer to provide a first result based on the accumulated first charge, and a second result based on the accumulated second charge, wherein the result from the logic circuit is determined at least based on shifting the first result and adding the shifted first result to the second result.
19. The apparatus of claim 15 , wherein each voltage pulse has a same constant voltage.
20. The apparatus of claim 15 , further comprising at least one integrator to provide a first result based on the accumulated first charge and a second result based on the accumulated second charge, wherein the result from the logic circuit is based on the first and second results.