Analog memories utilizing ferroelectric capacitors
A ferroelectric memory having a plurality of ferroelectric memory cells, each ferroelectric memory cell including a ferroelectric capacitor is disclosed. The ferroelectric memory includes read and write lines and a plurality of ferroelectric memory cell select buses, one select bus corresponding to each of the ferroelectric memory cells. Each of the ferroelectric memory cells includes first and second gates for connecting the ferroelectric memory cell to the read line and the write line, respectively, in response to signals on the ferroelectric memory cell select bus corresponding to that ferroelectric memory cell. A write circuit causes a charge to be stored in the ferroelectric capacitor of the ferroelectric memory cell currently connected to the write line, the charge having a value determined by a data value having at least three states. A read circuit measures the charge stored in the ferroelectric capacitor of the ferroelectric memory cell currently connected to the read line to generate an output value, the output value corresponding to one of the states.
1. A memory circuit comprising
a ferroelectric capacitor characterized by a stored charge that is stored in said ferroelectric capacitor when said ferroelectric capacitor is not powered, said stored charge representing a stored data value
a reset circuit that causes said stored charge to be equal to a predetermined stored charge;
a write circuit that receives a plurality of input data values after said reset circuit caused said stored charge to be equal to said predetermined stored charge, converts each of said plurality of input data values to a corresponding charge that is independent of said stored charge in said ferroelectric capacitor and causes said corresponding charges to be added to said stored charge; and
a read circuit that determines said stored charge and generates a sum data value indicative of a sum of said plurality of input data values.
2. The memory of claim 1 wherein said read circuit comprises
a capacitor reset circuit for setting said ferroelectric capacitor to a predetermined state of polarization; and
a circuit that measures a charge leaving said ferroelectric capacitor when said ferroelectric capacitor polarization is set to said predetermined state of polarization.
3. The memory of claim 1 wherein said read circuit comprises a circuit that measures an amount of charge needed to cause said ferroelectric capacitor to enter a predetermined state of polarization.
4. The memory of claim 1 wherein said ferroelectric capacitor is characterized by a maximum charge, Q max , that can be stored in said ferroelectric capacitor, and wherein said write circuit comprises
a capacitor charging circuit;
a write capacitor that can be alternately connected to said capacitor charging circuit and said ferroelectric capacitor, said write capacitor being connected to said capacitor charging circuit when one of said input data values is received and charged to a voltage that depends on said one of said input data values; and
a ferroelectric capacitor write circuit that disconnects said write capacitor from said capacitor charging circuit after said write capacitor has been charged and then connects said write capacitor to said ferroelectric capacitor such that a charge in said write capacitor is transferred to said ferroelectric capacitor, said charge being less than Q max for at least two of said input data values.
5. A method for storing data in a ferroelectric capacitor that stores a remnant charge, said method comprising:
resetting said ferroelectric capacitor to a predetermined polarization state, characterized by a reset remnant charge that is stored in said ferroelectric capacitor, said reset remnant charge being representative of an initial data value;
receiving a plurality of input data values;
converting each of said input data values to a corresponding charge that is independent of said remnant charge currently stored in said ferroelectric capacitor; and
causing each of said corresponding charges to be added to said remnant charge such that said remnant charge represents a sum of said initial data value and input data values after said corresponding charges have been added.
6. The method of claim 5 further comprising:
resetting said ferroelectric capacitor to a predetermined polarization state after said plurality of corresponding charges have been added to said remnant charge;
measuring a sum charge that flows from said ferroelectric capacitor during said resetting; and
converting said sum charge to an output data value equal to said sum of said initial data value and input data values.
7. The method of claim 5 further comprising:
measuring an amount of charge that must be added to said remnant charge in said ferroelectric capacitor to cause said ferroelectric capacitor to enter a predetermined state of polarization; and
determining an output data value equal to said sum of said initial data value and input data values from said measured amount of charge.