Voltage random access memory (VRAM)
An integrated circuit memory cell and voltage ladder design that adapts techniques typically applied to Static Random Access Memory (SRAM) circuits to implement a compact array of analog Voltage Random Access Memory (VRAM) locations. The memory cells in the VRAM each store a digital value that controls a corresponding switch. The switch couple a particular voltage from a set of voltages generated by the ladder, to be output when that location is enabled. Multiple analog output voltages are provided by simply providing additional rows of cells.
1. An analog Voltage Random Access Memory (VRAM) for storing a digital value in a memory location and causing a particular voltage to be output when that location is accessed, comprising
a resistor ladder network, coupled to at least one voltage reference, for providing a plurality of reference analog voltages at a plurality of nodes therein; and
a compact array of a plurality of cells, each cell containing a storage device and a corresponding analog reference voltage switch, the storage device connected to control the state of the analog reference located in the cell, and each such switch coupled between a node in the resistor ladder network corresponding to a pre-selected one of the reference analog voltages, and an analog voltage output point.
2. An apparatus as in claim 1 wherein the ladder comprises a first set of resistors connected in series to provide the set of reference analog voltages.
3. An apparatus as in claim 2 wherein additional resistors are arranged in parallel with one or more of the resistors in the first set to provide one or more fine reference analog voltages.
4. An apparatus as in claim 1 wherein the storage device in at least one cell stores a single digital bit.
5. An apparatus as in claim 4 wherein the single bit storage device is a pair of cross coupled inverter gates.
6. An apparatus as in claim 1 wherein multiple cells are connected to the same node in the resistor ladder network so as to be available to produce multiple analog output voltages.
7. An apparatus as in claim 1 wherein each storage device in the array of cells is addressable.
8. An apparatus as in claim 7 wherein each storage device in the array of cells is addressable by row and column decoders.
9. An apparatus as in claim 1 additionally comprising:
a first cell, having stored therein digital information to produce a coarse analog voltage;
a second cell, having stored therein digital information to produce a fine analog voltage; and
a difference amplifier, connected to receive the coarse analog voltage and the fine analog voltage, to produce the analog output voltage as a difference there between.
10. An apparatus as in claim 1 coupled to provide a plurality of reference voltages in a charge to digital converter.