Memory device with column select being variably delayed
A memory device ( 10 ) includes an array ( 12 ) of memory cells arranged in rows and columns. Preferably, each memory cell includes a pass transistor coupled to a storage capacitor. A row decoder ( 18 ) is coupled to rows of memory cells while a column decoder ( 14 ) is coupled to columns of the memory cells. The column decoder ( 14 ) includes an enable input. A variable delay ( 32 ) has an output coupled to the enable input of the column decoder ( 14 ). The variable delay ( 32 ) receives an indication (R/W′) of whether a current cycle is a read cycle or a write cycle. In the preferred embodiment, a signal provided at the output of the variable delay ( 32 ) is delayed if the current cycle is a read cycle compared to if the current cycle is a write cycle.
1. A memory device comprising:
a plurality of storage cells arranged in an array or rows and columns;
a plurality of bitlines, each bitline coupled to a plurality of storage cells along one of the columns;
a plurality of wordlines, each wordline coupled to a plurality of storage cells along one of the rows;
a column decoder having a plurality of control outputs, each of the control outputs coupled to a column of the array; and
a variable delay circuit coupled in series between the plurality of control outputs of the column decoder and the columns of the array, the delay circuit causing the control outputs of the column decoder to be variably delayed based upon a control input, such that the control outputs are delayed longer for a read cycle than for a write cycle.
2. The memory device according to claim 1 , wherein the amount of delay time generated by the variable delay circuit is programmable.
3. The memory device according to claim 1 , wherein the amount of delay time generated by the variable delay circuit is selected from only one of a first delay time or a second delay time only.
4. The memory device according to claim 1 , wherein each storage cell includes a transistor coupled in series with a storage capacitor.
5. The memory device according to claim 4 , wherein the memory device comprises a double data rate (DDR) synchronous DRAM (SDRAM).
6. The memory device according to claim 1 and further comprising:
a plurality of address pins;
a column address buffer with a plurality of inputs coupled to the plurality of address pins, the column address buffer further including an output coupled to an input of the column decoder;
a row decoder having a plurality of control outputs, each of the control outputs coupled to a row of the array; and
a row address buffer with a plurality of inputs coupled to the plurality of address pins, the row address buffer further including an output coupled to an input of the row decoder.
7. The memory device according to claim 1 , wherein the delay circuit comprises:
a clock input;
a first transmission gate with an input coupled to the clock input, the first transmission gate comprising an n-channel transistor coupled in parallel with a p-channel transistor;
a first delay element with an input coupled to an output of the first transmission case, the first delay element including at least one inverter;
a second transmission gate with an input coupled to an output of the first delay element, the first transmission gate comprising an n-channel transistor coupled in parallel with a p-channel transistor;
a second delay element with an input coupled to an output of the first delay element, the second delay element including at least one inverter;
a third transmission gate with an input coupled to an output of the second delay element, the third transmission gate having an output coupled to an output of the second transmission gate, the third transmission gate comprising an n-channel transistor coupled in parallel with a p-channel transistor; and
logic circuitry adapted to generate a delay time, wherein the amount of delay is variable by activating particular combinations of the first, second and third transmission gates.
8. The memory device according to claim 7 , wherein the amount of delay time generated by the variable delay circuit is selected from one of a first delay time or a second delay time only, wherein the first delay time corresponds to the first delay element and the second delay time corresponds to the first and second delay elements.
9. The memory device according to claim 7 , wherein the delay circuit includes two paths, a first one of the paths including the first and second delay elements, the delay circuit further including two tri-state drivers controlled by a signal received at the variable delay control input, the two tri-state drivers controlling the output of a signal from one of the two paths.
10. The memory device of claim 7 , wherein the delay circuit further comprises a fourth transmission gate with an input coupled to the clock input and an output coupled to the outputs of the second and third transmission gates, wherein the amount of delay is variable by activating particular combinations of the first, second, third and fourth transmission gates.
11. The memory device of claim 10 , wherein the outputs of the second, third and fourth transmission gates comprise an output of the variable delay circuit, the output of the variable delay circuit being coupled to a clock input of the column decoder.
12. The memory device according to claim 1 , wherein the variable delay comprises:
a first tri-state driver with an input coupled to the variable delay input, the first tri-state driver having an output coupled to the variable delay output;
a delay element with an input coupled to the variable delay input; and
a second tri-state driver with an input coupled to an output of the delay element, the second tri-state driver having an output coupled to the variable delay output.
13. The memory device according to claim 12 , wherein the delay element comprises a programmable delay.
14. The memory device according to claim 13 , wherein the delay element includes a plurality of delay segments, the delay element further including at least one control signal indicating which, if any, of the segments are included in a signal path between the variable delay input and the variable delay output.
15. The memory device according to claim 14 , wherein the delay element includes four delay segments.
16. The memory device according to claim 1 , wherein the variable delay comprises:
a first tri-state driver with an input directly connected to the variable delay input, the first tri-state driver having an output directly connected to the variable delay output;
a delay element with an input coupled to the variable delay input; and
a second tri-state driver with an input coupled to an output of the delay element, the second tri-state driver having an output coupled to the variable delay output.
17. The memory device according to claim 1 , wherein the control input receives an indication of whether a current cycle is a read cycle or a write cycle, the indication of whether the current cycle is a read cycle or a write cycle being based upon an externally received read/write signal.