| Since flash technology was matured enough, flash memories have dominated the non-volatile memory market, and they are facing limitation on further development. Rank modulation is a new data representation method which can be used in flash memories and other types of non-volatile memories. This data representation uses the relations of a group of memory cells levels, instead of using the absolute levels in conventional memory devices. Through this technology, there are many advantages to utilize the ranks of a group of memory cells, so rank modulation data representation technology provides a new option for memory devices to improve their performances. Because the data representation is different from the conventional method, retrieve data from the memory cell is one of the most important research topics. This research started from suggesting rank modulation memory architectures to provide practical memory architecture and required functions. Then, two different types of rank read circuits were designed and tested in this to read the data stored in cells. The rank read circuits was designed by Winner-Take-All(WTA) and current comparators which are able to compare levels. WTA is a circuit that is capable of identifying the highest input signal among multiple inputs. Current comparator is a device that compares two currents and outputs a digital signal indicating which is larger. The three different rank modulation architectures with different rank read schemes were designed and tested to explore performance in each system. In addition, retention test was performed to know the rank variation over time in aging device. The test results showed that the charge gain or loss is not individual mechanism, but much more block wide mechanism, which makes rank modulation more reliable memory device. |
