The Technology
Reading NAND Flash, as well as some other types of memory cells, entails applying a reference voltage, waiting, and then sensing a certain voltage to determine whether then threshold voltage of the cell was higher or lower than the reference voltage. (This relative value determines whether a capacitor is discharge, resulting in zero senses voltage, or not.) The waiting time is determined by the maximum discharge time, which in turn varies from chip to chip, among areas within a chip, with the age of a chip and its usage intensity, with temperature, etc. So, chip manufacturers specify a “safe” waiting time, which is often much longer than necessary.
Our approach entails waiting a shorter time before sensing the voltage. This may result in read errors but does not destroy the data. Moreover, we observe that the error is in one direction: a cell that appears discharge will remain discharged, so the only possible error is that a cell that appears not to have been discharged has not yet been discharged. Our solution to this is to store, along with an N-bit page of data, logN bits that serve as a signature. Specifically, the signature is the number of cells whose threshold voltage should be above the reference voltage. (This is known at writing time.) At read time, both the page and the signature are read, the number of above-reference cells is counted and is compared to the signature. It can be shown that they will match only if neither the data nor the signature are erroneous due to the premature sensing. Upon disagreement, the reading is repeated with a longer waiting time. So, at the cost of (logN X number of levels) additional bits, we can read much faster without introducing undetected error. Finally, information regarding the current appropriate waiting time for each page or group of pages can be maintained in the background, thereby adapting waiting time to the actual capabilities of the chip at any given time. The cost in negligible.
Advantages
- Faster reading time, adapted to the actual capabilities of a chip
Applications and Opportunities
- Any memory chip
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