Read Disturb 101
When choosing memory for applications, one consideration engineers weigh is the number of read and write operations that are expected. In some cases, applications have very few write cycles. For example, in gaming, it is usually only necessary for software to be loaded onto the memory card, and then only read operations are required when end users play the game. Although the absence of write operations means there are fewer demands on storage, read disturb errors are still possible. As such, it’s necessary for engineers to choose memory solutions that are optimized to reduce the likelihood of these read disturb problems.
Causes of Read Disturb Errors
Within NAND flash memory, read disturb errors generally happen for two reasons. One is a biasing condition that happens during read operations. The cells in NAND memory are strung together in a line that is similar to a multi-input NAND gate. When a read operation is initiated, the appropriate word line is biased to the read threshold voltage. The word lines not involved in the read operation are biased to high voltage simultaneously. This allows the selected cells’ state to move to the sense amplifier by creating a fully conducting NAND string. A side effect of biasing is the programming of the unselected word lines. As that same read cycle is performed repeatedly, the programming of the word lines will increase, ultimately triggering a sensing error.
Another common cause of read disturb errors is cross-coupling noise. Cross-coupling noise occurs in nearby cells, and it gets worse as usage of the memory device increases. Errors caused by cross-coupling noise also increase in high temperatures. As temperatures increase, charges to and from memory cells also increase.
Reducing the Risk of Read Disturb Errors
Just as there are two main causes of read disturb errors, there are two solutions that can be integrated into memory devices to reduce them. Devices can incorporate an ECC error detection capability that will trigger re-writes of important data any time a particular operational threshold is met.
Read wear leveling is another tool that can fight read disturb errors. Read wear leveling keeps track of read disturb errors per block of cells. When the count reaches a pre-determined number, the entire block of data is re-written to a different block, completely refreshing the block affected by the read disturb errors.
If you’re concerned about the impact of read disturb errors on your industrial application, discuss your needs with the product team at Delkin. We can provide custom storage solutions to meet your application’s requirements.