Bias for data retention in fuse ROM and flash memory
A storage device is provided that performs constant biasing in priority blocks, such as OTP memory blocks (fuse ROM) and flash memory blocks having a threshold number of P/E cycles. The storage device includes an OTP memory, a flash memory, and a controller. The OTP memory includes a block having a word line and a plurality of cells coupled to the word line. The flash memory includes another block having a word line and a plurality of cells coupled to this word line. The controller is configured to apply a constant bias to the word line of the OTP memory block and, in some cases to the word line of the flash memory block, between execution of host commands. As a result, lower bit error rates due to wider Vt margins may occur while system power may be saved through selective application of constant biasing.
1. A storage device, comprising:
a flash memory including a block, the block having a word line and a plurality of cells coupled to the word line; and
a controller configured to apply a constant bias to the word line between execution of host commands in response to a number of program/erase (P/E) cycles of the block being greater than or equal to a threshold.
2. The storage device of claim 1 , wherein the controller is configured to apply the constant bias to the word line following execution of a write command until execution of a read command.
3. The storage device of claim 1 , further comprising:
a one-time programmable (OTP) memory including a second block, the second block having a second word line and a plurality of cells coupled to the second word line;
wherein the controller is configured to apply another constant bias to the second word line between the execution of the host commands.
4. The storage device of claim 1 , wherein the controller is configured to determine whether the number of P/E cycles of the block exceeds the threshold.
5. The storage device of claim 4 , wherein the controller is configured to apply the constant bias to the word line in response to determining that the number of P/E cycles exceeds the threshold.
6. The storage device of claim 4 , wherein the block includes a second word line, and wherein the controller is configured to refrain from applying another constant bias to the second word line in response to determining that the number of P/E cycles does not exceed the threshold.
7. The storage device of claim 1 , wherein the constant bias is a function of the number of P/E cycles of the block.
8. The storage device of claim 1 , wherein the constant bias comprises a driven bias.
9. The storage device of claim 1 , wherein the constant bias comprises a floating bias.
10. The storage device of claim 9 , wherein the controller is configured to periodically refresh the floating bias.
11. A storage device, comprising:
a fuse read only memory (ROM) including a block, the block having a word line and a plurality of cells coupled to the word line; and
a controller configured to receive host commands from a host device, and to apply a constant floating bias to the word line between execution of the host commands.
12. The storage device of claim 11 , wherein the fuse ROM stores system information.
13. The storage device of claim 11 , further comprising:
a flash memory including a second block, the second block including a second word line and a plurality of cells coupled to the second word line;
wherein the controller is configured to determine whether a number of program/erase (P/E) cycles of the second block exceeds a threshold, and to apply another constant bias to the second word line between the execution of the host commands in response to determining that the number of PIE cycles of the second block exceeds the threshold.
14. The storage device of claim 11 , further comprising:
a charging circuit, wherein the controller is configured to apply a driven bias to the word line using the charging circuit.
15. The storage device of claim 14 , wherein the controller is configured to open the charging circuit to create a floating bias before applying the constant floating bias between the execution of the host commands.
16. The storage device of claim 15 , wherein the controller is configured to periodically close the charging circuit to refresh the floating bias after applying the constant floating bias between the execution of the host commands.
17. A storage device, comprising:
a one-time programmable (OTP) memory including a first block, the first block having a first word line and a plurality of cells coupled to the first word line;
a flash memory including a second block, the second block having a second word line and a plurality of cells coupled to the second word line; and
a controller configured to apply a first constant bias to the first word line and a second constant floating bias to the second word line between execution of host commands.
18. The storage device of claim 17 , wherein when applying the first constant bias to the first word line, the controller is configured to drive the first constant bias to the first word line using a charge pump.
19. The storage device of claim 18 , wherein the controller is configured, after charging of the cells coupled to the first word line, to float the first constant bias.
20. The storage device of claim 19 , wherein the controller is configured to drive the first constant bias to the first word line a period of time after floating the first constant bias.