PRELIMINARY
Recovering From Over-Erasure (Flash-Write Operation)
2.7 Recovering From Over-Erasure (Flash-Write Operation)
Generally, not all bits in the flash array have the same amount of charge re- moved with each erase pulse. By the time all bits have reached the VER1 read margin (and erase is complete), some of the bits in the array may be over- erased. They are said to be in depletion mode. If even one single flash cell is over-erased into depletion mode, it is always read as logic 1 and can corrupt the reading of other bits. This condition must be detected and corrected, be- cause it also inhibits reprogramming of the flash array.
The 'F20x/F24x flash array employs the flash-write operation to recover bits that are erased into depletion mode. The flash-write operation is similar to the erase operation in that it affects all bits in the array simultaneously. This en- ables recovery of multiple bits from depletion mode, but requires the flash- write operation to be followed by the clear and erase operations to restore the erase margin on all bits.
A flash-write pulse is the time during the flash-write operation between the set- ting and the clearing of the EXE bit (bit 0 of SEG_CTR). During the flash-write pulse, all array bits are affected simultaneously. (See Figure 2±1, Flash Memory Logic Levels During Programming and Erasing, on page 2-4 for an illustration of this mechanism.) Multiple flash-write pulses may be required to fully recover all bits in the array, and the application of flash-write pulses is con- trolled by the flash-write algorithm.
The flash-write operation uses the inverse-erase read mode and inverse- erase reference level to detect bits that are in depletion mode. For more infor- mation about the inverse-erase read mode, see section 2.4, Read Modes, on page 2-12.
PRELIMINARY
Flash Operations and Control Registers | 2-15 |
