Issue caused by brownout during Flash erase

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Issue caused by brownout during Flash erase

If there is an accidental power loss or supply voltage drop or unexpected reset on C90FL

Flash, (i.e., brownout) that occurs during a Flash erase operation, the Flash blocks that are

erased are left a not deterministic state, but it depends on the step where the erase

operation has been interrupted.

Typically to recover the Flash block(s) that are interrupted to a working state again, users

can simply perform an erase operation on the Flash block(s). However, there are two cases

that require special attention.

2.1

Recover from non-correctable ECC errors

If a brownout occurs during Flash erase operation the bits in the block(s) remain and non-

correctable ECC (error correcting code) errors appear. Note that in C90FL Flash, a single-

bit error correction and double-bit error detection (SEC-DED) ECC code are used.

For example, if the brownout occurs during the program step in the erase operation, many

Flash pages, including the corresponding ECC bits, are left programmed. Note that all zeros

are not a valid ECC codeword, and hence this causes non-correctable ECC errors when

they are reading those Flash block(s). Similarly the brownout can occur after the program

step but in the middle of the erase step.

Even at this state, the Flash block(s) can still be erased to recover it. However, some Flash

programmer tools (e.g. Lauterbach) may read Flash while executing a code from RAM

before performing an erase operation. As a result, it generates ECC error and hence an

exception, and if the Flash programmer tool does not have proper exception handler

implemented, it may cause the code execution to hang in the Flash programmer tool and

thus cause the erase operation to fail.

So to recover from this state, users need to be aware of possible Flash programmer tool

failure for erase operation caused by Flash ECC exception. Users can either use the

FlashErase function provided in the RPC56xx/SPC56xx C90FL Flash Standard Software

Driver to erase the interrupted Flash block(s), or use Nexus/JTAG debugger script to simply

toggle the low-level Flash register bits to perform the erase operation. Please refer to

Appendix A for an example Lauterbach Trace32 script for erasing C90FL Flash.

2.2

Recover from depleted bits

It is also possible that a brownout during Flash erase operation leave the bits in the Flash

block(s) erased at an over-erased or depleted state. For instance, if the brownout occurs

after the erase step but before the compaction and software program step to complete, it is

possible to be left in that state.

Depending on how much depleted the bits are, the excessive column leakage caused by the

bits might cause the following program operation to fail due to suppressed drain bias. Note

that the first step in an erase operation is a program step, and thus for this case, the erase

operation to recover the interrupted Flash block(s) might fail. This appears to users as if the

Flash block(s) cannot be erased and recovered.