Best logic to erase fewer bytes than sector size(minimum erasable size) in flash

Question

I am using Spansion's flash memory of 16MB. The sector size is 256KB. I am using the flash to read/write/delete 30 byte blocks(structures). I have found in the data sheet of the IC that minimum erasable size is 256KB. One way of deleting a particular block is to

1. Read the sector containing the block to delete into a temporary array.
2. Erase that sector.
3. Delete the required block in temporary array
4. Write back the temporary array into Flash.

I want to ask is there any better alternative logic to this.

Answer 1

There is no way to erase less than the minimum erasable sector size in flash.

However, there is a typical way to handle invalidating small structures on a large flash sector. Simply add a header to indicate the state of the data in that structure location.

Simple example:

• 0xffff Structure is erased and available for use.
• 0xa5a5 Structure contains data that is valid.
• 0x0000 Structure contains data that is not valid.

The header will be 0xffff after erasing. When writing new data to a structure, set the header to 0xa5a5. When that data is no longer needed, set the header to 0x0000.

The data won't actually be erased, but it can be detected as invalid. This allows you to wait until the sector is full and then clean up the invalid records and perhaps compact the valid ones.

Answer 2

Firstly, check the device datasheet again. Generally Spansion devices will let you have a 64kB page size instead of 256kB. This may or may not help you, but generally increased granularity will help you.

Secondly, you cannot avoid the "erase before write" cycle where you want to change bits from 0 to 1. However, you can always change bits from 1 to 0 on a byte-by-byte basis.

You can either rethink your current 3-byte structure to see if this is of any use to you, or move to a 32-byte size structure (which is a power of 2 and so slightly more sane IMO). Then, to delete, you can simply set the first byte to 0x00 from 0xFF which a normal erased byte will be set to. That means you'll end up with empty slots.

Like how a garbage collector works, you can then re-organise to move any pages that have deleted blocks on so that you create empty pages (full of deleted blocks). Make sure you move good blocks to a blank page before deleting them from their original page! You can then erase the empty page that was full of deleted, or re-organised blocks.

When you're working with flash memory, you have to think out your read/erase/write strategy to work with the flash you have available. Definitely work it out before you start coding or locking down memory structures, because generally you'll need to reserve at least one byte as a validity byte and usually you have to take advantage of the fact that you can always changes bits that are set to 1 to 0 in any byte at any time without an erase cycle.