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I would like to know the difference between 24-bit and 32-bit QSPI flash? why 24-bit address is used in flash?

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    \$\begingroup\$ Which QSPI chip? What does the datasheet tell about this? Have you compared 24-bit and 32-bit addressing modes? \$\endgroup\$ – Justme Nov 15 '20 at 14:33
  • \$\begingroup\$ The difference is 8 bit and 8 bits is the usual increment that memory chip tech jumps in so before 24 bits was available, a lot of devices would have been 16 bit addressing but who really cares and why? \$\endgroup\$ – Andy aka Nov 15 '20 at 14:42
  • \$\begingroup\$ QSPI Flash :- S25FL128S/S25FL256S. Datasheet didn't explain the use of addressing 24-bit and 32-bit mode. @Justme \$\endgroup\$ – SriGanesh D Nov 15 '20 at 15:51
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24 bits can address 16 megabytes. When a larger chip is made like 32 megabyte chip it must be addressed via 32 bits address. It might still have the 24-bit command to allow use of new and larger chips in older systems supporting only 24-bit commands. Also when the speed of the QSPI bus goes high enough, the 24-bit address can be sent with dummy padding bytes to allow time for the chip to fetch the data, but it can then use the faster bus to transfer it.

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  • \$\begingroup\$ what does it mean, "24 bits can address 16 megabytes"? 24 bits can address more than 16MB.! \$\endgroup\$ – SriGanesh D Nov 15 '20 at 16:09
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    \$\begingroup\$ @SriGaneshD 24 bits can address 2^24 = 16777216 units (often bytes, sometimes larger words). Using the relevant traditional (non-SI) system, one megabyte is 1024 (2^10) kilobytes and one kilobyte is 1024 (2^10) bytes. Using that convention, it takes 20 bits to address each byte in one megabyte and thus 24 bits can address 16 (2^4) megabytes. These days, the standard is to use mebibyte (MiB) to avoid confusion with SI prefixes, but the context makes Justme's usage clear IMO. See Binary Prefix \$\endgroup\$ – Extrarius Nov 15 '20 at 16:12
  • \$\begingroup\$ @SriGaneshD How much in your opinion 24 bits can address then? I don't think you will find many people agreeing with you though. \$\endgroup\$ – Justme Nov 15 '20 at 17:06
  • \$\begingroup\$ Ya. Agree with your point.@Justme \$\endgroup\$ – SriGanesh D Nov 15 '20 at 17:10

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