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Using Flash Memory

Source

Adapted from Using flash memory.

Classic Spectranet has 128K of flash memory. It is paged into the Z80 address space like SRAM, but writing it is different from writing RAM.

Flash versus RAM​

Flash is non-volatile and keeps its contents after power off. It behaves like ROM during normal reads, and ordinary memory writes do not change it.

Important flash properties:

  • erased bits read as 1
  • programming can change bits from 1 to 0
  • changing 0 back to 1 requires erasing a whole sector
  • the Am29F010 used by classic Spectranet has eight 16K erase sectors
  • each byte write or sector erase requires a command sequence

RAM is volatile and can be written byte by byte without an erase step.

Spectranet flash layout​

Flash occupies pages 0x00 to 0x1F. Pages 0x00 to 0x03 are reserved by Spectranet, and the last 256 bytes of page 0x1F store configuration such as MAC address, IP settings, and DHCP preference.

Erasing flash​

Because sectors are 16K, rewriting one 4K page requires preserving the other pages in the same sector, erasing the whole sector, and writing the required data back.

Erase and write code must run from RAM. If the CPU fetches instructions from flash during the command sequence, the operation can be disturbed or aborted. Classic Spectranet utilities copy flash programming code to RAM before executing it.

The sector erase sequence is:

  • write 0xAA to address 0x555
  • write 0x55 to address 0x2AA
  • write 0x80 to address 0x555
  • write 0xAA to address 0x555
  • write 0x55 to address 0x2AA
  • write 0x30 to the lowest address of the sector

Completion can be detected by polling the flash status bits, especially DQ7 and DQ5.

Writing flash​

Programming one byte uses this sequence:

  • write 0xAA to address 0x555
  • write 0x55 to address 0x2AA
  • write 0xA0 to address 0x555
  • write the byte to the target address

Each byte requires its own command sequence. Existing 0 bits cannot be changed back to 1 without erasing the sector, but additional 1 bits in the same byte can still be programmed to 0.