w25qxx

A no_std driver and simulator for the W25Q family of SPI flash chips (W25Q32, W25Q64, W25Q128, etc.).

Design

The Flash trait defines the core operations. Driver logic is written against the trait, so it works against both the MockFlash simulator and a real SPI backend.

pub trait Flash {
    fn read(&self, addr: u32, buf: &mut [u8]);
    fn write(&mut self, addr: u32, data: &[u8]);
    fn erase_sector(&mut self, addr: u32);
}

MockFlash

MockFlash<const CAPACITY: usize> simulates flash hardware constraints in memory:

• Write can only flip bits from 1 → 0 (&=). Writing to a non-erased location silently corrupts data, matching real hardware behavior.
• Erase resets an entire 4KB sector to 0xFF. Minimum erase unit — individual pages cannot be erased.
• Read has no size or alignment constraints. Panics if the read exceeds capacity (real hardware wraps around to 0x000000).

use w25qxx::{Flash, MockFlash, SECTOR_SIZE};

const N_SECTORS: usize = 16;
let mut flash = MockFlash::<{ SECTOR_SIZE * N_SECTORS }>::new();

flash.erase_sector(0x0000);
flash.write(0x0000, &[0xDE, 0xAD, 0xBE, 0xEF]);

let mut buf = [0u8; 4];
flash.read(0x0000, &mut buf);
assert_eq!(buf, [0xDE, 0xAD, 0xBE, 0xEF]);

Hardware constraints

Property	Value	Notes
Page size	256 B	Maximum write unit
Sector size	4 KB	Minimum erase unit (16 pages)
Block size	64 KB	16 sectors — erase_block not yet implemented
Endurance	100,000 erase cycles per sector

Flash memory model

Flash can only flip bits 1 → 0 on write. Flipping bits back to 1 requires erasing the entire sector. This means writing to an already-written location requires a read-modify-erase-write cycle:

1. Read the full 4KB sector into RAM
2. Modify the target bytes in the buffer
3. Erase the sector
4. Write all pages back

Next steps

• Implement W25Q128 SPI backend (impl Flash for W25Q128<SPI>)
• Add erase_block (64KB)