Can't write STM32 Flash with 8 bits

Question

I can't program the STM32 Flash with 8 bits of data. I have a STM32F103C8T6 and I'm trying to write in flash, I saw in HAL_FLASH libraries and I took some of the code to write in a single byte of the flash.

HAL_FLASH_Unlock();
/* Proceed to erase the page */
SET_BIT(FLASH->CR, FLASH_CR_PER);
WRITE_REG(FLASH->AR, FLASH_START_ADDR);
SET_BIT(FLASH->CR, FLASH_CR_STRT);

HAL_Delay(200);
CLEAR_BIT(FLASH->CR, FLASH_CR_PER);

SET_BIT(FLASH->CR, FLASH_CR_PG);
*((__IO uint8_t *)(FLASH_START_ADDR)) = 0x9;

HAL_Delay(200);
CLEAR_BIT(FLASH->CR, FLASH_CR_PG);

HAL_FLASH_Lock();

The loop is a simple LED blinking to see if STM get an error trying to do the flash thing. I can't write in flash with the line:

*((__IO uint8_t *)(FLASH_START_ADDR)) = 0x9;

When I change it to:

*((__IO uint16_t *)(FLASH_START_ADDR)) = 0x9;

It works, but I don't know why.

Answer 1

The flash hardware is built so that it writes 16 bits at a time, presumably with no option to write more or fewer.

The processor hardware is built so that it knows how wide a write is being requested, depending on the flavor of the store instruction (STR, STRB, STRH). It communicates the width of the write to the memory bus that is connected to the flash hardware.

Somewhere in the hardware -- I think it's the responsibility of the peripheral (i.e, the flash hardware) something detects that you're trying to do a byte write rather than a half word (16-bit) write, and flags an error. I'm not going to dig for the details, but I'm 99.44% sure that it gets 'seen' by the processor as an illegal memory access.

So, the bottom line is -- the flash hardware won't let you do that no way no how, so don't bother trying with that processor.