PM0073

Reading/programming the STM32Wxxx embedded Flash memory

Doc ID 17766 Rev 5

19/34

Erase procedure

The option byte erase sequence (OPTERASE) is as follows:

1.

Enable the FPEC clock.

2.

Check that no Flash memory operation is ongoing by reading the BSY bit in the

FLASH_SR register.

3.

Unlock the OPTWRE bit in the FLASH_CR register.

4.

Set the OPTER bit in the FLASH_CR register.

5.

Set the STRT bit in the FLASH_CR register.

6.

Wait for BSY to reset.

7.

Read the erased option bytes and verify.

2.3

Protections

The user area of the Flash memory can be protected against read by untrusted code. The

pages of the Flash memory can also be protected against unwanted write due to loss of

program counter contexts. The write-protection granularity is four pages.

2.3.1

Read protection

The read protection is activated by setting the RDP option byte, and then by applying a

system reset to reload the new RDP option byte.

Note:

If the read protection is set while the debugger is still connected through JTAG/SWD, apply a

POR (power-on reset) instead of a system reset (without debugger connection).

Once the protection byte has been programmed:

●

Main Flash memory read access is not allowed except for the user code (when booting

from main Flash memory itself with the debug mode not active).

●

Pages 0-3 are automatically write-protected. The rest of the memory can be

programmed by the code executed from the main Flash memory (for IAP, constant

storage, etc.); it is protected against write/erase (but not against mass erase) in debug

mode or when booting from the embedded SRAM.

●

All features linked to loading code into and executing code from the embedded SRAM

are still active (JTAG/SWD and boot from embedded SRAM) and this can be used to

disable the read protection. When the read protection option byte is altered to a

memory-unprotect value, a mass erase is performed.

●

Flash memory access through data read using JTAG, SWV (serial wire viewer), SWD

(serial wire debug) and boundary scan are not allowed.