STM32U535NET3QTR datasheet(23/278 Pages) STMICROELECTRONICS

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DS14217 Rev 1

23/278

STM32U535xx

Functional overview

3.4.1

Flash memory protection

The option bytes allow the configuration of flexible protections:

•

write protection (WRP) to protect areas against erasing and programming. Two areas

per bank can be selected with 8-Kbyte granularity.

•

RDP (readout protection) to protect the whole memory, has four levels of protection

available (see Table 3 and Table 4):

–

Level 0: no readout protection

–

Level 0.5: available only when TrustZone is enabled

All read/write operations (if no write protection is set) from/to the nonsecure flash

memory are possible. The debug access to secure area is prohibited.

Debug access to nonsecure area remains possible.

–

Level 1: memory readout protection

The flash memory cannot be read from or written to if either the debug features

are connected or the boot in RAM or bootloader are selected. If TrustZone is

enabled, the nonsecure debug is possible and the boot in SRAM is not possible.

Regressions from Level 1 to lower levels can be protected by password

authentication.

–

Level 2: chip readout protection

The debug features, the boot in RAM and the bootloader selection are disabled.

A secure secret key can be configured in the secure options to allow the

regression capability from Level 2 to Level 1. By default (key not configured), this

Level 2 selection is irreversible and JTAG/SWD interfaces are disabled. If the

secret key was previously configured in lower RDP levels, the device enables the

RDP regression from Level 2 to Level 1 after password authentication through

JTAG/SWD interface.

Note:

In order to reach the best protection level, it is recommended to activate TrustZone and to

set the RDP Level 2 with password authentication regression enabled.

Table 3. Access status versus protection level and execution modes when TZEN = 0

Area

RDP

level

User execution

(boot from flash memory)

Debug/boot from RAM/ bootloader(1)

Read

Write

Erase

Read

Write

Erase

Flash main memory

Yes

No(4)

Yes

N/A

System memory (2)

Yes

N/A

Option bytes(3)

1Yes

Yes(4)

N/A

Yes

Yes(4)

N/A

2Yes

No(5)

N/A

OTP

1Yes

Yes(6)

N/A

Yes

Yes(6)

N/A

2Yes

Yes(6)

N/A

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