STCOM05 datasheet(29/58 Pages) STMICROELECTRONICS

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STCOM

Description

Level 4: secure for customer loader

The JTAG is blocked and it is not possible to access the embedded Flash for the

external read or write operation via the JTAG, while the read and write access is

allowed to the CortexTM-M4 code (with the exception of the shadow sector). With

respect to the Level 2, there is no load or upgrade of the firmware image from the

external NVM. The firmware image shall be already present in the embedded Flash

(loaded in Level 1) and a customer application (secondary bootloader) can manage

upgrades.

Level 1*: secure erase and unlocking

The Cortex™-M4 is accessible through the JTAG and it's possible to download

firmware in the embedded Flash. Before the unlocking, the embedded Flash is fully

erased including the customer OTP data. The shadow sector is not accessible.

In order to respect these security requirements, the STCOM embeds a bootloader code.

The user can also configure the boot mode through the BOOT0/1 pins to select the proper

mode. The following boot modes are available:

Normal mode: this is the standard way of booting the code and eventually load (or

restore) a new image version from the external NVM.

Customer OTP write mode: in this mode the user can write its security keys and data

(e.g.: custom EUI48). After booting in this mode the security level is 2 (secure) or 4

(secure for customer loader) based on the selected option. In the security level 2 only

ciphered firmware can be loaded from an external memory while in the security level 4

the firmware upgrade is left to a customer bootloader.

Unlocking mode: this is the boot mode that enables again all the debug feature.

Low power mode: the minimal boot mode to guarantee low power operations. The

RTE is not enabled and the CortexTM-M4 code is executed as soon as possible at 24

MHz (the user can then scale the clock up or down).

The relationship between the security level and boot mode are shown in Table 5:

Table 5. Boot mode and security level relationship

Boot

mode

Level 1

Level 2

Level 3

Level 4

Level 1*

Normal

mode

Yes

If lock option

selected for some

area → go to level 3

Yes, but no update

is performed on

locked sector

Yes

(no update is

performed)

Yes

Customer

OTP write

mode

Yes → go to level

2 or 4

Unlocking

mode

Yes → go to level

Low power

mode

Yes

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