Arm Cortex-M4

Applicable for

STM32MP15x lines

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1 Article purpose

The purpose of this article is to:

• briefly introduce the Arm^® Cortex^®-M4 core and its main features
• indicate the level of security supported by this processor

2 Peripheral overview

The Arm Cortex-M4 is seen as a coprocessor on STM32MP15, where the Arm Cortex-A7 is the main processor that controls it. The Cortex-M4 is present across all the STM32MP15x lines.

2.1 Features

The Cortex-M4 is a 32-bit processor that belongs to the Armv7E-M architecture family. Armv7E-M corresponds to the ARMv7-M architecture, with DSP extension. Among a wide range of features, it includes a memory protection unit (MPU) and a single-precision floating point unit (FPU).

Refer to the STM32MP15 reference manuals for the complete list of features.

2.2 Security support

The Cortex-M4 does not support secure mode: it only supports a non-secure mode that defines the Cortex-M4 non-secure hardware execution context.

3 Peripheral usage and associated software

3.1 Boot time

The STM32CubeMP1 package execution startup on the Cortex-M4 coprocessor is controlled by the Cortex-A7, that is running the OpenSTLinux distribution.

The coprocessor startup can be done at two different stages of the boot chain:

• by U-Boot SSBL, as explained in the How to start the coprocessor from the bootloader article
• by Linux remoteproc, by default

Thanks to a specific OP-TEE trusted application (TA) running on the Arm^® TrustZone and to the ETZPC peripheral, it is possible to authenticate the Cortex^®-M4 firmware and install it on isolated MCU RAM to ensure its integrity during the execution. For details, please refer to How_to_protect_the_coprocessor_firmware article.

3.2 Runtime

3.2.1 Overview

The Cortex-M4 runs the STM32CubeMP1 package.

The Cortex-M4 operates as a coprocessor, either autonomously as any external microcontroller (such as an STM32F4) could, or communicating with the main processor (Cortex-A7) via the RPMsg communication pipe.

3.2.2 Software frameworks

Domain	Peripheral	Software components			Comment
		OP-TEE	Linux	STM32Cube
Ecosystem	Cortex-M4			STM32CubeMP1 package

3.2.3 Peripheral configuration

The Cortex-M4 configuration is done in two steps:

1. As the main processor of the system, the Cortex-A7 (running OpenSTLinux distribution) first takes care of the initialization of all system resources: supplies, clock tree, and so on.
2. The STM32CubeMP1 package then takes care of all the Cortex-M4 local configuration (NVIC, MPU, and so on). It can rely on the resource manager to modify system resources without interfering with the Cortex-A7.

3.2.4 Peripheral assignment

The Cortex-M4 is the Cortex-A7's coprocessor, so it cannot assign peripheral to its usage but it manages all the peripherals assigned to its context.

4 How to go further

Refer to the Arm website for more detailed information on this core.