1. Article purpose[edit source]
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[edit source]
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[edit source]
The Cortex-M4 is a 32 bits processor that belongs to 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 single-precision floating point unit (FPU).
Refer to the STM32MP15 reference manuals for the complete list of features.
2.2. Security support[edit source]
The Cortex-M4 does not support secure mode (Arm Trustzone) : it only supports a non-secure mode that is defining the Cortex-M4 non-secure hardware execution context.
3. Peripheral usage and associated software[edit source]
3.1. Boot time[edit source]
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 How to start the coprocessor from the bootloader article
- by Linux remoteproc, by default
3.2. Runtime[edit source]
3.2.1. Overview[edit source]
The Cortex-M4 is running STM32CubeMP1 package.
The Cortex-M4 operates as a coprocessor, either autonomously like any external microcontroller (such as a STM32F4) could do, or communicating with the main processor (Cortex-A7) via RPMsg communication pipe.
3.2.2. Software frameworks[edit source]
| Domain | Peripheral | Software frameworks | Comment | ||
|---|---|---|---|---|---|
| Cortex-A7 secure (OP-TEE) |
Cortex-A7 non-secure (Linux) |
Cortex-M4 (STM32Cube) | |||
| Ecosystem | Cortex-M4 | STM32CubeMP1 package | |||
3.2.3. Peripheral configuration[edit source]
The Cortex-M4 configuration is done in two times:
- As the main processor of the system, the Cortex-A7 (running OpenSTLinux distribution) is the first one taking care of all the system resources initialization : supplies, clock tree, etc.
- Then, the STM32CubeMP1 package takes care of all the Cortex-M4 local configuration (NVIC, MPU, etc.) and it can rely on the resource manager to modify system resources without interfering with the Cortex-A7
3.2.4. Peripheral assignment[edit source]
The Cortex-M4 is the Cortex-A7's coprocessor, so it cannot be assigned but it is managing all the peripherals assigned to its context.
4. How to go further[edit source]
Refer to Arm website[1] for more detailled information on this core.
5. References[edit source]
Arm® is a registered trademark of Arm Limited (or its subsidiaries) in the US and/or elsewhere. 