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FreeRTOS on the Cortex-M4 of a Colibri VF61

 
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FreeRTOS™

The NXP/Freescale VF6xx SoC which is the core of the Colibri VF61 module implements a heterogeneous asymmetric architecture. Besides the main CPU core based on the ARM Cortex-A5 processor, a secondary general purpose ARM Cortex-M4 core is available too. The secondary core typically runs a RTOS optimized for microcontrollers or a bare-metal application. Toradex provides FreeRTOS™, a free professional grade real-time operating system for microcontrollers, along with drivers and several examples which can be used on our Colibri VF61 platform. The FreeRTOS™ port is based on NXP FreeRTOS BSP for i.MX 7.

The FreeRTOS source code is available at: http://git.toradex.com/cgit/freertos-toradex.git/ branch colibri-vf61-m4-freertos-v8

Overview

The Cortex-M4 CPU core lives side by side with the Cortex-A5 based primary CPU core. Both CPU complexes have access to the same interconnect and hence have equally access to all peripherals (shared bus topology).

There are several types of memory available. The Cortex-M4 provides local memory (Tightly Coupled Memory, TCM), which is relatively small but can be accessed by the CPU without any latency. There are multiple OCRAM areas (On-Chip RAM, typically SRAM) which are relatively fast as well and slightly larger. The third option is the DDR3 based main memory. From a performance perspective one of the internal areas should be selected whenever possible.

A traditional microcontroller typically has internal NOR flash where the firmware is stored and executed from. This is not the case on Colibri VF61: There is no NOR flash where the firmware can be flashed onto. Instead, the firmware needs to be stored on the mass storage device such as SD-card or the internal NAND flash. The available mass storage devices are not "memory mapped", and hence application can not be executed directly from any of the cores (no eXecuted-In-Place, XIP). Instead, code need to be loaded into one of the available memory sections before the CPU can start executing it.

The Colibri VF61 always boots using the Cortex-A5 core. The core executes the internal boot ROM which typically loads a boot loader such as U-Boot. The boot loader allows loading the firmware from the mass storage device (e.g. NAND flash) into memory, and triggers the Cortex-M4 to start executing the firmware. To upgrade or replace a firmware, one can just replace the firmware binary on the mass storage device.

Memory areas

The two CPU platforms use a different memory layout to access individual sub systems. This table lists some important areas and their memory location for each of the cores side by side. The full list can be found in the Vybrid reference manual.

Region Size Cortex-A5 Cortex-M4 (System Bus) Cortex-M4 (Code Bus)
Boot ROM 96KB 0x00000000-0x00018000 0x00000000-0x00018000
DDR Address 2048MB (less for M4) 0x80000000-0xffffffff 0x80000000-0xdfffffff 0x00800000-0x0fffffff (maps to 0x80800000-0x8fffffff)
OCRAM 512KB 0x3f000000-0x3f07ffff 0x3f000000-0x3f07ffff 0x1f000000-0x1f7fffff
TCML 32KB 0x1f800000-0x1f808000 0x1f800000-0x1f808000
TCMU 32KB 0x3f800000-0x3f808000 0x3f800000-0x3f808000

Get the FreeRTOS Source Code

The FreeRTOS source code is available on our git server. Use git to clone the repository:

# git clone -b colibri-vf61-m4-freertos-v8 git://git.toradex.com/freertos-toradex.git

This table shows how the FreeRTOS BSP source code is structured:

Directory Content
doc/ NXP/Freescales FreeRTOS BSP Documentation
examples/ Application examples
examples/vf6xx_colibri_m4/ Examples ported to Toradex Colibri VF61
middleware/multicore/open-amp/ OpenAMP based RPMsg stack (remote messaging framework)
platform/ Driver library, startup code and utilities
platform/CMSIS/ Cortex Microcontroller Software Interface Standard (CMSIS) ARM Cortex®-M header files, DSP library source
platform/devices/VF6XX/linker/ Linker control files for each supported toolchain
platform/drivers/ Peripheral Drivers
platform/utilities/ Utilities such as debug console
rtos/FreeRTOS/ FreeRTOS Kernel folder