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VAR-SOM-MX7 - FreeRTOS for M4

1 Overview

The i.MX7 utilize both the ARM Cortex-A7 and Cortex-M4 cores for general-purpose programmable processing.

The dual-core architecture enables the device to run a rich operating system like Linux on the Cortex-A7 core and an RTOS like FreeRTOS on the Cortex-M4 core, to provide real-time responsiveness.

1.1 Multi-Core Architecture

The ARM Cortex-A7 and Cortex-M4 share the resources access using AHB/AXI buses.


Please note that:

  • Cortex-A7 always boots as the primary core.
  • Cortex-M4 does not have a boot ROM and at POR is not provided a clock
  • Cortex-A7 user code is responsible for the following:
    • Loading and authenticating Cortex-M4 firmware by HAB API or with Cortex-A7 firmware together as a unified image by boot ROM.
    • Launching the Cortex-M4 by enabling its clock and clearing its reset bit in System Reset Controller.

Further details are available in i.MX 7Dual Applications Processor Reference Manual

1.2 FreeRTOS BSP

The FreeRTOS BSP for i.MX 7Dual is a Software Development Kit that provides comprehensive software support for the i.MX 7Dual processor. The FreeRTOS BSP includes a set of peripheral drivers that aim at encapsulating peripheral register access and give users the maximum flexibility at same time, an open source event driven preemptive RTOS – FreeRTOS operating system (OS) and an open source multicore communication stack – RPMsg.

Demo and example applications are provided to demonstrate peripheral drivers, FreeRTOS kernel, RPMsg usage and to highlight the main features of the i.MX 7Dual processor. Next figure highlights the layers and features of the FreeRTOS BSP for i.MX 7Dual.

VAR-SOM-MX7 FreeRTOS BSP architecture.png

Here we describe how to use ARM GCC toolchain, but also DS-5 an IAR are officially supported following Getting Started with FreeRTOS BSP for i.MX 7Dual.

2 Prerequisites

Before starting, prepare a Yocto boot SD (Morty or newer).

3 Installing required packages

Download and install GNU-ARM bare-metal toolchain:

mkdir ~/var-som-mx7_m4-freertos
cd ~/var-som-mx7_m4-freertos
tar xvf gcc-arm-none-eabi-5_4-2016q3-20160926-linux.tar.bz2

Download FreeRTOS BSP for the SOM:

cd ~/var-som-mx7_m4-freertos
git clone -b freertos_bsp_1.0.1_imx7d-var01
cd freertos-variscite

4 Documentation

Original NXP documentation is available online or in the following folder:


5 Available demos

All of the Variscite examples are located under the following folder


Default M4 pin settings are used by demo applications:

function pin
debug UART (UART2) RX: J5.5 / TX: J5.7
Button (BACK) SW3 (GPIO1_IO11)
LED D10 (GPIO1_IO13)
CAN (CAN2) J14.1-2
I2C (I2C2) SCL: J14.7 / SDA: J14.9
ECSPI (ECSPI2) SCLK: J10.3 / MISO: J10.5 / MOSI J10.7 / CS0 J10.9

The available demos for VAR-SOM-MX7 are

path memory type
demo_apps/blinking_imx_demo TCM
demo_apps/hello_world TCM
demo_apps/hello_world_ddr DDR
demo_apps/hello_world_ocram OCRAM
demo_apps/low_power_imx7d/rand_wfi TCM
demo_apps/rpmsg/pingpong_bm TCM
demo_apps/rpmsg/pingpong_freertos TCM
demo_apps/rpmsg/str_echo_bm TCM
demo_apps/rpmsg/str_echo_freertos TCM
demo_apps/sema4_demo TCM
driver_examples/adc_imx7d TCM
driver_examples/ecspi/ecspi_interrupt/master TCM
driver_examples/ecspi/ecspi_interrupt/slave TCM
driver_examples/ecspi/ecspi_polling/master TCM
driver_examples/ecspi/ecspi_polling/slave TCM
driver_examples/flexcan/flexcan_loopback TCM
driver_examples/flexcan/flexcan_network TCM
driver_examples/gpio_imx TCM
driver_examples/gpt TCM
driver_examples/i2c_imx/i2c_interrupt_rtc_imx7d TCM
driver_examples/i2c_imx/i2c_polling_rtc_imx7d TCM
driver_examples/uart_imx/uart_interrupt TCM
driver_examples/uart_imx/uart_polling TCM
driver_examples/wdog_imx TCM

Almost all of the above demos are also available for iMX7 SabreSD.

You can build and run the demos following official NXP documentation for iMX7 SabreSD, available online or in the following pdf:


5.1 Memory types

The BSP allow linking using 3 different memory types: DDR, OCRAM, TCM.

Almost all the demos are provided for TCM.

Here is available a short summary of memory areas used by Cortex-M4 as described in related linker file.

memory type M4 memory area A7 memory area memory lentgh linker file
DDR 0x9FF00000-0x9FF07FFF 0x9FF00000-0x9FF07FFF 32kB MCIMX7D_M4_ddr.ld
OCRAM (part of OCRAM_128KB) 0x20210000-0x20217FFF 0x00910000-0x00917FFF 32kB MCIMX7D_M4_ocram.ld
TCM (TCML) 0x1FFF8000-0x1FFFFFFF 0x007F8000-0x007FFFFF 32kB MCIMX7D_M4_tcm.ld
data section (TCMU) 0x20000000-0x20007FFF 0x00800000-0x00807FFF 32kB shared among all linker files

All linker files are locate in folder


Further details about memory mapping are available in i.MX 7Dual Applications Processor Reference Manual paragraphs:

  • 2.1.2 Cortex-A7 Memory Map (pag 190)
  • 2.1.3 Cortex-M4 Memory Map (pag 192)

5.2 Special notes about I2C demos

Since we have no on-board sensors, under "driver_examples/i2c_imx" you will find 2 demos accessing the RTC.

The demos require disabling I2C2 from Linux dts. This also means that both WM8731 and OV5640 cannot be controlled on Linux side.

However this BSP aims to provide examples for testing, so if not needed, feel free to re-enable it on Linux side.

If you plan to use a different I2C bus for M4, just disable it in Linux dts and update I2C section in file:


6 Building a demo

For any demo just follow this steps:

cd ~/var-som-mx7_m4-freertos/freertos-variscite/examples/var-som-mx7_m4
cd <demo_folder>
cd armgcc
export ARMGCC_DIR=~/var-som-mx7_m4-freertos/gcc-arm-none-eabi-5_4-2016q3
./ > /dev/null

Then copy the ".bin" in the FAT32 partition of the boot SD.

7 Running a demo

A detailed step by step procedure to running each of the demos is available online or in the document


The default boot process leads U-Boot to auto detect the right dtb, assuming the M4 is not used.

To allow Cortex M4 accessing shared resources (RAM, SPI, I2C, CAN), a dedicated dtb must be loaded by U-Boot.

To enable Cortex M4 for VAR-SOM-MX7:

setenv use_m4 yes; saveenv

To disable Cortex M4 for VAR-SOM-MX7:

setenv use_m4 no; saveenv

Binary demos must be loaded to the memory type used for linking.

For all memory types the steps are the same, just the addresses are different.

load application image to memory
flush content to memory
start Cortex-M4 from memory

For DDR:

fatload mmc 0:1 0x9FF00000 demo_ddr.bin
dcache flush
bootaux 0x9FF00000


fatload mmc 0:1 0x00910000 demo_ocram.bin
dcache flush
bootaux 0x00910000

For TCM:

fatload mmc 0:1 0x007F8000 demo_tcm.bin
dcache flush
bootaux 0x007F8000

Alternatively, a dedicated "m4boot" script is already available for start M4 application, simply settings 2 variables: "m4image" and "m4bootdata".

The variable "m4image" must be initialized with the name of binary demo:

setenv m4image mydemo.bin

The variable "m4bootdata" must be initialized with the address of used memory:

setenv m4bootdata 0x9FF00000 # for DDR
setenv m4bootdata 0x00910000 # for OCRAM
setenv m4bootdata 0x007F8000 # for TCM

Finally, the "m4boot" command will do the work:

run m4boot

A real use case is available for Ping-Pong FreeRTOS demo.

8 Debugging a demo

8.1 JTAG interface

Cortex M4 debugging requires JTAG.

The JTAG Interface is exported through Test Points TP1/2/3/4/5 on VAR-SOM-MX7

TP signal description
TP1 TMS JTAG mode select
TP2 TDI JTAG data-in
TP3 TDO JTAG data-out
TP4 TCK JTAG clock


8.2 Debugging GUI

The only GUI currently supported to debug Cortex M4 applications is Lauterbach TRACE32.

The script to attach TRACE32 is available here:


Make sure that TRACE32 ICD (In-Circuit-Debugger) for ARM is installed and the Lauterbach debugger device supports ARM Cortex-A7 and ARM Cortex-M4 debugging.

Build a FreeRTOS application for the target, and change the default ELF load path in attach_imx7d_m4.cmm line 115: data.load.elf "<your ELF path>" /verify.

Connect the TRACE32 debugger device to the computer and the board (through JTAG). Run the TRACE32 ICD ARM debugger, and load attach_imx7d_m4.cmm by choosing "File -> Run Batchfile".

Run (GO) and debug the program with the single step (Step, Over, Next, Return) or break points.