1. Intro
This document contains instructions to run run the SAI low power audio demo on the i.MX 8M Plus EVK.
Here, the RPSMG to allows audio to be passed from the A53 cluster running Linux to the M7 core. The latter controls the on board WM8960 audio codec, which is connected to a 3.5 mm audio jack that allow us to play music using headphones.
I will show the necessary steps to make the demo work and will add some GStreamer examples to demonstrate the demo's capabilities.
TBD: update this with a nice diagram that depicts the A53 and M7 RPMSG channel.
2. Requirements
Hardware
- MX 8M Plus EVK
- Headphones with 3.5 mm audio jack
- Type-C power supply for i.MX 8M Plus EVK
- Micro USB to USB adapter cable
Software
- A recent prebuilt Linux BSP image from NXP.com ( we tested this on 5.15.35 and 5.15.5 releases)
- Windows 10 or Ubuntu 20.04 Workstation
- MCUXpresso SDK for i.MX 8M Plus ( available from: Welcome | MCUXpresso SDK Builder (nxp.com))
3. Reference documentation for this example
MCUXpresso SDK
[1] Getting Started with MCUXpresso SDK for EVK-MIMX8MP
Available within the MCUXpresso SDK package: \{INSTALL PATH}\SDK_X_X_X_EVK-MIMX8MP\docs
[2] SAI low power audio README file
Contains instructions for the SAI Low Power Audio Demo.
Available within the MCUXpresso SDK package: \{INSTALL PATH}\SDK_X_X_X_EVK-MIMX8MP\boards\evkmimx8mp\demo_apps\sai_low_power_audio
4. Downloading a pre-built Linux BSP image for the i.MX 8M Plus
I will make use of the prebuilt Linux Image for the i.MX 8M Plus EVK for demonstrating the demo works. At the moment of writing this time, I used the 5.15.32 release, although there are older releases like 5.10.5 that I tested and proved to work with no issues.
This SAI Low Power Audio Demo shall work for other processors on the i.MX 8M family. Although specific instructions ( e.g. load address for M-core binary load) might require some adaptation. For M-core load address, please refer to the specific MCUXpresso SDK documentation for each processor.
The prebuilt Linux image (5.15.32) for the i.MX 8M Plus EVK can be downloaded from here:
https://www.nxp.com/webapp/Download?colCode=L5.15.32_2.0.0_MX8MP&appType=license
You can download other releases from here: Embedded Linux for i.MX Applications Processors | NXP Semiconductors . Select a version and a board and select download.
5. Flashing the BSP image
If you are using an Ubuntu 20.04 workstation, I recommend you to flash the image using dd. For this, you can refer to the i.MX Linux User's Guide:
Section - 4.3.2 Copying the full SD card image - https://www.nxp.com/docs/en/user-guide/IMX_LINUX_USERS_GUIDE.pdf
sudo dd if=.wic of=/dev/sdx bs=1M && sync
NOTE: when using dd, ALWAYS, double check the of device that you are about to writing. Messing up with another location or partition will harm your system
If you are following this document on a Windows machine:
You can use the Universal Update Utility (UUU) to flash your image on either the board's eMMC or SD card.
Document named UUU.pdf shall serve as your reference guide for further instructions and flashing examples. It is available along with UUU binary here: https://github.com/NXPmicro/mfgtools/releases
Two examples are shown below for your convenience:
SD card flash
uuu -b sd_all bootloader rootfs.sdcard.bz2
eMMC flash
uuu -b emmc_all bootloader rootfs.sdcard.bz2
uuu uuu.auto
NOTE: UUU is also compatible with Ubuntu
NOTE: there are other engineers who like to use BalenaEtcher for flashing their BSP images. I have tested it and works on both Ubuntu and Windows 10 machines.
6. Preparing the BSP and booting up M7 core using U-Boot
I am writing this upon the instructions contained on the README file for the low power audio example [2]. Instructions ready to copy and paste will follow:
Instruct U-Boot to pass to the kernel the rpmsg device tree to enable communication between the A53 cluster and the M7 one:
u-boot=>setenv fdtfile imx8mp-evk-rpmsg.dtb
u-boot=>saveenvLoad the M7 example:
u-boot=>setenv mmcargs 'setenv bootargs ${jh_clk} console=${console} root=${mmcroot} snd_pcm.max_alloc_per_card=134217728'
u-boot=>saveenvNow, we need to load the M4 with the demo. Refer to [1] for further information. If running the BSP on an SD card, make sure the example binary is listed on the boot partition as follows:
fatls mmc 1:1You shall see something similar to this:
imx8mp_m7_TCM_sai_low_power_audio.bin
Open the serial terminal emulator for the M7. Out of the fourth ports listed when we plug the i.MX 8M Plus serial debug cable to the PC, the M7 is typically the last one listed.
I like doing the previous step so I can see the result of the next commands issued in U-boot to load the M7 image.
fatload mmc 1:1 0x48000000 imx8mp_m7_TCM_sai_low_power_audio.bin;
cp.b 0x48000000 0x7e0000 20000;
bootaux 0x7e0000Here is an screenshot that shows how the U-Boot's response should look:
That should have prompted the following message on the M7 terminal:
Now, let’s move to user space!
u-boot=> boot7. Testing the example using a simple GStreamer pipeline
As soon as the O.S. finishes booting. We can see that M7 terminal prompts the following:
Confirm that the WM8960 is listed as audio card as follows:
cat /proc/asound/cards
Make note of the list. The wm8960 is listed a the third sound card.
This is where I like to differ a bit from [2] and I suggest a quicker test in case of not having an audio file ready. We just simply use GStreamer to play an audiotest source. Please make sure to plug in your headphones onto the board’s 3.5 mm jack before.
The following GStreamer pipeline is using the WM8960 as an audiosink.
gst-launch-1.0 audiotestsrc ! alsasink device=hw:3NOTE: please be cautious and not put the headphones directly in your head at the first attempt. The sound can be too loud to some people.
This is what you should see on the M7 side:
Stop the GStreamer pipeline issuing CTRL + C. M7 shall warn you about that:
NOTE: you can use the aplay command to play audio as shown on [2]. However, I consider using a testsrc is much quicker and flexible for a quick test.
8. Additional information
Feel free to go ahead and tweak the GStreamer pipeline to change audio test source properties. audiotest src.
This command will let you know the available options:
gst-inspect-1.0 audiotestsrc
NOTE: you can navigate through the displayed list using the “d”key. Press “q’’ to quit.
For example:
For example, I am reproducing sound using a different setup based on the list above:
gst-launch-1.0 audiotestsrc freq=4000 volume=0.8 wave=8 ! alsasink device=hw:39. Errata and future updates
TBD:
- Add an example on how to define the default audio card and play the audio either using gst-play or building the pipeline using filesrc
- Comment on the limitations of the M7 core regarding sample rate and audio formats
