How to load bin to Cortex-M4 in Google Coral Dev

Currently I'm trying to load hello_world.bin to cortex m4 from u-boot in Google coral dev

What I've tried?

Bring up Coral dev target
- Followed steps in https://coral.ai/docs/dev-board/get-started/
- Able to get mendel OS prompt in /dev/ttyUSB0(host)->ttymxc0(target)
- Able to mdt shell
- Target ready with u-boot->kernel image->rootfs
SDK for i.MX8MQ
- Download SDK from https://mcuxpresso.nxp.com/
- For processor 'MIMX8MQ6xxxJZ' (auto selects Boards - EVK-MIMX8MQ)
- Build hello_world.bin for Cortex M4 (TCM based). Uses UART2 in BOARD_InitBootPins

$ tar -xzf SDK_2.8.0_MIMX8MQ6xxxJZ.tar.gz
$ cd boards/evkmimx8mq/demo_apps/hello_world/armgcc
$ export ARMGCC_DIR=<path to>/gcc-arm-none-eabi-9-2020-q2-update/
$ export PATH=$PATH:<path to>/gcc-arm-none-eabi-9-2020-q2-update/bin
$ ./build_debug.sh 
$ ls debug/hello_world.bin

Load hello_world.bin from u-boot
- I've placed my bin in tftpboot location.

Hit any key to stop autoboot:  0 
u-boot=> tftp 0x7e0000 192.168.0.33:/tftpboot/coral/boot/hello_world.bin
Using ethernet@30be0000 device
TFTP from server 192.168.0.33; our IP address is 192.168.0.133
Filename '/tftpboot/coral/boot/hello_world.bin'.
Load address: 0x7e0000
Loading: ####
         2.3 MiB/s
done
Bytes transferred = 16700 (413c hex)
u-boot=> bootaux 0x7e0000
## Starting auxiliary core at 0x007E0000 ...

But I'm not able to see any output on the second terminal ttyUSB1
Expected output should be Hello World on the second terminal

What I've Observed

In Coral-Dev-Board-baseboard-schematic.pdf
- According to Baseboard block diagram, UART2/3 is connected to MicroUSB via converter
  - But UART1(ttymxc0) is also available on micro USB
  - After kernel boot UART3(ttymxc0) is available on micro USB
- But after kernel boot, I noticed UART3 to be available on second enumerated port
- For example,

// On coral dev
root@wishful-zebra:~# ls -las /dev/ttymxc*
0 crw------- 1 mendel tty     207, 16 Aug 20 03:45 /dev/ttymxc0
0 crw-rw---- 1 root   dialout 207, 17 Aug 20 03:05 /dev/ttymxc1
0 crw-rw---- 1 root   dialout 207, 18 Aug 20 03:40 /dev/ttymxc2

root@wishful-zebra:~# stty -echo raw speed 115200 < /dev/ttymxc2
115200
root@wishful-zebra:~# echo "test" > /dev/ttymxc2 

// prints 'test' on /dev/ttyUSB1 host side
Welcome to minicom 2.7.1

OPTIONS: I18n 
Compiled on Aug 13 2017, 15:25:34.
Port /dev/ttyUSB1, 23:55:15

Press CTRL-A Z for help on special keys

test

Question

How to load firmware to Cortex M4 on Google Coral Dev?
Which i.MX SDK port to use for Google Coral Dev?
What are the UARTs available on the micro USB?
Which UART is used for Cortex M4 in Google Coral Dev

Solution

Here are the instructions to boot the imx8m M4 and run the TFLite Micro Hello World (outputs a sine wave) on it: https://coral.googlesource.com/mcuxpresso_sdk/+/refs/heads/master/boards/evkmimx8mq/demo_apps/hello_world_tflite/.

you'll need to sync the project repo and rebuild linux-imx and uboot-imx.

Output from the M4 core will be visible on the second UART that enumerated when you connected your serial port via USB.