This codebase is intended to use with the NXP blhost tool. In this instance, a Raspberry Pi 3B+ is used to communicate with an NXP RT685-EVK. The EVK is setup as described below so that the host (the Raspberry Pi) can toggle the necessary pins and the process can proceed without user intervention.

The Raspberry Pi is loaded with the latest version of Raspian, Bullseye. This uses version 3.9 of Python and version 2.01 of the NXP SPSDK. Since the Raspberry Pi does not contain onboard flash, the user must supply a microSD which can be used with the Raspberry Pi. To image the SD, the simplest way to proceed is by using the imager provided by Raspberry Pi. The image chosen for this implementation was the Bullseye. The imager selection is shown below

When the image is installed, update the system using the following:

sudo apt update
sudo apt upgrade -y
sudo apt-get install rpi.gpio

If there is any difficulty with the network, the IP address may have to be changed according to the interface or SSID. This can be done by right clicking on the desktop icon for the wireless network in the top right corner and setting the IP address appropriately.

The last install command above will install the python scripts for interacting with the onboard GPIO. The script provided uses to GPIO on the board to 1) reset the EVK and 2) toggle the ISP pin. The pin layout on the Raspberry Pi 3B is as follows:

For this application, we are using the GPIO pins located pins 3 and 5. In the application, the BCM numbering is used where pin 3 is referred to as GPIO 2 and is used to toggle the ISP line and pin 5 is GPIO 3 and is used to toggle the reset line.

As the outputs from the Raspberry Pi are 3.3 V when high, they must be reduced to the VDDIO range (1.8 V) for the NXP in order to not create issues with VCCIO. For the ISP2 line, the input is attached to the ground side of switch 5 which is tied to a 1 kOhm resistor. To reduce the input from 3.3 to 1.8, simply run the Raspberry Pi output through approximately 830 Ohms. For the reset line, use a voltage dvidier where R1 is 10 kOhms and R2 is 12 kOhms.

The UART port that the Raspberry Pi can be one of the following:

• /dev/ttyACM0
• /dev/ttyAMA0

NXP provides a set of python scripts to communicate with the bootloader via a host such as a PC or an SoC. The application, called blhost, is provided with the secure provisioning SDK, described as a "unified, reliable, and easy to use Python SDK library".

To install the SDK on the Raspberry Pi, run the following in a terminal to 1) create a Python virtual environment and 2) install the secure provisioning toolkit (SPSDK):

python3 -m venv .venv
source .venv/bin/activate
python -m pip install --upgrade pip
pip install spsdk

Once the scripts are installed and the virtual environment has been activated, the blhost command can be used directly from the command line to communicate with ROM bootloader once the ISP pins are configured properly.

Given the correct BOOT_CFG register settings, the system will boot based upon the in-system programming pins ISP0-2. The proper setting for the 4 bits in BOOT_CFG to boot from the ISP pins is 0x0. The pins for ISP are defined as follows:

• ISP0 - PIO1_15
• ISP1 - PIO1_16
• ISP2 - PIO1_17

The default setting of the Audio EVK and the standard EVK is to boot from FlexSPI Port B. The two settings for the ISP pins relevant to our application are as follows:

Unfortunately, the only pin that needs to be toggled is ISP2 and this pin is not available on any header. For access to ISP2, bring the ground side out of the ISP2 pin. The diagram below is from the RT685 EVK drawings and shows the layout and connections of SW5. Place the line that will be pulled out closest to R95 and set the position of switch 3 to the ON position (the default position).

The script communicates with the ROM bootloader via the Python scripts provided by the SPSDK. The serial communication is configured to run at 921.6 kbaud. The repo provides an image that when booted into, allows the user to toggle the blue LED using SW2. In order to properly load the SPL or any other application via blhost python script:

1. Hold MCU in reset. On the EVK, the reset pin can be routed to the Arduino header J29 pin 3. The signal is routed to the header by shorting JP14.
2. Set the pinout for ISP 2 from the default low to high. This pin is not available via any header on either EVK so the pin will have to be brought out likely from SW5.
3. Release reset.
4. Begin programming from via UART. I have chosen J5 which which is also routed through the USART header J16. This is the USART port used by ISP mode FlexComm 0. In order to use J16, disable the connection to the Link2 proble on J5 by shorting pins 2-3 on JP21.
5. Hold system in reset.
6. Set ISP2 back to it's default setting of low.
7. Release reset.

After this process, the system should boot into the application that has been flashed.

To run the script, first the virtual environment has to be activated. Then the script itself should be called as follows:

cd [path to spdk]/spsdk
source .venv/bin/activate
cd [path to scripts]

python src/test-py.py --comm_port /dev/ttyACM0 --baud 926100

In the call, both the comm port and baud rate values can be omitted if the user wishes only to use the default values as shown above.

When running the script, the flash config block (FCB) has to be properly configured so that the MCU can access the memory located there. After successfully configuring the block, the FCB is printed to screen and should look like the following:

FCB
==============================
46 43 46 42 00 04 01 56 00 00 00 00 00 03 03 00
01 00 01 00 01 06 00 00 00 00 00 00 01 00 02 00
01 07 00 00 01 0A 00 00 00 00 00 00 00 00 00 00
02 00 00 00 01 00 00 00 00 00 00 00 00 00 00 00
50 00 00 00 01 08 07 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 04 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
EC 07 13 07 20 0B 14 33 04 27 00 00 00 00 00 00
05 04 04 24 00 00 00 00 00 00 00 00 00 00 00 00
05 07 FA 07 20 0B 14 33 04 27 00 00 00 00 00 00
06 04 00 00 00 00 00 00 00 00 00 00 00 00 00 00
06 07 F9 07 00 00 00 00 00 00 00 00 00 00 00 00
21 07 DE 07 20 0B 00 00 00 00 00 00 00 00 00 00
72 04 00 04 00 04 03 04 00 04 01 20 00 00 00 00
72 04 00 04 00 04 02 04 00 04 01 20 00 00 00 00
DC 07 23 07 20 0B 00 00 00 00 00 00 00 00 00 00
12 07 ED 07 20 0B 04 23 00 00 00 00 00 00 00 00
72 04 00 04 00 04 00 04 00 04 01 20 00 00 00 00
60 07 9F 07 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 01 00 00 00 10 00 00 01 00 00 00 02 00 00 00
00 00 01 00 00 81 00 07 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

If the application is succefully written to, then the user should see at the end message "Flash successfully written to...".

The application(s) provided were built using the MCUXpresso IDE provided by NXP as described in the MCUXpresso Secure Provising Tool User Guide.

1. Set BOOT_HEADER_ENABLE to 0.
2. Build image. The image built will result in an axf file.
3. Use the axf with a start address of 0x80000 and build the image in the MCUXpresso Secure Provisioning application as show below

Once the image is built into a binary, this can be written to the proper place in flash using the script provided.