Notecard Outboard Firmware Update

An increasing number of MCUs produced in the last decade are shipped with their primary bootloaders in ROM, unmodifiable by any user operation. On these devices, including all modern ST Microelectronics and Espressif microcontrollers, when a RESET pin is asserted, the device enters this ROM bootloader. The bootloader can load and execute code from a variety of sources including Flash, RAM, UART, USB, I2C, or SPI. This ROM bootloader's behavior is controlled by actively probing those I/O ports and by sampling the state of "strapping pins" or specially locked "boot option bytes" in flash.

These manufacturer-provided ROM bootloaders present new alternatives for hardware designers - specifically, to perform firmware updates in a manner that is far more flexible in terms of language and RTOS, and far less vulnerable to inadvertent programming bugs.

Beginning with firmware version 3.5.1, the Blues Notecard is now capable of utilizing these capabilities of modern MCUs, performing firmware updates "from the outside", and not involving the firmware running on the MCU, whatsoever. It can update firmware regardless of RTOS or language, and can be used to switch between them, even modifying flash memory layout and partitioning any time after-the-fact, at the developer's discretion.

How It Works

By using the Notecard in conjunction with a modern MCU with a ROM bootloader, you can achieve a far more robust form of firmware update that we call Notecard Outboard Firmware Update.

At a high level the process works as follows:

You ensure your hardware is using the required wiring.
You enable Notecard Outboard Firmware Update on your Notecard.
You build your firmware image file.
You upload your firmware on Notehub.
The Notecard downloads the firmware, verifies it, and performs the update.

note

Notecard Outboard Firmware Update is only compatible with the following card.aux modes:

off
dfu
neo-monitor

Required Wiring

To take advantage of Notecard Outboard Firmware Update you must lay out several connections between the Notecard's AUX pins and your own host MCU's RESET, BOOT, and UART pins.

The following carriers have the required wiring available out of the box, and are ready-made for using Notecard Outboard Firmware Update:

If you're not using a ready-made carrier, you can still utilize Outboard Firmware Update on most modern STM32 and ESP32 hosts by connecting the following pins.

Pin Mapping Table:

Notecard	MCU
AUX1*	--
AUX3	B0
AUX4	RST
AUXRX	TX
AUXTX	RX
GND	GND

note

AUX1 - Not DFU in progress (NDFU).

AUX1 has no corresponding pin on the Host MCU; instead, it is used to drive an external multiplexor (or mux). AUX1 is active LOW when a DFU is in progress, otherwise it remains HIGH.

AUX1 is not enabled by default. It must be explicitly enabled by issuing a card.aux request and specifying "mode":"dfu".

Several examples of using Outboard Firmware Update on a variety of different hosts are available in our Notecard Outboard Firmware Update Examples GitHub repository.

Enabling Notecard Outboard Firmware Update

In order to use Notecard Outboard Firmware Update you must first configure your Notecard to receive firmware updates.

Firmware is uploaded to Notehub, then downloaded from Notehub to your Notecard, and finally flashed to your host MCU. Each of these steps must be enabled in order to enable Notecard Outboard Firmware Update.

Notehub to Notecard

Downloading firmware from Notehub to the Notecard is enabled by default. Although, you may wish to ensure the transfer is enabled by explicitly sending a dfu.status request to the Notecard.


{"req":"dfu.status","on":true}

Notecard to Host MCU

To allow the Notecard to flash the host MCU with the downloaded binary, use the card.dfu request (setting "name" to the architecture of the host (e.g. stm32, esp32) and "on" to true).


{"req":"card.dfu","name":"<host_mcu>","on":true}

note

By default, the Notecard expects STM32-based hosts to have a boot pin that's assumed to be active high, where high-logic voltage indicates Boot Mode, and low-logic voltage indicates Normal Mode.

If you are using an STM32-based host with a boot pin that's instead assumed to be active low, you can send the card.dfu request a "name" of "stm32-bi" (where "bi" stands for boot inverted), to ensure Notecard Outboard Firmware Update works correctly.


{"req":"card.dfu","name":"stm32-bi","on":true}

Now that you've enabled Notecard Outboard Firmware Update, you next need to prepare your firmware image file.

Building a Firmware Image File

In this section you'll learn how to build a firmware image file for use with Notecard Outboard Firmware Update.

Generate/Collect Binaries for your Target Platform

Building your firmware binary itself is not unique to Notecard Outboard Firmware Update. This is the standard creation of a firmware binary that will be deployed to a target device. The only thing new is we will be operating on this binary (.bin) file instead of immediately installing it on the target device.

When using the Arduino IDE, you can find the location of the binary in the final logs of the compilation step, as illustrated below.

Looking in the folder specified, we will find the .bin file alongside the .elf file mentioned in the build output.

Wrapping Binaries Using Binpack

Binpack Overview

The binpack utility is provided through the Notecard CLI. Binpack is used to create a thin wrapper around your binary, which both offers protection and enables optimization of binary installation.

Binpack Construction

The syntax of the binpack utility is as follows:


notecard -binpack <host_arch> <memory_addr>:<binary.bin> [<memory_addr>:<binary.bin> ...]

<host_arch> - Replace with the architecture of your host MCU. (See the card.dfu request's name argument for a list of possible values.)
<memory_addr> - The address* where the binary should be installed.
<binary.bin> - The binary file to package.

* Minimally, the page of memory associated with the address provided will be completely erased and rewritten.

Targeting an STM32 device and performing binpack on the Arduino example provided above would result in the following syntax:


notecard -binpack stm32 0x8000000:Example1_NotecardBasics.ino.bin

After the command executes, you will see output similar to the following:


2022-10-20-205150.binpack now incorporates 1 files and is 28999 bytes:

HOST: stm32
LOAD: Example1_NotecardBasics.ino.bin,0x08000000,0x70a8,0x70a8

Alternatively, targeting an ESP32 device and performing binpack on the Arduino example provided above would result in the following syntax:


notecard -binpack esp32 0x10000:Example1_NotecardBasics.ino.bin

The two changes worth noting, are...

The <host_arch> parameter changed from stm32 to esp32
The <memory_addr> parameter changed from 0x8000000 to 0x10000

Circuit Python Example

A simple binary (.bin) might be stored at 0x08000000 on an STM32, or at 0x10000 on an ESP32. However, let's focus on something slightly more complex, like CircuitPython. CircuitPython is typically configured as a 3-part image containing a UF2 secondary bootloader, a CircuitPython interpreter, and the CircuitPython scripts.

Creating a CircuitPython Script Binary

In order for a CircuitPython script to become a candidate for Binpack, it will first need to be translated from a text file into a binary compatible with the CircuitPython interpreter. To this end, Blues has published a utility, the CircuitPython Filesystem Builder, in order to transform scripts into binaries ready for the interpreter.

Once you have installed the tool following the steps in the README file, you can invoke the tool with the following syntax:


python3 main.py <directory> <output_filename>.cpy

<directory> - The directory containing the files to store in the filesystem
<output_file> - The file that will ultimately be packaged within the .binpack file

Example:


python3 main.py my_cp_app/ scripts.cpy

note

The .cpy extension is REQUIRED to facilitate the .binpack utility.

Binpack a CircuitPython 3-part Image


notecard -binpack stm32 0x8000000:tinyuf2-swan_r5-0.10.1.bin 0x8010000:circuitpython-swan_r5.bin 0x8100000:scripts.cpy

You can see from the call to the binpack utility, the UF2 bootloader will be loaded at 0x08000000, the CircuitPython interpreter will be loaded at 0x08010000, and the Python script will be loaded at 0x08100000.

Binpack a CircuitPython 2-part Image

Alternatively, CircuitPython can be flashed as a 2-part Image. This can be useful once you have stopped iterating on your firmware, and no longer plan to flash new firmware from a USB connected laptop. The syntax to perform this operation is shown below:


notecard -binpack stm32 0x8000000:circuitpython-swan_r5-nobootloader.bin 0x8100000:scripts.cpy

As you can see, the UF2 bootloader has been elided, and instead the CircuitPython interpreter will be loaded directly at 0x08000000 and the Python script will be loaded at 0x08100000.

Uploading Firmware to Notehub

Now that you've built your firmware image file and your Notecard is ready to receive it, your last step is to upload your firmware to Notehub and send it to your devices.

To do so, see our guide on managing host firmware, which shows you how to upload a binary to Notehub, and deploy that binary to individual devices or fleets of devices.

Applying the DFU Update to the Host

Once you've queued up a firmware update in Notehub, the Notecard detect a new host binary is available on its next sync and download the firmware (up to 1.5MB) into its own flash storage.

The Notecard will then perform a RESET on the host microcontroller, which places it into its ROM bootloader. Then, using a microcontroller-specific communications protocol, the Notecard reprograms the various areas in flash as directed by instructions within the firmware image file, verifies them via MD5 hashes, and restarts the MCU.

note

The firmware update process won't begin until the Notecard next syncs with Notehub. To expedite this process while prototyping, use the hub.set API to set "mode":"continuous" and "sync":true. These settings consume more power to maintain the continuous connection, and should only be used in production deployments on high-power devices.


{"req":"hub.set", "mode":"continuous", "sync":true}