At Bitcraze we like making decks! When we released the Crazyflie 2.0 we were really excited about the new deck connector we put on the platform. Using this, it was suddenly possible to expand the platform, adding new, more complex functionalities years after the initial release. It’s something that we really like about the Crazyflie. Over the years we’ve released a bunch of decks and also updated a few of them as new, improved sensors became available.
Deck limitations
As the number of decks has increased, along with their complexity, we’ve started noticing some limitations. We’ve touched on this a while back in this blog post where you can find more details, but here’s a short summary:
- Resource sharing in the STM32F4: Mainly DMA conflicts between different peripherals, like conflict for DMA using DSHOT on the Crazyflie 2.1 Brushless and the WS2812b driver for the LED-ring.
- Bus arbitration and performance: Some decks make excessive use of some buses, which can cause issues with certain combinations like the LPS deck and micro-SD card deck.
- Deck combinations and pins: As more interesting decks are released and we’re able to carry more weight, users want to combine more decks. Although we try to be smart with pin allocation there’s a limit on how you can combine the decks.
- MCUs on decks: As the complexity increases, separate MCUs are also included on the decks. Although working well for offloading the main MCUs on the Crazyflie, the complexity quickly increases both for usage and for development. This is something we’ve seen with the AI deck for instance, which contains 2 MCUs.
Like suggested in the original blog post, the solution will be to move more intelligence onto the decks by putting MCUs on them. These MCUs can then use the hardware on the decks without sharing resources with the main Crazyflie MCU and can also help process the data to make the communication with the Crazyflie more high level. But doing this will of course then make the issue raised in the last point even worse. To mitigate this we need to have better control over the decks. Our proposed solution for this is adding a new MCU to the decks which acts as a controller. Being able to switch on/off power, bootstrap and control various aspects of the deck. Some of the features include:
- Uniquely identify the deck and also identify the model of deck – This will replace the current 1-wire memory subsystem we have today
- Uses minimal amount of pins, basically only using the I2C pins
- Use the MCU pins as GPIOs or as various peripherals:
- GPIO expander enables switching on/off onboard power supply, simplifying bootloading and saving power. It also enables bootstrapping of onboard MCUs, which can be used for selecting what output pins to use for various functionality
- Bridge I2C to SPI/UART for bootloading of onboard MCU. This would of course be much slower than using the SPI/UART but would simplify bootloading and not need pins allocated on the expansion connector
- Bootstrapping the board makes it possible to to implement support for duplicate decks, for instance selecting what UART (1 or 2) on startup makes it possible for the driver on the Crazyflie to start up each deck and select what UART is used.
Our implementation
For our implementation of this system we have selected the STM32C011 MCU, as it comes in various packages from very small to slightly larger, so it’s possible to solder by hand. It also has the features we needed. To maximize the the flexibility of the system we don’t want to program in the I2C addresses in production. Even if you disregard the fact that the address might conflict with other I2C devices in the future, it will definitely conflict if you add two of the same type of deck.
To fix this we will automatically detect all the deck-ctrl MCUs on the bus and then move them to individual addresses. This will enable us to change the addresses in the future if there are collisions and also allows multiple instances of the same deck attached at the same time. Below is a video demonstrating this in action, implemented on custom development decks we’ve made for simplifying firmware development. Each deck runs the same deck-ctrl firmware and all 3 are auto-detected at startup.
The future
The current solution will be used on our next two upcoming decks: the new LED deck and the new Lighthouse deck. As we get closer to their release we will publish the firmware for the deck-ctrl MCU on GitHub. If the design turns out well we will keep rolling it out to future decks, increasing the flexibility of the deck expansion system even more.