One of the unique characteristic of the Crazyflie platform is the fact that decks (the Crazyflie expansion boards) are detected dynamically at startup. This makes the platform pretty much plug and play: plug a flow deck and you can fly autonomously with relative positioning, no configuration or recompilation required.

In this blog post we will present a new system we have implemented to identify and enumerate decks. This is intended to be the new default from now-on and can be used by anyone who wants to make a Crazyflie-compatible deck.

Current system: 1-Wire Memories

This is currently achieved using 1-Wire Memories. These memories can be discovered and addressed using a guaranteed unique serial number. This means that there can be as many memories as we want on the bus and they can all be discovered and read.

In the Crazyflie ecosystem, every deck has a 1-Wire memory that contains the identity of the deck. At startup, the Crazyflie discover and read all the memories which allows to discover all the decks and initialize the corresponding deck driver.

The future: DeckCtrl

The current system works very well but is has two major shortcommings:

• The 1-Wire memory we are using does not provides GPIO or any other control. It makes it impossible to control the deck “out-of-band” in order to switch it on/off or launch a bootloader for example.
• The 1-Wire memory has only one manufacturer and, because of a lot of reasons, we can only use a single model of it. At times it has caused us stress when the chip availability is low.

The new DeckCtrl-based system addresses these two problems by using a regular micro controller over I2C and provides support for GPIO as well as, in the future, UART, SPI and any other protocol that might be needed to handle the deck startup and configuration.

DeckCtrl is mainly a protocol definition. The current implementation is for STM32C011 micro controllers. While this could be implemented on any micro controllers, we will likely stick to the STM32C011 for the foreseeable future.

Deck discovery over I2C

One of the main innovations is the development of a new discovery and enumeration algorithm over I2C. Indeed, one of the main difference between I2C and 1-Wire is that 1-Wire memories are addressed using a 64Bit unique serial number while I2C uses 7 (or 10) bit addresses. This means that it is impractical to assign a unique I2C address to each device, and even if we tried to assign one address per deck type it would make it impossible to detect the same deck multiple times on a Crazyflie.

To solve that problem we designed a protocol that allows starting all DeckCtrl on the same address and enumerate them so that we can select all of them one by one, and then assign them a unique address on the bus. At a high-level this behaves very similarly to DHCP, where all devices will be assigned an address dynamically.

Once the decks have all been configured, it is possible read a memory to recover the deck identity and initialize deck drivers as we are currently doing.

Just that is a great step forward for Bitcraze: we are not dependent anymore to a single model of 1-Wire memory.

Deck life-cycle control

The second major improvement is what I called earlier “out-of-band” control of the deck. By that I mean that we can have control over the deck that is independent from the deck’s main micro controller chip. This is something we have long missed to, for example, be able to put decks in bootloader mode. The solution that has been used for the lighthouse deck is to always start in bootloader mode and then boot. This works, but has proven to be impractical for some use-cases and is not possible with all micro controllers.

The new DeckCtrl protocol defines space for GPIO control and the possibility for more in the future. This allows for example to switch ON and OFF a deck in software if one of the GPIO is used to control the deck power supply. We can also control the Reset and Boot pin of a micro controller in order to put it in bootloader mode.

Our goal there is to greatly simplify the design of more clever decks based on micro controllers. This will allow us, and anyone interested in making decks, to make more competent decks while making is easy to develop the firmware for them. For example, we will be looking at allowing to reprogram decks without having to restart the Crazyflie, which will make it much easier to develop and iterate.

Status

We have now finished the first iteration of the DeckCtrl protocol design and implementation. There is also a driver that exercises the GPIO part of it, this has been tested to work on many identical decks at the same time on a dev-deck we use for development:

This has been done in the context of the High-Power LED deck project. This future deck will feature a powerful RGBW LED which will be controlled by a micro controller onboard the deck. So it will be seen as an I2C device from the Crazyflie side. This design also shows the intent of DeckCtrl as this deck has 2 STM32C011: one implementing DeckCtrl and one application micro controller implementing the LED driver and other useful functionalities like color correction and temperature monitoring.