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Crazy RealTime Protocol

The protocol used to communicate with the Crazyflie is called the Crazy RealTime Protocol, or CRTP in short. It's a simple protocol mainly designed to suit the radio chip used on the Crazyflie but the protocol can also be used on other physical links.

This page and it's sub-pages describe the protocol itself, not the implementation of some of the more complex functions such as logging and parameters.

Each packet has a 1 byte header and can carry up to 29 bytes data payload. The header field has the following layout:

   7    6    5    4    3    2    1    0
|       Port        |  Link   |  Chan.  |


  • Port: Used to identify the functionality or task that is associated with this message
  • Link: Reserved for future use
  • Channel: Used to identify the sub-task/functionality


CRTP is implemented in 3 layers:

 |   Application/ports   |
 | CRTP packets handling |
 |       CRTP Link       |
  • CRTP Link is responsible for transferring the packets between the Crazyflie and the host. It will typically handles packet size and error detection.
  • CRTP Packet handling delivers the packet to the right subsystem in the Crazyflie and in the host control software.
  • Application/ports represents the subsystems that sends and receives messages.

Port allocation

Current port allocation:

Port Target Used for
0 Console Read console text that is printed to the console on the Crazyflie using consoleprintf
2 Parameters Get/set parameters from the Crazyflie. Parameters are defined using a macro in the Crazyflie source-code
3 Commander Sending control set-points for the roll/pitch/yaw/thrust regulators
5 Log Set up log blocks with variables that will be sent back to the Crazyflie at a specified period. Log variables are defined using a macro in the Crazyflie source-code
14 Client-side debugging Debugging the UI and exists only in the Crazyflie Python API and not in the Crazyflie itself.
15 Link layer Used to control and query the communication link

Physical layers

Below are details on how CRTP is implemented for different physical links. Currently only the radio link is working but there's some legacy support for the UART still in the code.

2.4 GHz NordicSemi Radio

The wireless interface will add additional header data like CRC when sending the packet and therefore the packet sent to the wireless interface is only a CRTP packet.

USB port

This is currently not implemented but it will potentially replace the serial port link. The use of this is to get CRTP at higher speeds than the radio can transmit.

Serial port

This is mostly for legacy support but has been used during development to log data at higher speeds. The support for using the serial port might be broken currently though.

|  0xAA  |  0xAA  | Header | Length | Data0  | Packet | CRC32  |
Field Byte Description
Start 0-1 Start token for synchronization (0xAAAA)
Header 2 The destination channel
Size 3 The size of the packet
Data 4-34 The data in the packet
CRC 35-36 The CRC16 of the packet
projects/crazyflie/crtp.1361513232.txt.gz · Last modified: 2015-07-15 16:30 (external edit)