The Crazyradio dongle is based on a Nordic semiconductor nRF24LU1 chip. The radio communication is done using the Nordic “Enhanced ShockBurst™” packet protocol in PTX mode with acknoledge. Variable size packet, from 1 to 32 bytes, can be send and acknoledged by the copter. The acknoledgement packet can contains a payload from 0 to 32Bytes.

This page document the protocol used in the version 0.40 of the Crazyradio dongle. Future version (up to 1.0) will be kept compatible.

The USB devices have the VID/PID couple 0x1915/0x7777.

The radio dongle being configured in PTX (emmiter) mode it sends packet to the copter and waits for the acknowledge. The acknowledge can contains a Payload which is the mean to receive data. If the auto acknowledge is disable there is no IN transaction.

To send a packet the following sequence must be followes:

• Send the packet to EP1_OUT. Its length should be between 1 to 32Bytes
• Read the ACK from EP1_IN. The first by is the transfers status and the following bytes are the content of the ACK payload, if any.

The status byte contains flags indicating the quality of the link:

Crazyradio vendor requests summary:

The nrf24L chips provides 126 Channels of 1MHz from 2400MHz to 2525MHz. The channel parameter shall be between 0 and 125 (if not the command will be ignored).

The radio channel is set as soon as the setup transaction is completed which takes about 1ms. The new frequency is going to be used for the following transfered packets. Default value for the radio channel is 2.

The packet sent by the radio contains a 5 bytes address. The same address must be configured in the receiver for the communication to work.

The address must follow the requirement of the section 6.4.3.2 of the nRF24LU1 documentation:

Addresses where the level shifts only one time (that is, 000FFFFFFF) can often be detected in
noise and can give a false detection, which may give a raised Packet-Error-Rate. Addresses
as a continuation of the preamble (hi-low toggling) raises the Packet-Error-Rate.

The default address is 0xE7E7E7E7E7.

Possible values for the datarate:

After sending a packet the radio automatically waits for an acknowledge. ARD and ARC permits to configure the delay the radio waits for the acknowledge and the number of time the transfer will be retried in case the acknowledge is not received in that delay.

The delay ARD depends of the length, in second, of the ack packed. This depends of the datarate and the Payload length contained in the Ack packet. The ARD can be configured either by step of 250us or by ack payload length. If the ack payload length is configured the time will be recalculated automatically even if the datarate is changed. To set the Ack payload lenght the bit 7 of Ard must be set (length | 0x80)

Possible values for ARD:

ARC configures the number of time the radio will retry a transfer if the Ack has not been received, it can be set from 0 to 15.

By default ARD=32Bytes (0xA0) and ARC=3.

By default Crazyradio is configured with auto ack enable. It means that after transmiting a packet the radio waits for an acknoledge from the receiver. This setting permits to deactivate waiting for the Ack packet so that the packet will be sent only one time and there are no guarantee that it has been correctly received.

The nRF24L radio chip provides a test mode in which a continous non-modulated sine wave is emited. This permits, among other things, to test the quality of the RF elements of the board. When this mode is activated the radio dongle does not transmit any packets.

while the continuous carrier mode is active it is possible to set channel and power to change the frequency and power of the emitted wave.

This command is used to launch the Nordic semiconductor USB bootloader. After sending this command a USB reset shall be emitted which will trigger the dongle to start the bootloader. After sending this command the Dongle is only waiting for a USB reset which means that any other commands or data will be ignored.

The bootloader is pre-loaded by Nordic Semi. in the nRF24LU1 chip at the address 0x7800. The Crazyradio firmware will jump to it when the sequence LAUCH_BOOTLOADER following by a USB reset is executed. The bootloader will appear at VID/PID of 0x1915/0x0101. See nRF24LU1 datasheet for the bootloader documentation.

A PC client for the bootloader is part of the Crazyflie ground station program.