Syslink is the protocol used between the STM32 and NRF51 in Crazyflie 2.0. It handles low level communication on the serial port.

## Physical format

In Crazyflie 2.0 syslink is transmitted on serial port at 1MBaud. It is a packet-based protocol.

  +-----------+------+-----+=============+-----+-----+
|   START   | TYPE | LEN | DATA        |   CKSUM   |
+-----------+------+-----+=============+-----+-----+

• START is 2 bytes constant, 0xBC 0xCF.
• TYPE defines the type of packet
• LENGTH and type are uint8_t, defines the data length.
• CKSUM is 2 bytes Fletcher 8 bit checksum. See rfc1146. calculated with TYPE, LEN and DATA.

Note: Unless otherwise specified, all numbers are encoded in low-endian form.

## Packet types

Packet types is defined in the syslink.h file (in the stm32 and nrf51 firmware).

Packets are organized in groups to ease routing in firmwares:

Group Name Description
0x00 RADIO Radio related packets. For data and configuration
0x10 PM Power management
0x20 OW One wire memory access

A packet type has its group in the high nibble and the type in the low nibble. In the rest of the page packet type are written with group.

• Type: 0x00
• Data format: Raw radio packet as sent in the air

This packet carries the raw radio packet. The NRF51 acts as a radio bridge. Because the NRF51 does not have much memory and the STM32 is capable of bursting a lot of data a flow control rules has been made: The STM32 is allowed to send a RADIO_RAW packet only when one RADIO_RAW packet has been received.

The NRF51 is regularly sending CRTP NULL packet or empty packets to the STM32 to get the communication working both ways.

Note So far RADIO_RAW is the only syslink packet that has flow control constrain, all other packets can be sent full duplex at any moment.

• Type: 0x01
• Data format: One uint8_t indicating the radio channel

This packet is used only in ESB mode.

Packet sent to the NRF51 to set the radio channel to use. The NRF51 then send back the same packet to confirm that the setting has been done.

NRF51 radio channel are spaced by 1MHz from 2400MHz to 2525MHz.

• Type: 0x02
• Data format: One uint8_t indicating the radio datarate.

This packet is used only in ESB mode.

Packet sent to the NRF51 to set the radio datarate to use. The NRF51 then send back the same packet to confirm that the setting has been done.

Possible datarate:

Value Datarate
0 250Kbps
1 1Mbps
2 2Mbps

• Type: 0x03
• Data format: One uint8_t at 0 for disable, >0 for enable.

Allows to put the nRF51 in continuous wave mode. If enabled the nRF51 will disable Bluetooth advertising and set its radio to emit a continuous sinus wave at the currently set channel frequency.

Warning Continuous wave is a test mode used, among other thing, during manufacturing test. It will affect other wireless communication like Wifi and should be used with care in a test environment

• Type: 0x03
• Data format: One uint8_t indicating the last received pachet RSSI

This packet is meaningful only in ESB mode.

Packet sent 100 times per seconds to the STM32. Contains the power value of the latest received packet. The value is the same as reported by the NRF51: between 40 and 100 which means measurement of -40dBm to -100dBm.

## Power management packets

• Type: 0x11
• Format: No data

When sent by the STM32 to the NRF51, the NRF51 switch OFF the system and goes in deep sleep.

• Type: 0x13
• Format:

• Flags: Bit0: Charging. Bit1: USB Powered.
• VBAT: IEEE single float. Battery voltage.
• ISET: IEEE single float. Charge current in milli-Ampers.

The SYSLINK_PM_BATTERY_STATE packet is sent 100 times per seconds to the STM32.