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In this article we will show how to make a basic UAVCAN node and explore it in UAVCAN GUI Tool using the Zubax Babel hardware.

From software point of view we will try to keep things as simple as possible. We will use only libcanard library which a very light-weight UAVCAN implementation and stm32 standard peripheral library for stm32f37x family of controllers. Using of different HALs and any type of RTOS is avoided on purpose, as this is very basic example.


So our goal is to make Babel act as UAVCAN node, then check if it can be seen in UAVCAN GUI Tool, explore raw CAN data in Bus monitor and basically get acquainted with UAVCAN in general and libcanard in particular

Couple words about UAVCAN. UAVCAN is obviously a CAN bus where at least two nodes should present. In our case one node will be Zubax Babel and the second node - UAVCAN GUI Tool. We will need the second Zubax Babel or any other slcan compatible converter to connect UAVCAN GUI Tool to our CAN bus.

Important note. CAN bus in general needs termination resistors connected on both sides of the line. If the line is very short (10-20 cm) only one resistor may be sufficient. Сonveniently Zubax Babel has software-programmable termination resistor. If CAN bus seems not working for some reason - please check if termination resistor is activated (it can be activated using the UAVCAN GUI Tool itself).


First of all hardware initialization must be performed as libcanard stm32 driver configures CAN peripheral only and avoids clocking and gpio configuration. hw_init() function is trivial and is left out of this text. However it may be found in the demo project here.

Some global definitions must be presented in the project in any way. Here are they:


Lets begin with initialization of libcanard

Libcanard initialization
CanardInstance g_canard;   					//The library instance
static uint8_t canard_memory_pool[1024]; 	//Arena for memory allocation, used by the library

static void swInit(void)
    int result = 0;
    CanardSTM32CANTimings timings;
    result = canardSTM32ComputeCANTimings(RCC_Clocks.PCLK1_Frequency, 1000000, &timings);
        __ASM volatile("BKPT #01"); 
    result = canardSTM32Init(&timings, CanardSTM32IfaceModeNormal);
        __ASM volatile("BKPT #01"); 
    canardInit(&g_canard,                           // Uninitialized library instance
               canard_memory_pool,                // Raw memory chunk used for dynamic allocation
               sizeof(canard_memory_pool),        // Size of the above, in bytes
               onTransferReceived,                // Callback, see CanardOnTransferReception
               shouldAcceptTransfer,              // Callback, see CanardShouldAcceptTransfer
    canardSetLocalNodeID(&canard, 100);

canardSTM32Init and canardSTM32ComputeCANTimings are stm32-specific driver functions intended to simplify CAN peripheral setup.

Important note: libcanard stm32 can driver does not use interrupts or DMA. Its up to user to decide if CAN interrupts are needed and to implement them.

Libcanard is a static library and does not uses heap, so it needs some memory for operation which user must give it manually. That is uint8_t canard_memory_pool[1024]. Libcanard also needs two functions that must be implemented by user:

  • shouldAcceptTransfer - this callback is called every time a transfer received to determine if it should be passed further to library or ignored. Here we should filter out all messages that are needed for our particular task
  • onTransferReceived - this callback is called every time a transfer is received and accepted in shouldAcceptTransfer. Its a good idea to put incoming data handlers here

Here are these functions:

bool shouldAcceptTransfer(const CanardInstance* ins,
                          uint64_t* out_data_type_signature,
                          uint16_t data_type_id,
                          CanardTransferType transfer_type,
                          uint8_t source_node_id)
    if ((transfer_type == CanardTransferTypeRequest) &&
        (data_type_id == UAVCAN_GET_NODE_INFO_DATA_TYPE_ID))
        *out_data_type_signature = UAVCAN_GET_NODE_INFO_DATA_TYPE_SIGNATURE;
        return true;
    return false;
void onTransferReceived(CanardInstance* ins, CanardRxTransfer* transfer)
    if ((transfer->transfer_type == CanardTransferTypeRequest) &&
    (transfer->data_type_id == UAVCAN_GET_NODE_INFO_DATA_TYPE_ID))

As it is obvious from shouldAcceptTransfer our node will accept only one type of messages:

  • UAVCAN_GET_NODE_INFO_DATA_TYPE_ID - this is a request that UAVCAN GUI Tool sends to all nodes that it discovers to get some data like name, software version, hardware version and so on from it. In fact this is optional, but supporting this type of messages is a good idea.

Besides receiving UAVCAN messages each node must also broadcast at least one type of messages periodically - NodeStatus (once in every 100-1000 ms should be fine). So let's make a function for that.

#define CANARD_SPIN_PERIOD    1000

void canard_service(void)
	static uint32_t service_time = 0;  
    if(get_uptime() < service_time + CANARD_SPIN_PERIOD) return; // to enter this function only once a period
    service_time = get_uptime();
    gpio_toggle(GPIOE, GPIO_Pin_8); //some indication 

    uint8_t transfer_id = 0;



To make node status message we will have to compose it manually. For that we will need three values:

  • Uptime in seconds.
  • Node health. Our node will always be 100% healthy.
  • Node mode. Our node will always be in operational mode.

These values have to be encoded according to NodeStatus message description:

void makeNodeStatusMessage(uint8_t buffer[UAVCAN_NODE_STATUS_MESSAGE_SIZE])
	uint8_t node_health = UAVCAN_NODE_HEALTH_OK;
	uint8_t node_mode   = UAVCAN_NODE_MODE_OPERATIONAL;
    memset(buffer, 0, UAVCAN_NODE_STATUS_MESSAGE_SIZE);
    uint32_t uptime_sec = (get_uptime() / 1000);
    canardEncodeScalar(buffer,  0, 32, &uptime_sec);
    canardEncodeScalar(buffer, 32,  2, &node_health);
    canardEncodeScalar(buffer, 34,  3, &node_mode);

After UAVCAN GUI Tool receives this message first time it will try to get more info about the new node, so we also have to implement handler that will form GetNodeInfo message and send it back to UAVCAN

#define APP_VERSION_MAJOR   					99
#define APP_VERSION_MINOR   					99
#define APP_NODE_NAME   						"com.zubax.babel.demo"
#define GIT_HASH								0xBADC0FFE
#define UAVCAN_GET_NODE_INFO_RESPONSE_MAX_SIZE  ((3015 + 7) / 8)

uint16_t makeNodeInfoMessage(uint8_t buffer[UAVCAN_GET_NODE_INFO_RESPONSE_MAX_SIZE])

    buffer[7] = APP_VERSION_MAJOR;
    buffer[8] = APP_VERSION_MINOR;
    buffer[9] = 1;  // Optional field flags, VCS commit is set
    uint32_t u32 = GIT_HASH;
    canardEncodeScalar(buffer, 80, 32, &u32); 

    const size_t name_len = strlen(APP_NODE_NAME);
    memcpy(&buffer[41], APP_NODE_NAME, name_len);
    return 41 + name_len ;

void canardGetNodeInfoHandle(CanardRxTransfer* transfer)
    uint16_t len = 0;
    len = makeNodeInfoMessage(buffer);
    int result = canardRequestOrRespond(&g_canard,

App architecture

As libcanard does not use any interrupts and because our intention to keep everything simple the application will be organised as ordinar cycle

int main(void)
    /*!< At this stage the microcontrollers clock setting is already configured, 
       this is done through SystemInit() function which is called from startup
       file (startup_stm32f37x.s) before to branch to application main.
       To reconfigure the default setting of SystemInit() function, refer to
       system_stm32f37x.c file */
    RCC_GetClocksFreq(&RCC_Clocks); //To make sure RCC is initialised properly
    SysTick_Config(SystemCoreClock / 1000); //To make systick event happen every 1 mS

The only interrupt used in the application is SysTick interrupt for uptime counter with 1 mS resolution.

void systickIsr(void);
uint32_t getUptime(void);

static uint32_t  uptime = 0;

uint32_t getUptime(void)
    return uptime;

void systickIsr(void)

As libcanard does not use any interrupts it is up to user when and how to receive and transmit UAVCAN messages. In this application we will constantly poll if any message was received by MCU CAN peripheral and process it. We will also poll if library has any new messages to transmit and manually extract them from the library and pass to CAN transmitter.

void canardSend(void)
    const CanardCANFrame* txf = canardPeekTxQueue(&g_canard); 
        const int tx_res = canardSTM32Transmit(txf);
        if (tx_res < 0)         // Failure - drop the frame and report
            __ASM volatile("BKPT #01"); 
        if(tx_res > 0)
        txf = canardPeekTxQueue(&g_canard); 
void canardReceive(void)
    CanardCANFrame rx_frame;
    int res = canardSTM32Receive(&rx_frame);
        canardHandleRxFrame(&canard, &rx_frame, get_uptime() * 1000);

Now its time to try the app. The demo IAR project may be found here. After you flash your Zubax Babel with it you must see green LED blinking once a second. If you then connect it to UAVCAN GUI Tool you should see something like this: 

It may be also useful to go to bus monitor and check if messages are coming properly 

Messages from node with ID 100 are present in the picture above. And they keep appearing once a second. This means everything works like planned.

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