Issue with STM32F4 CAN Bus Clock Settings

[LRPS_92]

Hi,

Following is my init code for stm32f4 CAN Bus Normal mode. I've Calculated the bus to work on bit rate of 500 kbps & 16 Tq's. If i connect two discovery boards with transceivers(TJA1050) on both sides, i can see CAN data getting transmitted & received. But, If i connect CAN Bus Analyzer to the harness i'm not seeing correct CAN Bus data traffic in my PC. I doubt my clock settings.

Kindly anyone confirm am i setting the CAN Bus bit rate of 500 kbps properly with my initialization?

System Clock source details(following values received usingfunction RCC_GetClocksFreq():

-------------------------------------------------------------------------------------------------------------------------

RCC Clock Source : 0x08 (PLL used as System clock) 

SYSCLK_Frequency: 168000000 

HCLK_Frequency: 168000000 

PCLK1_Frequency: 42000000 

PCLK2_Frequency: 84000000 

Init Code:

--------------

void can1_init()

{

can1_low_level_init();

hcan1.CAN_Prescaler = 21;

hcan1.CAN_Mode = CAN_Mode_Normal;

hcan1.CAN_SJW = CAN_SJW_1tq;

hcan1.CAN_BS1 = CAN_BS1_12tq;

hcan1.CAN_BS2 = CAN_BS2_4tq;

hcan1.CAN_ABOM = DISABLE;

hcan1.CAN_AWUM = DISABLE;

hcan1.CAN_RFLM = DISABLE;

hcan1.CAN_TTCM = DISABLE;

hcan1.CAN_TXFP = DISABLE;

hcan1.CAN_NART = DISABLE;

CAN_Init(CAN1, &hcan1);

pTxM.DLC = 8;

pTxM.ExtId = 0x0200;

pTxM.IDE = CAN_ID_EXT;

pTxM.RTR = CAN_RTR_DATA;

pTxM.Data[0] = 0x00;

}

[turboscrew]

With prescaler 21, your "CAN-clock" tics with 2 MHz (42000000 / 21). your Tq is then 500 ns.

If you have 16 Tqs per bit, the bit rate is 2 MHz/16 = 125 kbps.

[Tesla DeLorean · Accepted solution]

Although the example here is 2 MHz / 17 = 117647 bps

42,000,000 / 500,000 = 84

84/6 = 14

Perhaps try 1+10+3 or 1+11+2, prescale 6

[turboscrew]

Note, that the first phase is always 1 Tq - by the CAN specs and by the reference manual. It's not set in code, but it's there by default.

you only set the combined second-third phase and the fourth phase lengths.

[LRPS_92]

@turboscrew​  @Community member​  Thank you. I misunderstood CAN bus clock source as 168 MHz instead of 42 MHz.