CODEWITHSUNDEEP
stm32can-bus
CKocar
CKocar

Reputation: 586

What is the meaning of CANBUS function mode initilazing settings for STM32?

I want to understand meaning of the following function mode definition, there is explanation in the library. But I don't understand that because explanations are very short and not enough. I searched on the net I couldnt find any information about. 

CAN_InitStructure.CAN_TTCM = DISABLE;
    CAN_InitStructure.CAN_ABOM = DISABLE;
    CAN_InitStructure.CAN_AWUM = DISABLE;
    CAN_InitStructure.CAN_NART = ENABLE;
    CAN_InitStructure.CAN_RFLM = DISABLE;
    CAN_InitStructure.CAN_TXFP = ENABLE;

Upvotes: 1

Views: 6955

Answers (1)

Tagli
Tagli

Reputation: 2602

These are the names of the bits located in the CAN master control register (CAN_MCR). So, the proper source for their meaning is the reference manual. My following answer will be somewhat copy & paste from the reference manual, but I will try to explain these bits in detail.

TTCM (Time triggered communication mode): This bit activates the Time Triggered Communication (TTCAN) mode, which is an extension to the CAN standard. I don't know much about TTCAN, but as I understand, it assigns time windows to messages to satisfy some real-time requirements. So, normally this bit should remain 0.

ABOM (Automatic bus-off management): If the transmit error counter (TEC) becomes greater than 255, the CAN hardware switches to bus-off state. To recover, it must wait for the recovery sequence, 128 occurrences of 11 consecutive recessive bits. Only after that, the CAN hardware may return to the normal operating state. This bit controls the returning behavior. If it's 1, returning to normal state is automatic. Otherwise, software should make the request, provided that the recovery sequence has been observed.

AWUM (Automatic wakeup mode): The CAN module can be in one of 3 modes: Initialization mode, normal mode or sleep (low power) mode. Sleep mode is requested by the software. However, you have 2 options to exit sleep mode. If this bit is 0, then you have to exit sleep mode manually. You may enable CAN wakeup interrupt to inform you about bus activity, then exit the sleep mode in ISR. But if this bit is 1, the hardware returns to normal mode automatically when it detects bus activity.

NART (No automatic retransmission): Normally, CAN hardware retries to transmit a message if its previous attempts fail, because of arbitration lost etc. But if you make this bit 1, the transmitter does not retry. This is required when you use Time Triggered Communication (TTCAN). Otherwise, you should keep this bit 0.

RFLM (Receive FIFO locked mode): Your receive mailboxes have 3 levels depth, meaning that they can store maximum 3 messages before they are overrun. This bit controls what happens in case of mailbox overrun. Default behavior is to keep the oldest 2 messages and the newest one. For example, if you received 5 messages, the buffer keeps the messages 1, 2 & 5. However, if you make this bit 1, the mailbox keeps the messages 1, 2 & 3 and discards the new arrivals.

TXFP (Transmit FIFO priority): You have 3 transmit mailboxes. When you fill more than one, the hardware must decide which one to transmit first. Normally, one can assume that a message with a lower ID number is more important and should be transmitted first. But if you want to transfer them in a first-comes-first-served fashion for some reason, you need to make this bit 1. Of course, this is just a local priority. On the physical bus, the messages with lower ID always have priority.

Upvotes: 9

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