{"id":600,"date":"2024-12-08T13:38:30","date_gmt":"2024-12-08T08:08:30","guid":{"rendered":"https:\/\/blog.mshafeeq.com\/?p=600"},"modified":"2025-03-06T13:09:40","modified_gmt":"2025-03-06T07:39:40","slug":"stm32-blue-pill-can-communication-in-loopback-mode-a-register-level-implementation","status":"publish","type":"post","link":"https:\/\/blog.mshafeeq.com\/index.php\/2024\/12\/08\/stm32-blue-pill-can-communication-in-loopback-mode-a-register-level-implementation\/","title":{"rendered":"STM32 Blue Pill CAN Communication in Loopback Mode \u2013 A Register-Level Implementation"},"content":{"rendered":"\n

In my previous blog post, I covered the fundamental concepts of the CAN bus protocol. If you haven’t had the chance to read it yet, I suggest doing so as it will enhance your understanding of how the CAN bus operates within the STM32 Blue Pill and offer valuable tips for debugging the CAN bus during the development process. You can check it out here<\/a>.<\/p>\n\n\n\n

In this blog post, we will dive into a hands-on session with the CAN bus in loopback mode<\/strong>. For this, you only need a Blue Pill board\u2014no external hardware is required. We will explore what loopback mode is, discuss the different modes supported by the STM32 Blue Pill, and more in the upcoming sections.<\/p>\n\n\n\n

The STM32 Blue Pill’s CAN module, called bxCAN (Basic Extended CAN)<\/strong>, supports both standard identifiers (11-bit)<\/strong> and extended identifiers (29-bit)<\/strong>, compliant with the CAN 2.0 protocol. The CAN module features two FIFOs (FIFO0 and FIFO1)<\/strong> for receiving messages sent by other CAN nodes. Each FIFO has three stages<\/strong>, meaning it can hold up to three messages in its queue. For transmitting data, the module uses three mailboxes<\/strong>. Messages are sent from the mailboxes either based on their priority or in a FIFO order, depending on the configuration.<\/p>\n\n\n\n

Additionally, the module provides 14 filter banks<\/strong> that allow you to select specific packets from the received data. I will explain how to configure these filter banks in the upcoming sections. Below is a simplified block diagram of the CAN peripheral to help you visualize its components.<\/p>\n\n\n

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\"\"<\/figure><\/div>\n\n\n

The STM32 bxCAN<\/strong> module supports several test modes, which are useful for different debugging and testing scenarios:<\/p>\n\n\n\n

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  1. Loopback Mode<\/strong>: This mode allows you to configure and debug the CAN setup without requiring external hardware. (Explained in detail in this post.)<\/li>\n\n\n\n
  2. Silent Mode<\/strong>: Ideal for observing CAN bus traffic without actively participating or interfering with the communication.<\/li>\n\n\n\n
  3. Combined Loopback and Silent Mode<\/strong>: Enables offline testing without sending signals onto the CAN bus, combining the advantages of loopback and silent modes.<\/li>\n<\/ol>\n\n\n
    \n
    \"\"<\/figure><\/div>\n\n\n

    Filter bank configuration<\/h4>\n\n\n\n

    The STM32 Blue Pill’s CAN peripheral includes 14 filter banks<\/strong>, each designed to selectively process CAN messages based on their Identifier (ID)<\/strong>. Each filter bank consists of two 32-bit registers<\/strong>: FR1 and FR2. These registers allow you to configure filters in either 32-bit width<\/strong> or 16-bit width<\/strong> mode, with two main filtering options: List Mode<\/strong> and Mask Mode<\/strong>.<\/p>\n\n\n\n

    Filter Configuration Modes<\/h5>\n\n\n\n
    Mode<\/strong><\/td>Width<\/strong><\/td>Configuration Details<\/strong><\/td><\/tr><\/thead>
    List<\/strong><\/td>32-bit<\/strong><\/td>FR1 and FR2 are used to configure 2 IDs<\/strong> (1 in each register).<\/td><\/tr>
    <\/td>16-bit<\/strong><\/td>FR1 and FR2 are divided into two 16-bit sections, allowing configuration of 4 IDs<\/strong> (2 in each register).<\/td><\/tr>
    Mask<\/strong><\/td>32-bit<\/strong><\/td>FR1 configures the ID<\/strong>, while FR2 configures the mask<\/strong>. Bits set to 1 in the mask must match the ID; bits set to 0 are “don’t care.”<\/td><\/tr>
    <\/td>16-bit<\/strong><\/td>The first 16 bits of FR1 and FR2 configure the ID and mask for one filter. The remaining 16 bits configure the ID and mask for a second filter.<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

    This information conveyed in the Figure 230 of RM0008.<\/p>\n\n\n\n

    Registers used<\/h4>\n\n\n\n
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    1. CAN Control and Status Register<\/strong>\n
        \n
      • Clock configuration registers \u2013 RCC_APB1<\/span><\/li>\n\n\n\n
      • CAN Master Control Register \u2013 CAN_MCR<\/span><\/li>\n\n\n\n
      • CAN Master Status Register \u2013 CAN_MSR<\/span><\/li>\n\n\n\n
      • CAN Transmit Status Register \u2013 CAN_TSR<\/span><\/li>\n\n\n\n
      • CAN Bit Timing Register \u2013 CAN_BTR<\/span><\/li>\n\n\n\n
      • CAN Interrupt Enable Register \u2013 CAN_IER<\/span><\/li>\n\n\n\n
      • CAN Receive FIFO 0 Register \u2013 CAN_RF0R<\/li>\n<\/ul>\n<\/li>\n\n\n\n
      • CAN Filter configuration Registers<\/strong>\n
          \n
        • CAN Filter Master Register \u2013 CAN_FMR<\/span><\/li>\n\n\n\n
        • CAN Filter Mode Register \u2013 CAN_FM1R<\/span><\/li>\n\n\n\n
        • CAN Filter Scale Register \u2013 CAN_FS1R<\/span><\/li>\n\n\n\n
        • CAN Filter Activation Register \u2013 CAN_FA1R<\/span><\/li>\n\n\n\n
        • CAN Filter FIFO Assignment Register \u2013 CAN_FFA1R<\/span><\/li>\n\n\n\n
        • CAN Filter Bank Registers \u2013 CAN_FR1 & CAN_FR2<\/li>\n<\/ul>\n<\/li>\n\n\n\n
        • CAN Mailbox Registers<\/strong>\n
            \n
          • CAN TX Identifier Register \u2013 CAN_TIxR<\/span><\/li>\n\n\n\n
          • CAN Receive FIFO mailbox Identifier register \u2013 CAN_RIxR<\/span><\/li>\n\n\n\n
          • CAN Mailbox Data Length Control and Time stamp Register \u2013 CAN_TDTxR<\/span><\/li>\n\n\n\n
          • CAN Receive FIFO Data Length Control and Time stamp Register \u2013 CAN_RDTxR<\/span><\/li>\n\n\n\n
          • CAN Mailbox data low\/high register \u2013 CAN_TDLxR\/CAN_TDHxR<\/li>\n\n\n\n
          • CAN Receive FIFO data low\/high register \u2013 CAN_RDLxR\/CAN_RDHxR<\/li>\n<\/ul>\n<\/li>\n<\/ol>\n\n\n\n

            Hands-On<\/strong><\/h4>\n\n\n\n

            In this hands-on session, we will configure the CAN peripheral in loopback mode<\/strong> to send and receive messages. Follow these steps:<\/p>\n\n\n\n

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            1. Task<\/strong>: Send a message (e.g., 2 bytes) with ID 0x102<\/strong> after configuring the filter with: