can: introduce loop_delay

During the data phase of a FDCAN transmission only one node is
transmitting, all others are receivers. The length of the bus line has
no impact.
When transmitting via pin FDCAN_TX the protocol controller receives the
transmitted data from its local CAN transceiver via pin FDCAN_RX. The
received data is delayed by the CAN transceiver loop delay.
If this delay is greater than TSEG1 (time segment before sample point),
a bit error is detected. Without transceiver delay compensation, the bit
rate in the data phase of a FDCAN frame is limited by the transceiver's
loop delay.

Since this parameter is related to the transceiver used, there cannot be
a default value, and it must be explicitly defined with the
configuration variable CONFIG_FDCAN_DEVICE_TRANSCEIVER_LOOP_DELAY.

Signed-off-by: Gilles DOFFE <gilles.doffe@rtone.fr>

drivers/include/can/candev.h

@@ -82,6 +82,7 @@ struct candev {
     enum can_state state;                  /**< device state */
 #ifdef MODULE_FDCAN
     struct can_bittiming fd_data_bittiming;/**< device bittimings for FD CAN only */
+    uint16_t loop_delay;                   /**< CAN FD transceiver loop delay */
 #endif
 };
 

sys/Kconfig

@@ -7,6 +7,7 @@
 menu "System"
 
 rsource "auto_init/Kconfig"
+rsource "can/Kconfig"
 rsource "chunked_ringbuffer/Kconfig"
 rsource "congure/Kconfig"
 rsource "debug_irq_disable/Kconfig"

sys/auto_init/can/auto_init_periph_can.c

@@ -48,6 +48,9 @@ void auto_init_periph_can(void) {
         candev_dev[i].rx_inactivity_timeout = candev_params[i].rx_inactivity_timeout;
         candev_dev[i].tx_wakeup_timeout = candev_params[i].tx_wakeup_timeout;
 #endif
+#ifdef MODULE_FDCAN
+        candev_dev[i].loop_delay = candev_params[i].loop_delay;
+#endif
 
         can_device_init(_can_stacks[i], CANDEV_STACKSIZE, CANDEV_BASE_PRIORITY + i,
                         candev_params[i].name, &candev_dev[i]);

sys/can/Kconfig

@@ -0,0 +1,36 @@
+menu "CAN"
+    depends on USEMODULE_CAN || USEMODULE_FDCAN
+
+menu "FD CAN"
+
+config FDCAN_DEVICE_SET_TRANSCEIVER_LOOP_DELAY
+    depends on USEMODULE_FDCAN
+    bool "FD CAN transceiver loop delay"
+    default n
+    help
+        Allow to set FD CAN Transceiver loop delay.
+
+config FDCAN_DEVICE_TRANSCEIVER_LOOP_DELAY
+    depends on FDCAN_DEVICE_SET_TRANSCEIVER_LOOP_DELAY
+    int "FD CAN transceiver loop delay value"
+    range 0 1000
+    default 0
+    help
+      This parameter defines the loop delay introduced by the CAN transceiver
+      during transmission. The loop delay represents the time taken for the
+      transmitted signal to be looped back to the CAN controller. This delay
+      is introduced by the physical transceiver circuitry and may vary
+      depending on the transceiver model and other physical factors.
+
+      The value is typically measured in nanoseconds and should be set
+      according to the specifications provided by the transceiver manufacturer.
+      A higher loop delay can affect the timing of CAN message transmissions
+      and may need to be adjusted in systems with tight timing requirements.
+
+      If unsure, leave this value as 0 or refer to the hardware documentation
+      for the correct delay value.
+
+
+endmenu # FD CAN
+
+endmenu # CAN

sys/can/device.c

@@ -35,6 +35,16 @@
 #define ENABLE_DEBUG 0
 #include "debug.h"
 
+#ifdef MODULE_FDCAN
+/**
+ * The loop delay in CAN, especially in CAN FD with bitrate switching, affects synchronization due to increased data rates.
+ * The unit is nanoseconds.
+ */
+#ifndef CONFIG_FDCAN_DEVICE_TRANSCEIVER_LOOP_DELAY
+#error "CONFIG_FDCAN_DEVICE_TRANSCEIVER_LOOP_DELAY must be defined. This property can be found in the datasheet of the CAN transceiver in use. The unit is nanoseconds."
+#endif /* CONFIG_FDCAN_DEVICE_TRANSCEIVER_LOOP_DELAY */
+#endif /* MODULE_FDCAN */
+
 #ifndef CAN_DEVICE_MSG_QUEUE_SIZE
 #define CAN_DEVICE_MSG_QUEUE_SIZE 64
 #endif
@@ -236,6 +246,13 @@ static void *_can_device_thread(void *args)
 
     candev_dev->ifnum = can_dll_register_candev(candev_dev);
 
+#if defined(MODULE_FDCAN)
+    if (candev_dev->loop_delay == 0) {
+        candev_dev->loop_delay = CONFIG_FDCAN_DEVICE_TRANSCEIVER_LOOP_DELAY;
+    }
+    dev->loop_delay = candev_dev->loop_delay;
+#endif
+
     dev->driver->init(dev);
     power_up(candev_dev);
 

sys/include/can/device.h

@@ -57,6 +57,9 @@ typedef struct candev_params {
 #if defined(MODULE_CAN_TRX) || defined(DOXYGEN)
     can_trx_t *trx;                  /**< transceiver to set */
 #endif
+#if defined(MODULE_FDCAN)
+    uint16_t loop_delay;             /**< CAN FD transceiver loop delay */
+#endif
 #if defined(MODULE_CAN_PM) || defined(DOXYGEN)
     uint32_t rx_inactivity_timeout;  /**< power management rx timeout value */
     uint32_t tx_wakeup_timeout;      /**< power management tx wake up value */
@@ -74,6 +77,9 @@ typedef struct candev_dev {
 #if defined(MODULE_CAN_TRX) || defined(DOXYGEN)
     can_trx_t *trx;   /**< transceiver attached to the device */
 #endif
+#if defined(MODULE_FDCAN)
+    uint16_t loop_delay;    /**< CAN FD transceiver loop delay */
+#endif
 #if defined(MODULE_CAN_PM) || defined(DOXYGEN)
     uint32_t rx_inactivity_timeout; /**< Min timeout loaded when a frame is received */
     uint32_t tx_wakeup_timeout;     /**< Min timeout loaded when a frame is sent */