sys/can: improve SJW calculation

sys/can/device.c

@@ -37,11 +37,11 @@
 
 #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 */
 
@@ -187,7 +187,7 @@
     }
 
     if (candev_dev->last_pm_update == 0 ||
             value > (candev_dev->last_pm_value - (ztimer_now(ZTIMER_USEC) - candev_dev->last_pm_update))) {
         candev_dev->last_pm_value = value;
         candev_dev->last_pm_update = ztimer_now(ZTIMER_USEC);
         ztimer_set(ZTIMER_USEC, &candev_dev->pm_timer, value);
@@ -424,7 +424,7 @@
  * @return the computed sample point from @p tseg1 and @p tseg2
  */
 static uint32_t update_sample_point(const struct can_bittiming_const *btc, uint32_t spt_nominal,
                                     uint32_t tseg, uint32_t *p_tseg1, uint32_t *p_tseg2, uint32_t *p_spt_error)
 {
     uint32_t best_spt = 0;
     uint32_t min_spt_error = UINT32_MAX;
@@ -537,7 +537,7 @@
         uint32_t rate = clock / (brp * nbt);
         rate_error = max(timing->bitrate, rate) - min(timing->bitrate, rate);
         if (rate_error > min_rate_error) {
             DEBUG("timing->rate=%" PRIu32 ", rate=%" PRIu32 ", rate_error=%" PRIu32 " > min_rate_error=%" PRIu32 ", continuing\n",
                   timing->bitrate, rate, rate_error, min_rate_error);
             continue;
         }
@@ -565,7 +565,7 @@
         }
     }
 
     DEBUG("computed values: min_rate_error=%" PRIu32 ", min_spt_error=%" PRIu32 "\n", min_rate_error, min_spt_error);
 
     if (min_rate_error) {
         rate_error = min_rate_error * 1000 / timing->bitrate;
@@ -581,25 +581,33 @@
     timing->phase_seg1 = tseg1 - timing->prop_seg;
     timing->phase_seg2 = tseg2;
 
-    if (!timing->sjw || !timing_const->sjw_max) {
-        timing->sjw = SJW;
-    }
-    else {
-        if (timing->sjw > timing_const->sjw_max) {
-            timing->sjw = timing_const->sjw_max;
+    /* this paper http://www.oertel-halle.de/files/cia99paper.pdf might help to understand SJW */
+    if (!timing->sjw) {
+        if (!timing_const->sjw_max) {
+            /* fallback if no device max value is known */
+            timing->sjw = SJW;
         }
-        if (timing->sjw > tseg2) {
-            timing->sjw = tseg2;
+        else {
+            /* try to find the max_value start at max */
+            timing->sjw = timing_const->sjw_max;
         }
     }
-
+    if (timing->sjw > timing->phase_seg2) {
+        /* SJW shall not be bigger than phase segment 1 */
+        timing->sjw = timing->phase_seg2;
+    }
+    if (timing->sjw > timing->phase_seg1) {
+        /* SJW shall not be bigger than phase segment 1 */
+        timing->sjw = timing->phase_seg1;
+    }
     timing->brp = best_brp;
 
     timing->bitrate = clock / (timing->brp * (CAN_SYNC_SEG + tseg1 + tseg2));
 
-    DEBUG("bitrate=%" PRIu32 ", sample_point=%" PRIu32 ", brp=%" PRIu32 ", prop_seg=%" PRIu32
-          ", phase_seg1=%" PRIu32 ", phase_seg2=%" PRIu32 "\n", timing->bitrate, timing->sample_point,
-          timing->brp, timing->prop_seg, timing->phase_seg1, timing->phase_seg2);
+    DEBUG("clock=%" PRIu32 ", bitrate=%" PRIu32 ", sample_point=%" PRIu32 ", brp=%" PRIu32
+          ", prop_seg=%" PRIu32", phase_seg1=%" PRIu32 ", phase_seg2=%" PRIu32", sjw=%" PRIu32 "\n",
+          clock, timing->bitrate, timing->sample_point, timing->brp,
+          timing->prop_seg, timing->phase_seg1, timing->phase_seg2, timing->sjw);
 
     return 0;
 }