Merge pull request #41 from attie-argentum/baudrate-calcs

Revise baudrate calculations

Author: pazi88

STM32_CAN.cpp

@@ -545,8 +545,16 @@ void STM32_CAN::setBaudRateValues(CAN_HandleTypeDef *CanHandle, uint16_t prescal
   uint32_t _TimeSeg1 = 0;
   uint32_t _TimeSeg2 = 0;
   uint32_t _Prescaler = 0;
+
+  /* the CAN specification (v2.0) states that SJW shall be programmable between
+   * 1 and min(4, timeseg1)... the bxCAN documentation doesn't mention this
+   */
+  if (sjw > 4) sjw = 4;
+  if (sjw > timeseg1) sjw = timeseg1;
+
   switch (sjw)
   {
+    case 0:
     case 1:
       _SyncJumpWidth = CAN_SJW_1TQ;
       break;
@@ -557,12 +565,9 @@ void STM32_CAN::setBaudRateValues(CAN_HandleTypeDef *CanHandle, uint16_t prescal
       _SyncJumpWidth = CAN_SJW_3TQ;
       break;
     case 4:
+    default: /* limit to 4 */
       _SyncJumpWidth = CAN_SJW_4TQ;
       break;
-    default:
-      // should not happen
-      _SyncJumpWidth = CAN_SJW_1TQ;
-      break;
   }
 
   switch (timeseg1)
@@ -660,73 +665,67 @@ void STM32_CAN::setBaudRateValues(CAN_HandleTypeDef *CanHandle, uint16_t prescal
   CanHandle->Init.Prescaler = _Prescaler;
 }
 
-void STM32_CAN::calculateBaudrate(CAN_HandleTypeDef *CanHandle, int baud)
-{
-  /* this function calculates the needed Sync Jump Width, Time segments 1 and 2 and prescaler values based on the set baud rate and APB1 clock.
-  This could be done faster if needed by calculating these values beforehand and just using fixed values from table.
-  The function has been optimized to give values that have sample-point between 75-94%. If some other sample-point percentage is needed, this needs to be adjusted.
-  More info about this topic here: http://www.bittiming.can-wiki.info/
-  */
-  int sjw = 1;
-  int bs1 = 5; // optimization. bs1 smaller than 5 does give too small sample-point percentages.
-  int bs2 = 1;
-  int prescaler = 1;
-  uint16_t i = 0;
-
-  uint32_t frequency = getAPB1Clock();
-
-  if(frequency == 48000000) {
-    for(i=0; i<sizeof(BAUD_RATE_TABLE_48M)/sizeof(Baudrate_entry_t); i++) {
-      if(baud == (int)BAUD_RATE_TABLE_48M[i].baudrate) {
-        break;
-      }
-    }
-    if(i < sizeof(BAUD_RATE_TABLE_48M)/sizeof(Baudrate_entry_t)) {
-      setBaudRateValues(CanHandle, BAUD_RATE_TABLE_48M[i].prescaler,
-                                   BAUD_RATE_TABLE_48M[i].timeseg1,
-                                   BAUD_RATE_TABLE_48M[i].timeseg2,
-                                   1);
-      return;
-    }
-  }
-  else if(frequency == 45000000) {
-    for(i=0; i<sizeof(BAUD_RATE_TABLE_45M)/sizeof(Baudrate_entry_t); i++) {
-      if(baud == (int)BAUD_RATE_TABLE_45M[i].baudrate) {
-        break;
-      }
-    }
-    if(i < sizeof(BAUD_RATE_TABLE_45M)/sizeof(Baudrate_entry_t)) {
-      setBaudRateValues(CanHandle, BAUD_RATE_TABLE_45M[i].prescaler,
-                                   BAUD_RATE_TABLE_45M[i].timeseg1,
-                                   BAUD_RATE_TABLE_45M[i].timeseg2,
-                                   1);
-      return;
+template <typename T, size_t N>
+bool STM32_CAN::lookupBaudrate(CAN_HandleTypeDef *CanHandle, int baud, const T(&table)[N]) {
+  for (size_t i = 0; i < N; i++) {
+    if (baud != (int)table[i].baudrate) {
+      continue;
     }
+
+    /* for the best chance at interoperability, use the widest SJW possible */
+    setBaudRateValues(CanHandle, table[i].prescaler, table[i].timeseg1, table[i].timeseg2, 4);
+    return true;
   }
 
-  while (sjw <= 4) {
-    while (prescaler <= 1024) {
-      while (bs2 <= 3) { // Time segment 2 can get up to 8, but that causes too small sample-point percentages, so this is limited to 3.
-        while (bs1 <= 15) { // Time segment 1 can get up to 16, but that causes too big sample-point percenages, so this is limited to 15.
-          int calcBaudrate = (int)(frequency / (prescaler * (sjw + bs1 + bs2)));
-
-          if (calcBaudrate == baud)
-          {
-            setBaudRateValues(CanHandle, prescaler, bs1, bs2, sjw);
-            return;
-          }
-          bs1++;
-        }
-        bs1 = 5;
-        bs2++;
+  return false;
+}
+
+void STM32_CAN::calculateBaudrate(CAN_HandleTypeDef *CanHandle, int baud)
+{
+  uint8_t bs1;
+  uint8_t bs2;
+  uint16_t prescaler;
+
+  const uint32_t frequency = getAPB1Clock();
+
+  if (frequency == 48000000) {
+    if (lookupBaudrate(CanHandle, baud, BAUD_RATE_TABLE_48M)) return;
+  } else if (frequency == 45000000) {
+    if (lookupBaudrate(CanHandle, baud, BAUD_RATE_TABLE_45M)) return;
+  }
+
+  /* this loop seeks a precise baudrate match, with the sample point positioned
+   * at between ~75-95%. the nominal bit time is produced from N time quanta,
+   * running at the prescaled clock rate (where N = 1 + bs1 + bs2). this algorithm
+   * prefers the lowest prescaler (most time quanter per bit).
+   *
+   * many configuration sets can be discarded due to an out-of-bounds sample point,
+   * or being unable to reach the desired baudrate.
+   *
+   * for the best chance at interoperability, we use the widest SJW possible.
+   *
+   * for more details + justification, see: https://github.com/pazi88/STM32_CAN/pull/41
+   */
+  for (prescaler = 1; prescaler <= 1024; prescaler += 1) {
+    const uint32_t can_freq = frequency / prescaler;
+    const uint32_t baud_min = can_freq / (1 + 5 + 16);
+
+    /* skip all prescaler values that can't possibly achieve the desired baudrate */
+    if (baud_min > baud) continue;
+
+    for (bs2 = 1; bs2 <= 5; bs2 += 1) {
+      for (bs1 = (bs2 * 3) - 1; bs1 <= 16; bs1 += 1) {
+        const uint32_t baud_cur = can_freq / (1 + bs1 + bs2);
+
+        if (baud_cur != baud) continue;
+
+        setBaudRateValues(CanHandle, prescaler, bs1, bs2, 4);
+        return;
       }
-      bs1 = 5;
-      bs2 = 1;
-      prescaler++;
     }
-    bs1 = 5;
-    sjw++;
   }
+
+  /* uhoh, failed to calculate an acceptable baud rate... */
 }
 
 uint32_t STM32_CAN::getAPB1Clock()

STM32_CAN.h

@@ -189,6 +189,9 @@ class STM32_CAN {
     void      initializeBuffers(void);
     bool      isRingBufferEmpty(RingbufferTypeDef &ring);
     uint32_t  ringBufferCount(RingbufferTypeDef &ring);
+
+    template <typename T, size_t N>
+    bool      lookupBaudrate(CAN_HandleTypeDef *CanHandle, int Baudrate, const T(&table)[N]);
     void      calculateBaudrate(CAN_HandleTypeDef *CanHandle, int Baudrate);
     void      setBaudRateValues(CAN_HandleTypeDef *CanHandle, uint16_t prescaler, uint8_t timeseg1,
                                                               uint8_t timeseg2, uint8_t sjw);