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SYNC_TIME is 3 quanta in LPC17xx CAN bit time calculation

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git-svn-id: https://nuttx.svn.sourceforge.net/svnroot/nuttx/trunk@4310 7fd9a85b-ad96-42d3-883c-3090e2eb8679
This commit is contained in:
patacongo
2012-01-18 19:08:15 +00:00
parent 9b5078804f
commit c67bc69d86
1 changed files with 33 additions and 34 deletions
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59
nuttx/arch/arm/src/lpc17xx/lpc17_can.c
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@@ -1049,7 +1049,7 @@ static int can12_interrupt(int irq, void *context)
* *
* |<----------------- NOMINAL BIT TIME ----------------->| * |<----------------- NOMINAL BIT TIME ----------------->|
* |<- SYNC_SEG ->|<------ BS1 ------>|<------ BS2 ------>| * |<- SYNC_SEG ->|<------ BS1 ------>|<------ BS2 ------>|
* |<---- Tq ---->|<----- Tbs1 ------>|<----- Tbs2 ------>| * |<--- 3*Tq --->|<----- Tbs1 ------>|<----- Tbs2 ------>|
* *
* Where * Where
* Tbs1 is the duration of the BS1 segment * Tbs1 is the duration of the BS1 segment
@@ -1059,13 +1059,13 @@ static int can12_interrupt(int irq, void *context)
* Relationships: * Relationships:
* *
* baud = 1 / bit_time * baud = 1 / bit_time
* bit_time = Tq + Tbs1 + Tbs2 * bit_time = 3*Tq + Tbs1 + Tbs2
* Tbs1 = Tq * ts1 * Tbs1 = Tq * ts1
* Tbs2 = Tq * ts2 * Tbs2 = Tq * ts2
* Tq = brp * Tpclk1 * Tq = brp * Tcan
* *
* Where: * Where:
* Tpclk1 is the period of the APB clock (PCLK = CCLK / 4). * Tcan is the period of the APB clock (PCLK = CCLK / CONFIG_CAN1_DIVISOR).
* *
* Input Parameter: * Input Parameter:
* priv - A reference to the CAN block status * priv - A reference to the CAN block status
@@ -1077,7 +1077,8 @@ static int can12_interrupt(int irq, void *context)
static int can_bittiming(struct up_dev_s *priv) static int can_bittiming(struct up_dev_s *priv)
{ {
uint32_t canbtr; uint32_t btr;
uint32_t nclks;
uint32_t brp; uint32_t brp;
uint32_t ts1; uint32_t ts1;
uint32_t ts2; uint32_t ts2;
@@ -1086,37 +1087,35 @@ static int can_bittiming(struct up_dev_s *priv)
canllvdbg("CAN%d PCLK: %d baud: %d\n", priv->port, canllvdbg("CAN%d PCLK: %d baud: %d\n", priv->port,
CAN_CLOCK_FREQUENCY(priv->divisor), priv->baud); CAN_CLOCK_FREQUENCY(priv->divisor), priv->baud);
/* Try to get 14 quanta in one bit_time. That is based on the idea that the ideal /* Try to get 16 quanta in one bit_time. That is based on the idea that the ideal
* would be ts1=6 nd ts2=7 and (1 + ts1 + ts2) = 14. * would be ts1=6 nd ts2=7 and (3 + ts1 + ts2) = 16.
* *
* bit_time = Tq*(1 +ts1 + ts2) * bit_time = Tq*(3 + ts1 + ts2)
* nquanta = bit_time/Tq * nquanta = bit_time/Tq
* nquanta = (1 +ts1 + ts2) * Tq = brp * Tcan
* nquanta = (3 + ts1 + ts2)
* *
* bit_time = brp * Tpclk1 * (1 + ts1 + ts2) * bit_time = brp * Tcan * (3 + ts1 + ts2)
* nquanta = bit_time / brp / Tpclk1 * nquanta = bit_time / brp / Tcan
* = PCLK1 / baud / brp * brp = Fcan / baud / nquanta;
* brp = PCLK1 / baud / nquanta;
* *
* Example: * First, calculate the number of CAN clocks in one bit time: Fcan / baud
* PCLK1 = 42,000,000 baud = 1,000,000 nquanta = 14 : brp = 3
* PCLK1 = 42,000,000 baud = 700,000 nquanta = 14 : brp = 4
*/ */
canbtr = CAN_CLOCK_FREQUENCY(priv->divisor) / priv->baud; nclks = CAN_CLOCK_FREQUENCY(priv->divisor) / priv->baud;
if (canbtr < 14) if (nclks < 16)
{ {
/* At the smallest brp value (1), there are already fewer bit times /* At the smallest brp value (1), there are already too few bit times
* (CAN_CLOCK / baud) is already smaller than our goal. brp must be one * (CAN_CLOCK / baud) to meet our goal. brp must be one and we need
* and we need make some reasonalble guesses about ts1 and ts2. * make some reasonalble guesses about ts1 and ts2.
*/ */
brp = 1; brp = 1;
/* In this case, we have to guess a good value for ts1 and ts2 */ /* In this case, we have to guess a good value for ts1 and ts2 */
ts1 = (canbtr - 1) >> 1; ts1 = (nclks - 1) >> 1;
ts2 = canbtr - ts1 - 1; ts2 = nclks - ts1 - 3;
if (ts1 == ts2 && ts1 > 1 && ts2 < 16) if (ts1 == ts2 && ts1 > 1 && ts2 < 16)
{ {
ts1--; ts1--;
@@ -1124,15 +1123,15 @@ static int can_bittiming(struct up_dev_s *priv)
} }
} }
/* Otherwise, nquanta is 14, ts1 is 6, ts2 is 7 and we calculate brp to /* Otherwise, nquanta is 16, ts1 is 6, ts2 is 7 and we calculate brp to
* achieve 14 quanta in the bit time * achieve 16 quanta in the bit time
*/ */
else else
{ {
ts1 = 6; ts1 = 6;
ts2 = 7; ts2 = 7;
brp = (canbtr + 7) / 14; brp = (nclks + 8) / 16;
DEBUGASSERT(brp >=1 && brp < 1024); DEBUGASSERT(brp >=1 && brp < 1024);
} }
@@ -1142,7 +1141,7 @@ static int can_bittiming(struct up_dev_s *priv)
/* Configure bit timing */ /* Configure bit timing */
canbtr = ((brp - 1) << CAN_BTR_BRP_SHIFT) | btr = ((brp - 1) << CAN_BTR_BRP_SHIFT) |
((ts1 - 1) << CAN_BTR_TESG1_SHIFT) | ((ts1 - 1) << CAN_BTR_TESG1_SHIFT) |
((ts2 - 1) << CAN_BTR_TESG2_SHIFT) | ((ts2 - 1) << CAN_BTR_TESG2_SHIFT) |
((sjw - 1) << CAN_BTR_SJW_SHIFT); ((sjw - 1) << CAN_BTR_SJW_SHIFT);
@@ -1152,11 +1151,11 @@ static int can_bittiming(struct up_dev_s *priv)
* to spikes on the bus-line). * to spikes on the bus-line).
*/ */
canbtr |= CAN_BTR_SAM; btr |= CAN_BTR_SAM;
#endif #endif
canllvdbg("Setting CANxBTR= 0x%08x\n", canbtr); canllvdbg("Setting CANxBTR= 0x%08x\n", btr);
can_putreg(priv, LPC17_CAN_BTR_OFFSET, canbtr); /* Set bit timing */ can_putreg(priv, LPC17_CAN_BTR_OFFSET, btr); /* Set bit timing */
return OK; return OK;
} }