CAN-FD w/ BRS and DR >= 2MB

[mickeyl]

I'm using MCP2518fd + TCAN334GD. With this combination I should get data rates up to 5MB going. In reality though, it only works up to a data rate of 1MB. If I configure more, it no longer sends anything out. Any idea?

[Emandhal]

Hello,

Can I have your configuration for the MCP?
The TCAN334GD support 5Mbit/s but we can't check if the device is the good one because TCAN334 and TCAN334GD have the same marking.
Is it possible for you to check the TxD signal between the MCP and the TCAN ?

[mickeyl]

Schematics and Wirings are correct. I have verified with CAN-FD adapters and commercial CAN-FD ECUs. But I have solved the problem, it was in the timing computation (in particular TDC):

• Data-bit timing search bounds are wrong in upstream (DTQBIT_MAX uses nominal limits).
• Data segment clamping also uses nominal constants (NTSEG* instead of DTSEG*).
• Sample point is fixed at 80% for all bitrates; Linux/CiA use stepped targets (875/800/750 permille), which matters at higher data rates.
• TDC setup diverges from Linux/ACAN:
• Uses SSP = BRP * DTSEG1 (missing DSYNC).
• Uses MANUAL_MODE for lower data rates.
• Uses TDCV = SSP - TDCO in a way that is not what Linux/ACAN do for AUTO mode.
• Should enable AUTO TDC only for DBRP 1 or 2, set TDCO to data sample point, and keep TDCV=0.

So to make CAN-FD data bitrates >= 1M working with your driver, you need to apply the following patch.

diff --git a/MCP251XFD.c b/MCP251XFD.c
index 2be127e..72f860d 100644
--- a/MCP251XFD.c
+++ b/MCP251XFD.c
@@ -838,8 +838,10 @@ eERRORRESULT MCP251XFD_CalculateBitTimeConfiguration(const uint32_t fsysclk, con
 
   //--- Calculate Nominal segments --------------------------
   pConf->NBRP = BestBRP - 1;                                        // ** Save the best NBRP in the configuration **
-  uint32_t NTSEG2 = BestNTQbits / 5;                                // The Nominal Sample Point must be close to 80% (5x20%) of NTQ per bits so NTSEG2 should be 20% of NTQbits
-  if ((BestNTQbits % 5) > 2) NTSEG2++;                              // To be as close as possible to 80%
+  uint32_t NominalSamplePointPermille = 875u;                       // Match Linux/CiA defaults for NRZ
+  if (desiredNominalBitrate > 800000u) NominalSamplePointPermille = 750u;
+  else if (desiredNominalBitrate > 500000u) NominalSamplePointPermille = 800u;
+  uint32_t NTSEG2 = ((BestNTQbits * (1000u - NominalSamplePointPermille)) + 500u) / 1000u; // nearest-to-target sample point
   if (NTSEG2 < MCP251XFD_NTSEG2_MIN) NTSEG2 = MCP251XFD_NTSEG2_MIN; // Correct NTSEG2 if < 1
   if (NTSEG2 > MCP251XFD_NTSEG2_MAX) NTSEG2 = MCP251XFD_NTSEG2_MAX; // Correct NTSEG2 if > 128
   pConf->NTSEG2 = NTSEG2 - 1;                                       // ** Save the NTSEG2 in the configuration **
@@ -857,14 +859,16 @@ eERRORRESULT MCP251XFD_CalculateBitTimeConfiguration(const uint32_t fsysclk, con
   if (desiredDataBitrate != MCP251XFD_NO_CANFD)
   {
     pConf->DBRP = BestBRP - 1;                                        // ** Save the best DBRP in the configuration **
-    uint32_t DTSEG2 = BestDTQbits / 5;                                // The Data Sample Point must be close to 80% (5x20%) of DTQ per bits so DTSEG2 should be 20% of DTQbits
-    if ((BestDTQbits % 5) > 2) DTSEG2++;                              // To be as close as possible to 80%
-    if (DTSEG2 < MCP251XFD_NTSEG2_MIN) DTSEG2 = MCP251XFD_NTSEG2_MIN; // Correct DTSEG2 if < 1
-    if (DTSEG2 > MCP251XFD_NTSEG2_MAX) DTSEG2 = MCP251XFD_NTSEG2_MAX; // Correct DTSEG2 if > 16
+    uint32_t DataSamplePointPermille = 875u;                          // Match Linux/CiA defaults for NRZ
+    if (desiredDataBitrate > 800000u) DataSamplePointPermille = 750u;
+    else if (desiredDataBitrate > 500000u) DataSamplePointPermille = 800u;
+    uint32_t DTSEG2 = ((BestDTQbits * (1000u - DataSamplePointPermille)) + 500u) / 1000u; // nearest-to-target sample point
+    if (DTSEG2 < MCP251XFD_DTSEG2_MIN) DTSEG2 = MCP251XFD_DTSEG2_MIN; // Correct DTSEG2 if < 1
+    if (DTSEG2 > MCP251XFD_DTSEG2_MAX) DTSEG2 = MCP251XFD_DTSEG2_MAX; // Correct DTSEG2 if > 16
     pConf->DTSEG2 = DTSEG2 - 1;                                       // ** Save the DTSEG2 in the configuration **
     uint32_t DTSEG1 = BestDTQbits - DTSEG2 - MCP251XFD_DSYNC;         // DTSEG1  = DTQbits - DTSEG2 - 1 (DSYNC)
-    if (DTSEG1 < MCP251XFD_NTSEG1_MIN) DTSEG1 = MCP251XFD_NTSEG1_MIN; // Correct DTSEG1 if < 1
-    if (DTSEG1 > MCP251XFD_NTSEG1_MAX) DTSEG1 = MCP251XFD_NTSEG1_MAX; // Correct DTSEG1 if > 32
+    if (DTSEG1 < MCP251XFD_DTSEG1_MIN) DTSEG1 = MCP251XFD_DTSEG1_MIN; // Correct DTSEG1 if < 1
+    if (DTSEG1 > MCP251XFD_DTSEG1_MAX) DTSEG1 = MCP251XFD_DTSEG1_MAX; // Correct DTSEG1 if > 32
     pConf->DTSEG1 = DTSEG1 - 1;                                       // ** Save the DTSEG1 in the configuration **
     uint32_t DSJW = DTSEG2;                                           // Normally DSJW = DTSEG2, maximizing DSJW lessens the requirement for the oscillator tolerance
     if (DTSEG1 < DTSEG2) DSJW = DTSEG1;                               // But DSJW = min(DPHSEG1, DPHSEG2)
@@ -873,15 +877,23 @@ eERRORRESULT MCP251XFD_CalculateBitTimeConfiguration(const uint32_t fsysclk, con
     pConf->DSJW = DSJW - 1;                                           // ** Save the DSJW in the configuration **
 
     //--- Calculate Transmitter Delay Compensation ----------
-    if (desiredDataBitrate >= 1000000)                                // Enable Automatic TDC for DBR of 1Mbps and Higher
-         pConf->TDCMOD = MCP251XFD_AUTO_MODE;                         // ** Set Automatic TDC measurement compensations for transmitter delay variations
-    else pConf->TDCMOD = MCP251XFD_MANUAL_MODE;                       // ** Set Manual; Don�t measure, use TDCV + TDCO from register
-    const uint32_t SSP = BestBRP * DTSEG1;                            // In order to set the SSP to 80%, SSP = TDCO + TDCV (Equation 3-10 of MCP25XXFD Family Reference Manual). SSP is set to DBRP * (DPRSEG + DPHSEG1) = DBRP * DTSEG1
-    uint32_t TDCO = SSP;
-    if (TDCO > MCP251XFD_TDCO_MAX) TDCO = MCP251XFD_TDCO_MAX;         // Correct TDCO if > 63
+    // Match Linux/ACAN behavior:
+    // - enable AUTO TDC only when DBRP is 1 or 2
+    // - use absolute TDCO at data sample point (no loop-delay subtraction)
+    // - keep TDCV at 0 in AUTO mode
+    const bool EnableAutoTDC = (BestBRP == 1u) || (BestBRP == 2u);
+    pConf->TDCMOD = EnableAutoTDC ? MCP251XFD_AUTO_MODE : MCP251XFD_TDC_DISABLED;
+
+    const uint32_t SamplePointSSP = BestBRP * (MCP251XFD_DSYNC + DTSEG1); // SSP target at data sample point: DBRP * (DSYNC + DTSEG1)
+    uint32_t TDCO = 0u;
+    uint32_t TDCV = 0u;
+    if (EnableAutoTDC)
+    {
+      TDCO = SamplePointSSP;
+      if (TDCO > MCP251XFD_TDCO_MAX) TDCO = MCP251XFD_TDCO_MAX;       // Correct TDCO if > 63
+    }
+
     pConf->TDCO = TDCO;                                               // ** Save the TDCO in the configuration **
-    uint32_t TDCV = SSP - TDCO;                                       // TDCV is the remaining of SSP: TDCV = SSP - TDCO (Equation 3-10 of MCP25XXFD Family Reference Manual)
-    if (TDCV > MCP251XFD_TDCV_MAX) TDCV = MCP251XFD_TDCV_MAX;         // Correct TDCV if > 63
     pConf->TDCV = TDCV;                                               // ** Save the TDCV in the configuration **
     pConf->EDGE_FILTER = true;                                        // ** Edge Filtering enabled, according to ISO 11898-1:2015 **
   }
diff --git a/MCP251XFD.h b/MCP251XFD.h
index ce70bd6..d83f4a7 100644
--- a/MCP251XFD.h
+++ b/MCP251XFD.h
@@ -143,7 +143,7 @@
 #define MCP251XFD_DSJW_MIN      (   1 ) //!< Min DSJW
 #define MCP251XFD_DSJW_MAX      (  16 ) //!< Max DSJW
 #define MCP251XFD_DTQBIT_MIN    ( MCP251XFD_NSYNC + MCP251XFD_NTSEG1_MIN + MCP251XFD_NTSEG2_MIN ) //!< Min DTQ per Bit (1-bit SYNC + 1-bit PRSEG + 1-bit PHSEG1 + 1-bit PHSEG2)
-#define MCP251XFD_DTQBIT_MAX    ( MCP251XFD_NTSEG1_MAX + MCP251XFD_NTSEG2_MAX + 1 )               //!< Max DTQ per Bit (49-bits)
+#define MCP251XFD_DTQBIT_MAX    ( MCP251XFD_DTSEG1_MAX + MCP251XFD_DTSEG2_MAX + 1 )               //!< Max DTQ per Bit (49-bits)

I have cross-referenced this w/ the Linux driver and ACANFD from Pierre Molinaro. With this patch, I can talk just fine with 8MB data rate:

[mickey]🌐 meteor in ~ [⏱ 29s]
❯ sudo ip link set can0 down; sudo ip link set can0 up type can bitrate 500000 dbitrate 8000000 fd on && candump -e -x can0
 (000.000000)  can0  RX B -  123  [01]  12                        '.'

[Emandhal]

Hi,

Sorry for the delay. You are right, the code is wrong.
After investigations and tests, I see others possibilities of possible improvements that I have done in an another driver for the TCAN4550.
I will tests and submit them if they are worth it else just apply your patch.

Thank for your help