STM32H7 FDCAN 8 Mbit/s setup / docum. unclear SYNC_SEG

[LCE]

Heyho,

I don't get the FDCAN on a STM32H735g-DK working with 8 Mbit/s.

I feed the FDCANs with 80 MHz so that the "time quantum" is 12.5 ns to make 8 Mbit possible with the integers.

I use the recommended kvaser website for calculations:

https://www.kvaser.com/support/calculators/can-fd-bit-timing-calculator/#/canfd

/* clock / timing */
 
/* nominal = arbitration phase
 *	NBTP
 * 	80 MHz: tq = NominalPrescaler x (1/fdcan_ker_ck)
 *		->  tq = 12.5 ns
 *	data rate = 1 / ((SyncSeg + Seg1 + Seg2) * tq)
 *		with SyncSeg = 1
 *	0.5 Mbps = 160 * tq
 *		-> TimeSeg1 + TimeSeg2 = 159
 */
	phCan->Init.NominalPrescaler = 1;		/* tq = NominalPrescaler x (1/fdcan_ker_ck) */
#if( 1 )
/* 5000 ppm */
	phCan->Init.NominalTimeSeg1 = 138;		/* NTSEG1 1..256, NominalTimeSeg1 = Propagation_segment + Phase_segment_1 */
	phCan->Init.NominalTimeSeg2 = 21;		/* NTSEG2 1..128 */
	phCan->Init.NominalSyncJumpWidth = 20;		/* NSJW   1..128 should be < NTSEG2 */
#else
/* 500 ppm */
	phCan->Init.NominalTimeSeg1 = 153;		/* NTSEG1 1..256, NominalTimeSeg1 = Propagation_segment + Phase_segment_1 */
	phCan->Init.NominalTimeSeg2 = 6; 		/* NTSEG2 1..128 */
	phCan->Init.NominalSyncJumpWidth = 3; 		/* NSJW   1..128 should be < NTSEG2 */
#endif
 
/* data = data phase with bit rate switching
 *	DBTP
 * 	80 MHz: tq = DataPrescaler x (1/fdcan_ker_ck)
 *		->  tq = 12.5 ns
 *	data rate = 1 / ((SyncSeg + Seg1 + Seg2) * tq)
 *		with SyncSeg = 1
 */
	phCan->Init.DataPrescaler = 1;		/* DBRP 1..32 */
#if CAN_FD_DATA_8MB
/* 8 Mbps = 10 * tq
 *	-> TimeSeg1 + TimeSeg2 = 9
 */
	phCan->Init.DataTimeSeg1 	= 5;		/* NTSEG1 1..32, NominalTimeSeg1 = Propagation_segment + Phase_segment_1 */
	phCan->Init.DataTimeSeg2 	= 4;        /* NTSEG2 1..16 */
	phCan->Init.DataSyncJumpWidth 	= 4;        /* NSJW   1..16 should be < NTSEG2 */
#else
/* 2 Mbps = 40 * tq
 *	-> TimeSeg1 + TimeSeg2 = 39
 */
	phCan->Init.DataTimeSeg1 	= 24;		/* NTSEG1 1..32, NominalTimeSeg1 = Propagation_segment + Phase_segment_1 */
	phCan->Init.DataTimeSeg2 	= 15;       /* NTSEG2 1..16 */
	phCan->Init.DataSyncJumpWidth 	= 14;       /* NSJW   1..16 should be < NTSEG2 */
#endif

0.5 Mbit in arbitration phase is working (checked bit rate on scope), so is data rate of 2 Mbit/s (checked bit rate on scope).

When using the 8 Mbit settings, the peripheral "hangs" after the first output, which doen's show any sign of BSR / 8 Mbit.

Any ideas?

Or even better any examples how to get to 8 Mbit/s ?

[LCE]

I just checked the register values again and they were wrong, so above are the "correct" values.

Still the same problem:

2 Mbps perfect,

8 Mbps not working

Any ideas?

Is the input clock fdcan_ker_ck = 80 MHz too low?

Shouldn't be, ST's 2 Mbps examples use 20 MHz, and at VOS1 max FDCAN clock is 100 MHz.

Edit: 4 Mbps is working with:

DBRP  = 1

DTSEG1 = 12

DTSEG2 = 7

 DSJW  = 5

[LCE]

Documentation, RM0468, latest edition:

61.4.4 Bit timing, pg. 2528

Synchronization segment (SYNC_SEG): a bit change is expected to occur within this

time segment, that has a fixed length of one time quantum (1 x tq).

DBTP reg description, page 2567:

Therefore the length of the bit time is (DTSEG1 + DTSEG2 + 3) tq for programmed values

Same descr. in NBTP.

So, which one is correct, or are these 2 different things?

@Imen DAHMEN​ please?

Edit: Same in G4 RM

[LCE]

Anybody getting 8 Mbps on a H735 ?

At least someone from @ ST ever got that working?

I tried all kinds of timings, and I can't imagine that the CCU is needed, especially when communicating only with another on-chip FDCAN with the same timing settings.

Or this the problem, trying to have 2 FDCANs running with 8 Mbps at the same time?

[LCE]

This is almost funny...

When I set FDCAN1 Mode to FDCAN_MODE_EXTERNAL_LOOPBACK, it sends with 8 Mbps, and FDCAN2 receives data correctly.

When I set FDCAN2 back to FDCAN_MODE_NORMAL, no 8 Mbps on the scope, no Rx on FDCAN2.

Then I get in FDCAN1:

PSR = 0000050F
        TDVC = 0 mtq
        PXE  = 0
        REDL = 0
        RBRS = 0
        RESI = 0
        DLEC = 05 -> error: bit 0
        BO   = 0
        EW   = 0
        EP   = 0
        ACT  = 01 idle
        LEC  = 07 -> error: no change
ECR = 00010008
        CEL = 1 CAN error log
        RP  = 0
        REC = 0 RX err
        TEC = 8 TX err

[LCE]

8 Mbit still not working (except ext loopback).

Checked again RM and DS, no specs concerning data rate.

Anybody?

@Peter BENSCH​  maybe?

[Peter BENSCH]

@Community member​ Thank you for your trust, but for once, due to lack of experience, I have nothing to contribute here.

Regards

/Peter

[LCE]

@Peter BENSCH​  Thanks for your reply, at least something! :)

Maybe you can forward this question to your FDCAN experts?

It's really strange, everthing working fine up to 4 Mbit/s on both peripherals, but as I described: 8 Mbit works only in external loopback mode.

[Imen.D]

Hello @LCE (Community Member)​ ,

Thank you for your contribution.

I will escalate this issue internally to the FDCAN experts and I'll make sure to post more details here as soon as I hear back from our team.

Imen

[LCE]

@Imen DAHMEN​ 

Thanks!

Anyway, note the documentation bug in DBTP and NBTP registers.

I'm pretty sure now that the +1 for the bit time is the correct value (instead of the +3).