[Ovmsdev] OVMS Poller module/singleton

[Michael Balzer]

Simon,

Am 25.01.25 um 21:00 schrieb Simon Ehlen via OvmsDev:
> Are adjustments/improvements also necessary at 125KBPS for CAN2/3?

I've had a look at SAE J2284-1 for the 125 kbit timing, and it seems for 
this speed, the esp-idf, the mcp_can lib and our current driver are 
outside the spec.

SAE recommends a sync jump width of 3 for low (nq=8) and 4 for higher 
resolutions, and single sampling at 62.5% (range 58.3…69.2%).

The esp-idf driver would use a SJW of 3 (would better be 4 as nq=20) and 
single sampling at 80% (very late). The Arduino lib uses SJW=2 (too low) 
and triple sampling at 75% (late).

Our current driver uses a SJW of 1 (much too low) and triple sampling at 
56.25% (too early):
     case CAN_SPEED_125KBPS:
       cnf1=0x03; cnf2=0xf0; cnf3=0x86;
       // BRP=3, PRSEG=1, PS1=7, PS2=7, SJW=1, BTLMODE=1, SAM=1, SOF=1, 
WAKFIL=0 → Sample 3x at 9/16 = 56,25%

(Note: the current comment in the source is incorrect, the tq values 
need each to be read as +1)

Possible SAE solutions for the MCP2515:

a)
     case CAN_SPEED_125KBPS:
       // SAE J2284-1:
       // BRP=3, PRSEG=3, PS1=6, PS2=6, SJW=4, BTLMODE=1, SAM=0, SOF=1, 
WAKFIL=0 → Sample point at 10/16 = 62.5%
       cnf1=0xc3; cnf2=0xaa; cnf3=0x85;
b)
     case CAN_SPEED_125KBPS:
       // SAE J2284-1:
       // BRP=3, PRSEG=3, PS1=7, PS2=5, SJW=4, BTLMODE=1, SAM=0, SOF=1, 
WAKFIL=0 → Sample point at 11/16 = 68.8%
       cnf1=0xc3; cnf2=0xb2; cnf3=0x84;

Please try.

Btw, if anyone would like to experiment: the Microchip timing calculator 
needs to be set to 16 MHz (that's our MCP2515 clock), and you need to 
add 0x80 to the cnf3 byte generated (that's the SOF flag, cannot be set 
in the UI).

Regards,
Michael

-- 
Michael Balzer * Am Rahmen 5 * D-58313 Herdecke
Fon 02330 9104094 * Handy 0176 20698926

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