[Ovmsdev] J1850 support research

[Craig Leres]

I've been doing some research on a possible J1850 expansion board; I've
looked at the (nice!) work that Mark and Marko have done on TPMS and
SWCAN expansion boards.

It looks like the TPMS expansion adds the k-line (LIN bus) to the
previously unused pin 1 of the DB9. I think I can also use pin 1 for the
J1850 bus.

The J1850 protocol chip I'm using (HIP7010) uses SPI as does the SWCAN
board. The ESP32 docs say there are four SPI devices. Two are used for
flash tasks leaving HSPI and VSPI. I believe ovms uses HSPI for SPI ram
access and VSPI is used to run the can buses (and SWCAN when present).

The problem I see is that the HIP7010 must be SPI master; unlike I2C I
don't think you can have more than one SPI bus master.

I think this means my best approach is to build a J1850 to canbus
protocol bridge and just encapsulate J1850 frames in can frames? I think
I could still feed the J1850 bus on pin 1 of the DB9. This way the
protocol bridge would only need the HD25 connector to do everything:
J1850, 5V, vehicle 12V (needed by the J1850 transceiver), and can bus lines.

Am I correct in talking myself out of an expansion board?

		Craig