[Ovmsdev] OVMS v3 First Board Layout

[Stephen Casner]

Mark,

I like the reliability of sticking with the 5V CAN transceiver.  Why
not structure the power system to use one switching power supply to go
from +12V to 5V and another to go from 5V (originating from +12V or
from USB) to 3.3V?  The Raspberry Pi typically gets power from USB;
the Model B+ needs both 3.3V and 1.8V which are provided by a tiny,
high-efficiency dual buck converter chip.  I tested pulling an extra
100mA from 3.3V and did not notice any significant heat from any of
the power supply components.

See the following for a description of the old RPi circuit that used a
simple but hot regulator and a comparison to the new circuit:

https://learn.adafruit.com/introducing-the-raspberry-pi-model-b-plus-plus-differences-vs-model-b/power-supply

That article also mentions (but does not fully explain) the use of a
MOSFET instead of a diode for power supply routing.

Regarging external connectors, I did not know a second connector was
planned.  However, using a higher-density connector makes sense these
days.  I've handled many DB-25 connectors for RS-232 serial in years
past, but those seem positively gigantic now.

BTW, I think it is a DE9-M, not DB9-M.

                                                        -- Steve

On Thu, 30 Mar 2017, Mark Webb-Johnson wrote:

>
> I’m trying to finalise the OVMS v3 final board layout, with the factory in China. We have some questions and seek your opinions:
>
> CAN transceivers / power
>
> Overall, the OVMS v3 system runs at 3.3V. We have two power supply sources: USB (where we use a 5V -> 3.3V regulator), and +12V vehicle power (where we use a +12V -> 3.3V switching power supply, to be as energy efficient as possible). Diodes are used for reverse-polarity protection as well as coping with the situation where both usb and vehicle power is applied simultaneously.
>
> Our problem is with the CAN transceivers. I’m used to the MCP2551 (been using it for a decade or more), but that is 5V so greatly complicates the power supply arrangements at the +12V side. We can switch to something like the SN65HVD233 transceiver that works at 3.3V.
>
> But, I am concerned about comments I am reading about 3.3V CAN transceivers and their inability to meet the ISO11898 dominant condition requirement of 3.5V. From my understanding, these 3.3V CAN transceivers get around this by driving CAN-L to 1V, to still get the differential of about 2V (recessive condition?). My concern is compatibility.
>
> What do people think about this? Any recommendations?
>
> External Connectors
>
> The idea is to retain the existing DB9 connector, with the same basic pin arrangement:
>
> DB9-M   Signal
> 3       Chassis/Power GND
> 2       CAN-L (primary)
> 7       CAN-H (primary)
> 4       CAN-L (alternate CAN)
> 5       CAN-H (alternate CAN)
> 9       +12V Vehicle Power
>
> That leaves pins #1, #6, and #8 free for expansion uses. It gives us compatibility with existing OVMS cables.
>
> We would then add a second connector. The suggestions here are DB15 normal density, DB25 normal density, or DA-26 high density. My preference is the DA-26 (as DB25 is the old parallel printer style connector and very bulky). As well as power lines, expansion cards could wire to this connector to expose external inputs/outputs.
>
> What do people think about the DA-26 connector? I’m suggesting a female version (as power is carried there, and I don’t want the pins to get pushed together for a short).
>
> Note that we’ve also got a micro-usb socket, as well as space for GPS and GSM/GNS antennas.
>
> Other than that, we are good to go. Things have stabilised now with Espressif, so we can proceed with building developer boards.
>
> Regards, Mark.
>
>