[Ovmsdev] OVMS v3 First Board Layout
mark at webb-johnson.net
Thu Mar 30 14:18:44 HKT 2017
Thanks for your quick reply.
My concern is that historically we’ve had a lot of power wastage in OVMS v2 (and v1 before that). We used LM style power regulators, and those burned off the 12V->5V difference as heat. Kind of like to Raspberry Pi A and B (but in our case 12V->5V is being burned off, not 5V->3.3V). Cheap, clean and simple, but very ‘lossy’.
Moving to a 12V->5V pre-conversion, then 5V->3.3V from both USB and 12V sides, seems a simple way to go, but what sort of impact on efficiency does that sacrifice?
The other concern is noise. For the GSM modem (and presumably ESP32, although I haven’t seen so much concern raised there), we need to keep things very clean.
Cost wise, the 3.3V transceivers are about 3+ times the price of the venerable MCP2551.
Regarding the extra connector, the idea is to provide a base module with base functionality. Then have a plug-in architecture (using SIP strips) that will allow an expansion board to be added. That expansion board should be able to connect to the microprocessor, do whatever it needs to do, then expose itself on the extra connector pins. The idea is to have two of these - one for the optional modem, and one for general expansion.
P.S. In the above I say 12V, but it is more like 14V - 15V in modern cars.
> On 30 Mar 2017, at 1:33 PM, Stephen Casner <casner at acm.org> wrote:
> 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:
> 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.
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