[Ovmsdev] OVMS CAN-USB Call for Assistance

HONDA S-2000 s2000 at sounds.wa.com
Thu Sep 5 03:57:20 HKT 2013

On Sep 4, 2013, at 01:02, Mastro Gippo wrote:
> 2013/9/4 HONDA S-2000 <s2000 at sounds.wa.com>:
>> If we're going to consider ARM, then I suggest that someone take  
>> the time to
>> find out what chips (or circuits) are good candidates for the voltage
>> translation. My hunch is that I/O pins on the ARM do not directly  
>> connect to
>> the 12V CAN bus, especially considering that transients of more  
>> than 40V
>> might appear. That said, perhaps an ARM solution can be  
>> accomplished with
>> the same low chip count as the proposed PIC32 solution.
> What are you talking about? No mcu connects directly to these
> voltages, they all need a transceiver. Also, there is no way we can
> effectively replace the 2551 with passives.. On the other hand, there
> are a bunch of equivalents out there from major companies like TI,
> that may or may not cost less.
> I'd say that the usb connector+MCU+mcp2551+a bunch of passives and a
> crystal is all we need, now it's just a matter of choosing the right
> mcu.

What are you talking about? I never suggested replacing the 2551 with  
passives. I mentioned that a handful of transistors and probably a  
voltage reference would be needed. Sorry I didn't state things  
strongly enough.

As for the MCP2551 in connection with an ARM, if you know of existing  
libraries that already exist for that combination, then please point  
to them.

Note that many modern processors do not even need a crystal, even for  
USB function. The PIC32MX795 is one example of a processor that does  
not need a crystal to operate as a USB Device. Considering the size  
and/or cost of a crystal, this is probably a significant advantage.  
In other words, cheap crystals are quite large, and small crystals  
are relatively expensive, so there's nothing as good as being able to  
leave out the crystal altogether.


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