[Ovmsdev] OVMS CAN-USB Call for Assistance
mark at webb-johnson.net
Thu Sep 5 10:22:45 HKT 2013
Looking at the wholesale pricing, and given that we have 5V already and lots of experience with MCP2551, I think I'll stick with that for the initial design and then see what the factory says.
At the moment, I'm trying to make a rough design using PIC32MX795F512H+MCP2551 - DB9 at one end, USB at the other. No final decision on microcontroller yet, but I just want to get something to the factory to firm up estimates on what this is going to cost. I'll tell the factory that the SN65HVD233 is an option for them, if it is easier/cheaper.
The PIC32 is completely overkill for this, and will most likely be 50% of the BOM cost. But, I really want lots of RAM on this thing. If we drop RAM requirement to 64KB, we can save a few bucks, but it really doesn't seem worthwhile.
Regarding the oscillator, every design I've ever done uses an external oscillator. I know the PICs (and others now) have internal FSCs, but am concerned about timing accuracies for CAN and USB. The data sheet says the internal FSC can be used for USB (and incidentally is always used for 48MHz USB when in suspend mode). Anyone have any experience with this? My gut feeling is to wire the board for an 8MHz external crystal (and two caps), and then see if we can get it working reliably with that disabled. If we end up not needing it, we can leave it unpopulated at manufacture time (saving a buck).
On 5 Sep, 2013, at 4:01 AM, HONDA S-2000 <s2000 at sounds.wa.com> wrote:
> The MCP2551 requires a minimum Vdd of 4.5V to ensure ISO-11898 specifications, but USB Vbus is only guaranteed to be 4.01V under certain conditions like at the end of a bus-powered hub.
> There are small 5V boost regulators that work with minimal passives, some even with just capacitors and no inductors, but it will add to the size and cost.
> Maybe the SN65HVD233 is a better choice. It only tolerates 36V spikes, not 42V, but I have no idea how likely either of those high voltages are on a CAN bus in real vehicles.
> The SN65HVD233 will tolerate 7V on Vcc, but 3.6V is the maximum recommended. I almost suggested that it should run directly from USB Vbus, but that is perhaps not wise. I'm sure the 3.3V LDO will handle the load of the CPU and CAN transceiver.
> On Sep 4, 2013, at 06:49, Mark Webb-Johnson wrote:
>> I'm used to the mcp2551, but it is a 5V device. The pic32mx795 has both CAN port pins 5V tolerant, and levels for tx seem sufficient for mcp2551 to rx. The 5V is not too much of an issue, as we have that from USB, but it would be good to be just 3.3V.
>> Anyone know of a 3.3V equivalent to the mcp2551? SN65HVD233? Or, just stay with mcp2551?
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