Just a quick request: there should be enough space on the pcb to add a
bluetooth module. Please add at least the footprint for
[http://www.ebay.it/sch/i.html?_odkw=hc-05&_sop=15&_osacat=0&_from=R40&LH_PrefLoc=2&_trksid=p2045573.m570.l1313&_nkw=hc-05+-74hc05&_sacat=0]
This would add the need for an independent power supply, but as I
recall you were already planning to place the footprints on the pcb.
I found myself in need for a wireless connection a lot of times,
usually to get a quick log with my phone. For example, this weekend
I'm doing a few tests with a tesla model s and I'd love a more stealth
method to get the data.
About being stealth, I found myself using ELM327 adapters to get CAN
data more than a few times, but I have to include a lot of filtering
due to the slowness of the adapter. That said, the STN1170 is an
"elm327 on steroids", and may have enough horsepower to log unfiltered
can data, while being a very good diagnostic tool to interface with
Torque. http://batman.homelinux.com/blog/stn1170-bluetooth-obdii-adapter/
I still have not tested it, but maybe will soon. Single chips are
available for 10$ in single quantities.

Regards
MG


2013/9/5 HONDA S-2000 <s2000@sounds.wa.com>:

I like the MCP2551, but I don't think it's guaranteed to work without a 5V
boost regulator. The USB specifications are clear that the 4.5V minimum
needed is not always guaranteed to be present.

As for the crystal, it's all about the accuracy of the internal oscillator.
USB calls out a precision in the specification, and as long as you meet that
you're fine. Older chips had an internal oscillator that was not precise
enough. This newer PIC32MX796 has a 0.9% oscillator, meaning that it's
plenty accurate enough for USB. I trust Microchip, in that they don't claim
their internal oscillator will work for chips that can't do it, but this one
can.

That said, there's nothing wrong with putting the traces on the board (other
than the space they take up). I'm partial to smaller SMD parts like the ECS
ECS-120-20-30B-DU, but those are more expensive than the large canned
crystals.

Does the CAN spec (ISO-11898) say what the required precision is for the
clock?

The SN65HVD235 has a bus listen-only feature that seems like a really safe
option for a project like this. I imagine that the device will mostly be
listening to CAN messages and not attempting to inject potentially dangerous
messages into an unknown vehicle. Supposedly, the listen feature also helps
with autobaud.

Brian



On Sep 4, 2013, at 19:22, Mark Webb-Johnson wrote:

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).

Regards, Mark.

On 5 Sep, 2013, at 4:01 AM, HONDA S-2000 <s2000@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.

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