[Ovmsdev] OVMS CAN-USB Call for Assistance

Mastro Gippo gipmad at gmail.com
Mon Sep 9 23:25:02 HKT 2013


After a quick search I found this:
http://www.ti.com/product/lp2992
with a vdo of less than 450mV and a max input voltage of 16V, it may allow
us to just use 2 diodes to auto-select the highest power source. Maybe a
similar product can be found on the china market with similar features but
cheaper.
The switch idea is good too, but a bit less practical IMHO.
Regards
MG


2013/9/9 Mark Webb-Johnson <mark at webb-johnson.net>

>
> I'm working on a rough eagle sketch based on the UWB32 as a starting
> point. Just something to get a pricing indication and that we can then see
> how other design could compare. It is a very simple circuit.
>
> 3x LEDs - 1 red, 1 green, 1 blue (power). 1 push-switch to be held down on
> plug-in to put the unit into firmware-update mode.
>
> I'm asking the factory for costing for 2 CAN ports. Extra cost looks to be
> about US$1-US$2 for the extra port, but it may be worth it. It does allow
> some pretty neat bridging tricks, as well as increases the flexibility of
> logging. It can be done with a single extra can transceiver chip that
> doesn't take much space (8 pin IC only), and we can use already spare pins
> on the DB9.
>
> The UWB32 approach is to use a LM317 style voltage regulator, to take the
> 5V USB to 3.3V. But, if we're going to add pinouts for bluetooth /
> expansion / whatever, and with the previous discussion regarding power from
> the 12V car side, I thought it worthwhile investigating if there is a
> simple way of doing this from a selectable power source.
>
> Here is the simple UWB32 style circuit for 5V USB -> 3.3V, that I
> currently have in my design:
>
>  The UWB32 has a separate LM317 and selection switch, for 7.6V-15V, using
> this circuit:
>
>
> Seems to be the UWB32 is using two LM317 - one for between 7.6V and 15V
> going down to 5V, and then another to convert the 5V (USB or stepped down
> from 7.6V-15V) to 3.3V.
>
> It seems overly complicated, but I see the issue with finding something
> cheap and simple to operate over the range (4.75V-15V) that we require.
> Most of the designs I have seen use dual LM317 (or something like it).
>
> Anyone seen anything simple so that we could have 12V car (say 9V-15V) and
> 5V (4.75V-5.25V according to spec) USB go to a slide switch to select
> source, then a 4.75V-15V step down to stable 3.3V? Have the switch on the
> side - slide it towards the DB9 for car power, or towards 5V for USB power
> (but never both). It would be a pretty neat solution and allow for
> unattended logging. This is, of course, assuming we use SN65HVD235 to avoid
> the need for 5V anywhere on the board.
>
> Regards, Mark.
>
> On 5 Sep, 2013, at 10:47 AM, HONDA S-2000 <s2000 at sounds.wa.com> wrote:
>
> 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 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.
>
>
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