CAN TRX and 3V3 systems

[UAVflightdirector]

I am mounting 5V CAN tranceivers. If you need compatibility with 3V
systems, you will need a different tranceiver.

The CAN physical layer will remain the same and will be 100%
compatible with the 5V system on the other end.

[Adam Barrow]

Matt,
as I understand it, this means I could use a separate 3V CAN transceiver to say attach to the UDB4 (assuming it were 3V) but still use the same 5V transceiver on the new expansion module? That's a really nice feature if so, since there's a chance that the UDB4 will be 3v3. If I've misunderstood, what would be required to use the new interface boards with a 3v3 controller?

Thanks,
Adam Barrow

[UAVflightdirector]

Adam,

Your first understanding is correct. Only the CAN traceiver on the
little board need to be changed. It is an industry standard pinout so
there are no problems finding a replacement.

It was the dsPIC33 on UDB4 that made me think of this.

Matt

[Adam Barrow]

Matt,
thanks for the clarification. In my case, I'm thinking I would probably just build an extra transceiver board when the time came, to have one for 5v systems and one for 3v3 systems. It's great to know however, that moving between the two (5v and 3v) environments won't be a whole lot of drama.

Regards,
Adam Barrow

[J. L.]

Couldnt you just use a logic level converter if you need lower than 5v
or am i not understanding something properly?

[UAVflightdirector]

The 100% most important reason for not using a LLT is:
The PCBs are sat on my desk rather than in design.

and also:
CAN tranceivers are really cheap (<1euro)
The complete board is really cheap (~4euro)
There are loads of spare trx PCBs

ALL: IF YOU HAVE A 3V SYSTEM AT THE AUTOPILOT. Please let me know and
I will see what I can do for it.

Matt

On Oct 13, 10:37 pm, "J. L. " <vwyodap...@gmail.com> wrote:

[J. L.]

I was meaning running the LLV converter between your AP and the servoboard.

[UAVflightdirector]

It sounds like you are not fully understanding what CAN is. My
apologies if I am reading you wrong. Here is a quick description for
everyone anyway:

The CAN physical connection does not use standard logic levels. It
uses a differential signalling system.

CAN is a little like I2C in that it is open collector driven.

The two bus lines are named CANH and CANL. These two lines are
terminated by 120ohm at each end. CANH gets pulled up, CANL gets
pulled down. They get pulled by about 1V in each direction.

A dominant bit is when the two signal lines are pulled apart.
Recessive is when they are not pulled apart.

The CAN receiver has a big common mode range (-7V to 12V) so a good
ground is not needed. You will see that the CAN tranceiver boards do
not have a bus ground connection. This is ok. You will connect the
ground with the power supply system somewhere. If you dont connect
the power supplies together, there is a little resistor that should be
fitted to the tranceiver board.

The CAN tranceiver board translates between the standard logic level
of your microcontroller and the CAN physical layer.
Why not put CAN directly on the microcontroller?
The CAN voltages are big compared to the process capability for a
processor.
The CAN trx has to dissipate quite a bit of power in a fault
condition.
The cost of integrating it with the micro would be huge compared to a <
$1 part in volume.

Regards Matt

On Oct 14, 6:25 am, "J. L. " <vwyodap...@gmail.com> wrote:
> On Wed, Oct 13, 2010 at 9:05 PM, UAVflightdirector

[J. L.]

No you were right I was thinking of it in the wrong way. Thanks for
the proper explanation for me, hardware stuff is not my forte. :)
Still excited to start playing with em. Great work from what I have
seen so far.

[Paul G.]

All the can pins on Microchip controllers are either by default 5v
tolerant or can be PPS'd onto 5v tolerant pins. The transceiver specs
are such that they should work as is with a 3.3v micro. Mostly you
don't need to worry about this. There does however have to be a common
ground. Isolated transceivers do exist but they isolate the logic side
from the bus side, the bus side of all the nodes should operate with a
common ground. The only way I could see this be a problem if each
board had its own battery supply - in that case one may have to
connect the two grounds.

Paul

On Oct 13, 10:14 pm, "J. L. " <vwyodap...@gmail.com> wrote:
> On Wed, Oct 13, 2010 at 9:53 PM, UAVflightdirector