Quote:
Originally Posted by dcb
Easy cowboy  Elm has been decided for 1.0, it is universal, easy to implement, and being through hole it is very "kit"able. We are going to work on the price AFTER getting something out there.
for the post elm obd version, we hope to be as low cost as possible, but have not abandoned the arduino platform yet. ishiyakazuo is looking into a software only (duino + a couple transistors) ISO implementation. Maybe you want to sort out the bare bones CAN details? It would be good if there is a bare minimum through hole couple transistors software only CAN version as well, as far as price and assembly go. |
Agreed. Elm is a good starting point. I'll probably skip 1.0 myself.
As far as bare-bones CAN, I'm not convinced it's a good idea. Hardware alone wouldn't just be "a couple of transistors", but rather quite a number of discretes (at least two Zeners, Cs, Rs, probably an op-amp). You'll need to worry about slew-rate, shielding the Arduino from transients and all that good stuff, and -- more importantly -- shield the car's CANbus from the Arduino. Just imagine what might happen if EMI within the Arduino is amplified and makes it to the CANbus, or if a transient melts your termination and shorts the bus... the CAN operates on a shared medium, and might well disable the car if choked.
And that's the next point... collision detection, arbitration, bitstuffing and phase-locking are all non-trivial problems that have folks much smarter than us scratching their heads. I don't think a 16 Mhz 168 is fast enough to handle 500kbs... even at 20 MHz you only have 40 cycles per bit to decode what's coming across the wire, and if that's enough, the uP is then fully utilized.
Logically, then, you might be looking for a second uP to handle the UI, and there you are out another $3-$4, along with the discretes... you might as well go with an available and proven solution.
Reading ahead (while I'm writing this) it appears ishiyakazuo was going the AT90-route. He is correct on the combined cost of MC+mega**8 solution vs. the AT90. Do keep in mind that the prototyping cost of the MC solution is considerably lower -- breadboard, RBBB, two DIP chips, done. No mussing with adapters or break-out boards. Ditto for reproducibility -- heck, you can wire-wrap this baby or stick it on a proto-shield.
I don't think we'll need 64K -- a large portion of the 12K-or-so in use now comes from the libraries you linked in. 32K should be plenty to do what you're attempting to accomplish here. I do have two precious ATmega328s here (thanks for the samples), and it's a perfect drop-in replacement for the 168. I'll pull the code and check the baseline this weekend.
BTW, good call on the Altoids tins
The custom PCB to populate a la carte is certainly a very well "kit"able option. I'm curious to see how the differential bus compares to, say, an RS485, where many solid interfaces are available.