18f4550 CAN bus (SOLVED)

705150 · Joined: 25 Apr 2013 Posts: 14

hi there, i'm working on a project. I have 4 pics connected in CAN bus (3 x 18f25k80 and 18f46k80) with mcp2551.
I want to be able to read what those pics are talking in CAN bus with my laptop. I have an 18f4550 to create a USB to Serial port on laptop, but i also need to connect the 18f4550 to the CAN bus.

Now, the question is: can i simulate a CAN controller (software CAN bus) on a 18f4550 so i can read/write on CAN bus?

my compiler is 5.008

thank you in advance

Ttelmah · Joined: 11 Mar 2010 Posts: 19545

Remember you are going to need an IC to generate the required signals. Canbus uses differential signalling, not simple logic levels. Software Can, is not going to work without this conversion, and the timings would prevent a chip from doing anything else (something like a USB interrupt would destroy the timings). So, simplest solution is to use an IC that does both. Something like the MCP2515. Driver for this (or it's older brother the 2510), is in the drivers directory (can-mcp251x).

705150 · Joined: 25 Apr 2013 Posts: 14

Of course CAN bus can not be driven directly, i am using mcp2551 for that task. However, mcp2551 delivers logic 5v signals at its exits (Rx Tx) and differentials at its inputs (CanL CanH). I was referring from the 2551 side to the 18f4550, if i can simulate a CAN bus driver.

Ttelmah · Joined: 11 Mar 2010 Posts: 19545

The driver has to do so much work. I'd say it might be doable, but the chip would end up having to do _nothing_ else. You have very tight timing requirements (which means nothing else interrupting), and then you would need code to do the scanning for conditions etc.. Realistically your PIC would end up not being useful for anything else.
You'd need to implement the complete state machine, and CRC, together with error handling etc.. At a very low Can rate, it might just about be possible, but will fill most of the PIC's memory, and at higher rates it just won't be able to cope.
Given the cost of the can interface chip, and that this includes the interface as well, it's silly to not use it.

705150 · Joined: 25 Apr 2013 Posts: 14

I have these chips lying around the house. I wanted to minimize the number of components, but i guess i will have to use both the 18f4550 for usb to serial and 18f25k80 for the CAN with mcp2551. Thank you for your reply, have a nice day.

Ttelmah · Joined: 11 Mar 2010 Posts: 19545

You are then adding another chip. Just use the 8550, and the MCP2551. You don't need another PIC.

RF_Developer · Joined: 07 Feb 2011 Posts: 839

705150 · Joined: 25 Apr 2013 Posts: 14

thank you Rf_developer for your helpfull reply, i have another project for the future where the pic is on 3.3v and need it to connect to a 5v canbus so the mcp2561 does come handy

newguy · Joined: 24 Jun 2004 Posts: 1909

newguy · Joined: 24 Jun 2004 Posts: 1909

My go-to choice for CAN transceiver is the Texas Instruments SN65HVD233MDREP.

705150 · Joined: 25 Apr 2013 Posts: 14

The thing is i used a 18f25k80 at 3.3v vdd but i also connected the mcp2551 at the 3.3v, but it behaved rather chaotic and damaged 2 can drivers until i switched to 5v at which its curently working fine right now.

But i have a rfm73 2.4ghz tranceiver that works at 3.3v spi that i have to use later. This is for a car alarm and the rfm73 is for the remote. I have to find a way to connect them all togheter.

Thank you all for your help so far, its good to find nice people here.

newguy · Joined: 24 Jun 2004 Posts: 1909

The MCP2551 is rated for 5V only - that's why it acted erratically. Use a CAN transceiver rated for 3.3V operation and your entire project can use a 3.3V power supply rail....no need for 5V at all.

It also needs to be clearly restated: you can connect any CAN project/PCB to any other CAN project/PCB and it doesn't matter if one uses a 5V system and the other a 3.3V system power supply.

705150 · Joined: 25 Apr 2013 Posts: 14

thank you all again for your reply, i hope to be able to return the help some day, for now