AVR-Chat

Hi Chuck,
 
Well done on the progress. Good to hear you went straight to CAN.
 
I have one comment on using interrupts that you might need to be aware of.
This won't apply if all your code for the MCP2515 is done in the interrupt
handler and you are not using SPI for anything else.

The issue you have is when you want to send CAN data. If you have interrupts
setup for receive, which is the most likely case, then you can't be checking
the MCP2515 in any of your foreground code to send data unless you have some
form of critical section around the SPI stuff. My first attempt at writing
this was long and messy, I'll try again.
 
The reason for his is that if you might be in the middle of sending an SPI
command to the MCP2515 in your foreground code to setup the register to read
or write to.
 
Then as you are about to read the register with the next command the
interrupt handler fires and changes the register pointer to get the received
data and then it exits the interrupt handler.
 
Now when you came back out, your foreground task would be looking at the
same register the interrupt handler just accessed and not the one you wanted
or had previously setup.
 
The issue I faced with the controller was the fact that I didn't get a
transmit interrupt until I actually sent something and it fired the
interrupt when that data was transmitted. I ended up switching off
INTERRUPTS whilst I was writing to the device which was only when I needed
to send data which on the design at the time was once per second. It only
had to be disabled for the SPI calls.
 
Hope this info helps.
 
Dave...
---
Very funny Scotty, now beam down my clothes!!!
---


 
From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of Chuck Hackett
Sent: 05 January 2011 05:53
To: AVR-Chat@yahoogroups.com
Subject: [AVR-Chat] Project status report and question ...
 
  
- I am now writing the firmware. My two challenges have been that I am using
FreeRTOS for the first time and, due to the recommendations of list members,
I
bypassed my initial plan (start with RS-485 and switch to CAN later) and
went right
to CAN. I included an MCP2515 CAN controller (SPI interface) and MCP2551 CAN
bus
driver on the board. It took me about 4 days to get the MCP2515 to cooperate
-
mostly due to a dumb (aren't they all?) programming error of course. On the
positive side, I have lot of 'instrumentation' in place as a result of the
"bug
chase". I now have simple test messages (w/o interrupts) going between two
controllers (over 5" of wire :-) ). In the process I have gained a basic
understanding of how to work with the MCP2515 and I'm about to convert it to
interrupt driven. So far I have not discovered any connections (traces) I
needed
that I did not include in the board design, so, so far, the boards won't
have any
ugly "cut/reroute" of traces. 


Cheers,

Chuck Hackett




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