SPI and 18F4550

[HenrikOlsson]

Hi,
As far as I can see there's nothing in your code that actually sends any data anywhere. In the SENDCANDATA you pull the CS line low, then you assign some values to a couple of variables, then you pull CS line high again. There's nothing to actually shift any data out of the PIC using either SHIFTOUT/SHIFTIN or the MSSP module.

Is there any chance you've looked at some code for a PIC with a built in CAN tranceiver? Is that perhaps where the TXBNDLC etc variables are coming from? If so, then those are probably registers for that particular PICs internal CAN tranceiver and simply creating variables with the same name will obviously not get anything out of your 2550 and into an external CAN tranceiver.

/Henrik.

[Christopher4187]

I was thinking the same thing last night about sending data but I really don't know how the SPI function works. Here is an excerpt from the MCP2515 data sheet (stand alone CAN IC) where I got the TXBNDLC variable from:

[HenrikOlsson]

Yeah, but that register is in the MCP2515 - you're only assigning values to variables in the PIC, there's currently nothing that transfers the data TO the register in the MCP2515.

Look at the SPI section of the datasheet for the MCP2515, under Write instruction it says

The Write instruction is started by lowering the CS pin. The Write instruction is then sent to the MCP2515 followed by the address and at least one byte of data.

Then, looking at table 12-1 we can see that the Write instruction is (in binary) 00000010 or 2 in decimal. As you can see in your screenshot from the datasheet the adress of the TXBnDLC register(s) in the MCP2515 is $35, $45 and $55 respectively (apparently there's three of them, I haven't read enough yet to understand exactly what they do).

So to set the first of the TXBnDLC registers you need to shift out the WriteCommand (2), the adress ($35) and the actual data (7). The SPI section of the datasheet also tells us the device supports mode 0,0 and 1,1 and the timing diagram shows that it expects (and sends) MSB first. So, using SHIFTOUT the above would look something like

INCLUDE "modedefs.bas"

Write_Cmd CON %00000010    ' Value for the Write Command
TXB1DLC CON $35                  ' Adress of the 1st TXBnDLC register in the MCP2515

LOW CS
SHIFTOUT DataPin, ClkPin, MSBFIRST, [Write_Cmd, TXB1DLC, 7]
HIGH CS

The example I linked to earlier and the article gadelhas wrote shows you how to set up and use the MSSP module.

/Henrik.

[Christopher4187]

Henrik,

You've made it a little clearer for me, thanks. I'm going to work with this today to see if I can get it to work. Something is strange with the demo boards I've received from Microchip. I can't erase the chip and even blinking an LED can be a task. I'm not even sure if the program I'm compiling is even getting to the 18F4550 at certain times. When I make the same circuit on a breadboard it doesn't give me any problems so I may have two issues at the same time, which makes things even more difficult.

Chris

[Christopher4187]

I'm getting closer. I couldn't figure out what was going on so I pulled out my scope and it seems pretty clear......at least for this step. This is a picture of the scope with a known good program (I'm using the same board for the working program and my program so I know the hardware is good). Yellow is the data and blue is the CS line.



Now look at the scope with my program:



So my signal looks like it's inverted. They speak about this in the BAUDCON, TXSTA and RXSTA settings but I tried adjusting them and it didn't help. Any ideas?

EDIT - It was under the SSPSTAT.6. The signal is correct now but still no CAN data. At least I'm getting closer......

[HenrikOlsson]

Ok, what exactly are we looking at?
My guess is the MOSI pin on the PIC (Ch1) and the CS line (Ch2)?

Without seeing your code it's a bit hard to guess what you're actually doing but what I can say is that BAUDCON, TXSTA and RXSTA (if we're talking about the PIC) are control registers for the USART and has nothing to do with the either the MSSP module or the SHIFTOUT/SHIFTIN - whatever you're actually using. Post the code ;-)

/Henrik.