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jmaudley
Joined: 01 Feb 2011 Posts: 32 Location: 53
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MCP2515 - trying to set CLKOUT prescaler to clock/4 |
Posted: Thu Sep 11, 2014 2:34 am |
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Hi,
I'm digging through the CCS code 'can_mcp2510' trying to find out where I can modify the CANCTRL register to adjust the CLKOUT freq.
Unfortunately, I'm struggling to find where this is set and I don't really understand the CCS code.
I'm assuming that can_set_mode (CAN_OP_CONFIG); is the function that sets this, but where is it and where is the definition for CAN_OP_CONFIG ?
It says in the comments that some of the defines are in the can_mcp2510.h file which confuses me even more.
I don't really get why can_mcp2510.h is included within can_mcp2510.c.
Also, should I be using can_mcp251X instead of 2510. The CCS CAN dev kit has an MCP2515 chip but uses the 2510 code.
I did a diff on the codes and there are some slight changes to CTR register I think.
Any guidance would be appreciated.
PS - It all works I just need to change the CLKOUT div which I use as the clock for my main PIC. |
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Ttelmah
Joined: 11 Mar 2010 Posts: 19553
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Posted: Thu Sep 11, 2014 3:39 am |
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It's included, since otherwise it wouldn't get read....
On CAN_OP_CONFIG, not quite.
CAN_OP_CONFIG, is the _command_ to put the chip into config mode, to accept the device configuration. It is defined in the .h file, which generates an enum, with values 4,3,2,1, for CAN_OP_CONFIG, CAN_OP_LISTEN, CAN_OP_LOOPBACK, and CAN_OP_NORMAL.
The actual configuration is sent after the chip is switched to this mode.
The key is the can_set_baud function. This is called immediately after this mode is selected.
If you look at the start of this, you see:
new_CNF1.brp=CAN_BRG_PRESCALAR;
which sets the prescaler (why they spell it with an A, don't ask me.....).
and if you look in the .h file, you see:
Code: |
#IFNDEF CAN_BRG_PRESCALAR
#define CAN_BRG_PRESCALAR 4 //baud rate generator prescalar (def: 4) ( Tq = (2 x (PRE + 1))/Fosc )
#ENDIF
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So it is set to '4', _unless it is already defined_
So all you need to do, is define the value you want, before loading the can file.
So:
Code: |
#define CAN_BRG_PRESCALAR what_you_want
#include <can_mcp2510.c>
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Putting the number you want, in 'what_you_want'.
The keyword to the chip, is 'compatible'. If you look at the header of the code, you see it says:
MCP2510 (and compatable) (again CCS spelling...). The 2515, is a direct update to the 2510. The 251x, and 2510 files seem to be almost duplicates of one another (no idea why...). |
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jmaudley
Joined: 01 Feb 2011 Posts: 32 Location: 53
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Still confused |
Posted: Thu Sep 11, 2014 5:51 am |
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Sorry, still a bit confused.
Should it really be the baud rate generation prescaler.
If it is I don't understand how 4 equates to system clock/8.
Shouldn't it be the clkout pin prescaler?
clkpre:2
Also don't understand how 2 equates to divide by 8. |
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Ttelmah
Joined: 11 Mar 2010 Posts: 19553
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Posted: Thu Sep 11, 2014 6:25 am |
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Got you. You want to use this for something.
The driver doesn't actually use it. It is always 'reloaded' with whatever it is already defined as, which gives /8.
Code: |
void can_set_mode(CAN_OP_MODE mode) {
struct struct_CANCTRL old_CANCTRL;
struct struct_CANSTAT new_CANSTAT;
memset(&old_CANCTRL,mcp2510_read(CANCTRL),1);
old_CANCTRL.reqop=mode;
//add this line
old_canctrl.clkpre=2; // /4
mcp2510_write(CANCTRL, (unsigned int8)old_CANCTRL);
do {
memset(&new_CANSTAT,mcp2510_read(CANSTAT),1);
//old_CANCTRL=mcp2510_read(CANCTRL);
} while (new_CANSTAT.opmode != mode);
}
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Replace the can_set_mode routine in can-mcp2510.c, with the above. |
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jmaudley
Joined: 01 Feb 2011 Posts: 32 Location: 53
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Working |
Posted: Thu Sep 11, 2014 7:27 am |
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You are a star!!
Thank you. |
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