topic Re: CAN RTR feature in S12 / MagniV Microcontrollers

CAN RTR feature

BFS — Thu, 10 May 2007 22:20:49 GMT

Hi,

We tried to use RTR feature. Sending RTR frames is OK, but I can't send RTR answer frame automatically (by hardware support).

MSCAN user manual says that:

This flag reflects the status of the Remote Transmission Request bit in the CAN frame. In the case of

a receive buffer, it indicates the status of the received frame and supports the transmission of an

answering frame in software. In the case of a transmit buffer, this flag defines the setting of the RTR

bit to be sent.

What is the meaning of software? (Does freescale MSCAN not support automatic RTR answering?)

Thanks in advance.

Re: CAN RTR feature

Lundin — Wed, 16 May 2007 15:56:21 GMT

Think of it... how would the hardware know how to respond to a RTR frame? Are you suggesting that there should be an 8 byte hardware data buffer for every possible identifier on the bus...? What would be the point in letting the hardware do this? Maybe you would get a slight gain in speed, I see no other purpose.

Also, correct me if I'm wrong, but I believe that the MSCAN hardware signals for a new frame after it has checked the End-of-frame. That gives the app at least 3 CAN-bus cycles of intermission frame space to react and produce a high-priority frame. Unless you have a very high baudrate on the bus, that should be no match for the CPU. With a standard baudrate of 125kbps and a HCS12 running with 8MHz, you'd have 384 cpu ticks to finish the task.

Re: CAN RTR feature

DanielM — Thu, 17 May 2007 22:15:58 GMT

automatic RTR reply is supported by FlexCAN modules (e.g. on power PC or new DSC devices) but not by the simpler MSCAN module

Daniel

Re: CAN RTR feature

BFS — Wed, 23 May 2007 20:37:51 GMT

Dear Lundin,

You said : Think of it... how would the hardware know how to respond to a RTR frame?

For example PIC18F2480 says that:

23.2.3.1 Programmable TX/RX and

Auto-RTR Buffers

The ECAN module contains 6 message buffers that can be programmed as transmit or receive buffers. Any of

these buffers can also be programmed to automatically handle RTR messages.

Another example that Daniel says that : automatic RTR reply is supported by FlexCAN modules (e.g. on power PC or new DSC devices) but not by the simpler MSCAN module

So these means that hardware RTR is possible but not in MSCAN (that was the right answer!)

You said : Are you suggesting that there should be an 8 byte hardware data buffer for every
possible identifier on the bus...?

Dear Lundin I don't suggest anything. There was a simple question and that was about supporting or not supporting hardware RTR. The answer is also very simple. "No hardware support" ,as freescale's technical support said:

There is no internal HW mechanism which recognize data/remote frame. It has to be done by user SW. by Ladislav

My Problem was not in receiving these remote frames but in interrupting the CPU by these frames. However, thanks for your interest and trying to help with this question.

Re: CAN RTR feature

sunil99 — Fri, 01 Jun 2007 14:12:14 GMT

hi ,

I am facing problem while handling interrupts in MC9S12XDP512 for codewarrior IDE i am not using XGATE.the linker file generated by it is not supporting the vector.c file which contains interrupt handling routines.so i need to write my own linker file can u suggest me ways how to write my own linker file to keep my own files and routines of interrupt in linker file in desired locations so please suggest me ways i am facing this problem from a weeek please show me some path.

thanks in advance

bye

sunil

Re: CAN RTR feature

Alban — Fri, 01 Jun 2007 15:16:50 GMT

Sunil posted a new thread with this question.
See using code warrior for MC9S12XDP512 how to write my own linker file for it. i am not using XGATE