CAN transceiver interfacing to uC- resistors on RXD ,TXD lines

[Akshatha Venkatesh]

could someone please tell me why series resistors are used on the TXD, RXD lines of a CAN transceiver while interfacing it with a microcontroller. I've read a few application notes where they suggest not using the series resistors since they would cause some delay, but a few others suggest using 1kohm resistors as described in the application note below,(AN10211 TJA1040 high speed CAN transceive).How exactly is the value 1kohm chosen, any idea?

Any elaborate explanation on how this exactly works would be helpful, or links on anything I should read about to understand this better. Also I read in one of TI's SN65HVDA1040A-Q1 EMC-Optimized High-Speed CAN transceiver datasheet [URL='https://i.stack.imgur.com/6p5AV.png'] ,[/URL]
[URL='https://i.stack.imgur.com/hI44z.png']
[URL='https://i.stack.imgur.com/hI44z.png'] that R1, R2,R3,R4 in the above figure are used to limit current of digital lines. I'm just wondering how exactly would I know if I need these resistors or not and are they used for better EMC performance or to limit current . Also are these resistors used for line termination ?[/URL][/URL]

 

[Harald Kapp]

How exactly is the value 1kohm chosen, any idea?

By rule of thumb and experience. The exact value is not critical and depending on the layout (e.g. length of traces, number of pcb layers incl. possibly ground planes etc.) other values may offer the same or better effect. Here a compromise between reduction of EMC and preservation of signal integrity has to be made.

R1, R2,R3,R4 in the above figure are used to limit current of digital lines.

Not really. In a CMOS circuit the static current on digital lines is almost negligible. What these resistors do is together with the capacitances of the wiring and the attached inputs they form a low pass filter. This filter will reduce higher order harmonics (by reducing the slope of fast digutal edges) and thus wil reduce EMC emissions.