Hello,
I am working with a System-on-Module based on the BeagleBone Black and I need to connect a TFT LCD screen that goes in the front panel of a rackmounted 19’’ enclosure. The cable has to run a distance of about 15cm and I am concerned about EMI stuff.
My question is: what is the appropiate line impedance for the RGB signals? What about termination resistor values?
I’ve seen that the TI Sitara Starter Kit has 33ohm termination resistors, but I am not sure how/why that value is chosen and how relates to line impedance. I know for example that for Ethernet you need 100ohm differential and for MII is generally 50ohm single ended. Is there a standard value for
Any suggestions will be appreciated!
Regards,
Francisco
Depends on the LCD, distance, capacitance, and the connector impedance of the SOM. The SOM supplier should be able to answer that question via an application note.
Gerald
33 ohms is a good starting point. Once you have your board operating, use a high speed digital oscilloscope to measure the waveforms on the LCD lines. If they are square with no or little ringing, then the value is fine. If you see ringing, start by increase the resistor value and if it gets worse, then lower the resistor value. This iterative approach is the best solution if you don’t have SI tools to model the circuits. There are several tools you can use to model these circuits:
https://www.mentor.com/pcb/hyperlynx/
http://www.awrcorp.com/products/microwave-office
https://en.wikipedia.org/wiki/HFSS
Regards,
John
These signals are also the boot pins. You need to check the schematic of the SOM to see what resistor values they are using to terminate the signals. I would buffer the signals before connecting them to a LCD panel.
Gerald