Michael Christie

HCS12: Resistance for SCI loopback

Discussion created by Michael Christie on Sep 4, 2007
Latest reply on Sep 7, 2007 by Daniel Lundin
Hi all,
I'm working on a project with an HCS12 (MC9S12DP512) MCU.  One of the SCI ports of the MCU will normally be used for communication.  However, there will be times when we want to loopback the TX port to the RX port (it's a bit of a long story whywe want to do this.)  When we do this loopback, we need the SCI to function so that the MCU will receive the data that it sends.
Rather than just short the two ports together, I want to put a resistor between them to protect against the software wrongly configuring the ports and causing a logic conflict (firstly to protect the ports but also to keep the power consumption to a minimum should this occur.)
My question is, is there anything I need to be wary of when doing this?  I'm looking at using a 10K resistor to restrict the current to 0.5mA in worst case scenario.  It's worked fine in the testing that I've done but I want to be sure that I haven't missed anything.  I haven't found anything to suggest that there is a limit on the resistance you can place on an SCI line.  The only issue I can think of is the capacitance of the line and RX port which, for 115kbs communcation, shouldn't be affected by 10K resistance.