I'm an FAE for Freescale Semiconductor. We have an Applications Note for PPP on an HC08 class microcontroller which describes exactly the application KM is attempting to solve.
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The source code can be downloaded from the following Freescale microcontroller product page:
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for "an2120sw"
If the MC68HC908GR8 doesn't have the right peripheral set for KM, there are plenty of other options to choose from that will run this code with little or no modification.
-Gary