System Clock Calculation Confusion

Discussion created by GARY OLMSTEAD on Jul 23, 2009
Latest reply on Jul 24, 2009 by GARY OLMSTEAD

Hi --


I have a MCF52259 that will have Ethernet, USB, and RS-485 ports.  I would like to use a single clock source for all three.  USB requires 48 MHz exactly, Ethernet seems to be happy with anything over 50 MHz, RS-485 should be 57600, although adjacent rates will do if necessary.  So, I need to balance MFD, RFD, and CCHR.


My problem is that I can't find values for MFD, RFD and CCHR that yield a value between 50 and 80 MHz, and will also yield an integer remainder after being divided by 32 for the baud rate generator.  I have two questions for you.


One, what is the purpose of Table 7-6 on page 136 of the Reference Manual?  It shows the ratios for every value of MFD divided by every value of RFD, but that ratio isn't used in the calculation of Fsys, or anyplace else I can find.  Is the table a red herring, or is the formula for Fsys wrong?


Two, the Fsys calculation appear to generate very few useful values.  In particular, Fsys for RFD = 4 to 7 is anywhere from zero to a few kiloHertz.  Fsys for RFD of 0 or 1 quickly runs over 80 MHz.  Is this correct, or is the formula for Fsys wrong, or did I mess up the calculation?  I put it in a spreadsheet, but I have no idea how to post it here.



Gary Olmstead