We have been using a crystal as the input to the 26M oscillator on the i.MX27. We are attempting to modify the design to use an external oscillator input instead. The external oscillator is input on the EXTAL26M pin.
What I am unclear about is the correct setting of the OSC26M_DIS bit. I was under the impression from the documentation that this bit should be set to 1 when using an external oscillator. However, this results in the system locking up when I restart the MPLL to use the new clock settings. When I leave this bit set to 0, everything seems to operate correctly, however that is not what I expect to happen. With an external oscillator, what is the correct setting for this bit? Is there something I am missing in order to disable the internal oscillator?
Here is the CSCR register setting:
CSCR = 0x33378101; //This works. OSC26M_DIS = 0
CSCR = 0x33378109; //This fails. OSC26M_DIS = 1