This question has come up during SpeedWay trainings so I thought I would post this here for everyone to see.
Q: In the Zynq PS Config tool, the UART clock setting is locked out. You mentioned modifying this clock setting from the default of 100MHz back to 50MHz in the SpeedWay training. How was this done?
A: The UART clock can only be modified by using software later on to change the PS register settings OR by hand-editing the ps7_system_prj.xml file while in the XPS stage of design of a PlanAhead project. This is what I had to do for the Zynq Linux SpeedWay since the hardware project is built upon the Zynq Intro SpeedWay platform. Since by default UART clock is set to 100MHz, in order to change it, I simply hand edited the LED_Controller\LED_Controller.srcs\sources_1\edk\system\data\ps7_system_prj.xml file and set the property back to 50MHz. The properties in this XML file get rolled out to the ps7_init.tcl, ps7_init.c, and ps7_init.h files when you do the SDK Export design step.
So the rule of thumb in this file is that if you want to use a default setting for a peripheral, you do not need a line entry the tools simply use the default value. If you want to modify a property from the default value, simply hand edit the XML file and make your entry. Caution, the last time I checked, this file is a lot like an MHS file in that if you screw something up you might get an error message, but the tools will not tell you what or where a problem was encountered. To give you an idea of what you are dealing with, here is a modification to the UART clock property in this file:
Why we need modified the requested frequency from the default 100 MHz to 50 MHz?