I'd recommend you start by designing shields and/or capes.
Designing a complete system from scratch has three components, the system design, verification of the design, and then bring-up. Its very frustrating there in the middle. If you want to practice take a built system, and the data sheets for the parts, and write an operating system for it. A lot of chicken/egg problems. Things like "Ok see if the HW bootloader is working by loading a loop which toggles this line which I can watch using an oscilloscope." Real, starting from nothing into something stuff.
Designing shields (Arduino) and capes (BeagleBone) gives you a "known" working system so that you can prototype various peripherals you want in the final system. This is what manufacturer evaluation boards are actually for. They have the ability to be configured into all of the supported modes so that you can try them out prior to building them. A number of systems just lift the schematic from the Evaluation board, delete the parts they don't want, add the parts they do, and voila system "design."
Staying below 20mhz is a good idea until you get a handle on what happens to signals as they travel across a circuit board. Working with LVDS (low voltage differential signalling) lines only makes sense when you see what a 50mhz line looks like 4" from the processor on an unterminated 3 mil trace. :-)
Also if you make useful shields/capes and re-sell them then you can earn some money to buy the tools to make the more complex systems. You will also discover that a 500Mhz oscilloscope that is 15 years old is nearly as expensive as a 500Mhz oscillocope that is new because well if it works and is calibrated, then it works. Kind of like machine tools, they have intrinsic value.
Heh... forget making sense, you can't even tackle some LVDS systems until you start thinking about your "observer effect" when you try to measure signals, i.e. the impedance of your probes...
Designing a complete system from scratch has three components, the system design, verification of the design, and then bring-up. Its very frustrating there in the middle. If you want to practice take a built system, and the data sheets for the parts, and write an operating system for it. A lot of chicken/egg problems. Things like "Ok see if the HW bootloader is working by loading a loop which toggles this line which I can watch using an oscilloscope." Real, starting from nothing into something stuff.
Designing shields (Arduino) and capes (BeagleBone) gives you a "known" working system so that you can prototype various peripherals you want in the final system. This is what manufacturer evaluation boards are actually for. They have the ability to be configured into all of the supported modes so that you can try them out prior to building them. A number of systems just lift the schematic from the Evaluation board, delete the parts they don't want, add the parts they do, and voila system "design."
Staying below 20mhz is a good idea until you get a handle on what happens to signals as they travel across a circuit board. Working with LVDS (low voltage differential signalling) lines only makes sense when you see what a 50mhz line looks like 4" from the processor on an unterminated 3 mil trace. :-)
Also if you make useful shields/capes and re-sell them then you can earn some money to buy the tools to make the more complex systems. You will also discover that a 500Mhz oscilloscope that is 15 years old is nearly as expensive as a 500Mhz oscillocope that is new because well if it works and is calibrated, then it works. Kind of like machine tools, they have intrinsic value.