I want to put 256 linear feedback shift registers on a FPGA and each LFSR will have just two tap positions for the XNOR feedback and each register is 63 cells . I don't care if the LFSR'S are not maximal length. The catch is I want the tap positions for all these shift registers to be easily reconfigurable. How difficult/easy is it to do this?
Linear Feedback Shift Registers on FPGA’s
fpgahdlprogrammable-logic
Related Solutions
I'd like to place these additional CPLDs on a different PCB. This has the advantage that I can simply extend the device when I need. On smaller harnesses, a single 114 test-point PCB will suffice. On larger ones, I can cascade.
There are multiple levels of modularization which you can aim for. Where you want to stop depends on your specific use case. At the most basic level, the hardware must be designed such that you can select the number of modules in use after the design stage. The difficulty of changing the number of modules, space available, desired software complexity (and available space for software, especially on a CPLD) and the system cost will be key factors in your decision.
Hardware
The simplest and cheapest way to do this is to build one PCB, (You don't need multiple PCBs for modular design!) and put footprints for your desired maximum number of CPLDs on the PCB. If you need more IO, you can then solder down another CPLD. Obviously, this isn't something you'd want to do very often.
At the next stage, you'd want to build daughtercards so that you can more easily add and remove modules. You asked:
But what would be the best way to actually connect the PCBs together?
This depends on your system architecture and number of modules. If you know you'll never want more than, say, 3 modules at any one time, just put three connectors on the main board. These can be edge connectors, or stackable connectors, or whatever you like that doesn't require wires. If the number of sub-modules is too large to fit connectors for each on one PCB, then you should consider stacking (if your bus can handle the fanout of your maximum number of modules) or daisy-chaining (if you need to buffer the signal or vertical space is limited) the modules.
Plenty of connectors are designed for this purpose; check the "Board-to-Board" section of your favorite distributor or manufacturer, and many are designed for extremely low crosstalk and high frequency - 500kHz is nothing, unless you're using PTH 0.1" breakaway headers and have fast-changing signals (even then, you're probably OK). Check the mating strength of your connectors just to be sure, but if you only have a few pins, the footprint of your interconnection doesn't carry the stresses well, or the system will be subject to vibration, you'll need standoffs. It's often wise to design the interface in such a way that different modules can be designed to interface with the motherboard in the future. Pins are cheap, give yourself a couple spares just in case!
Software
If your number of modules supports it, you can simply add a slave select line for each module. This isn't really a software solution, but I wanted to mention it.
If you don't mind programming every CPLD differently, you could build the system such that the microcontroller sees it as one giant shift register (which you've suggested). If you added or removed a module, that module's address space would simply be wasted, and extra time would be used transmitting to addresses which don't exist. Each module would need to 'know' its address space, though, which would make programming the complete system a struggle.
A more versatile solution is to use software addressing to access each sub-module. In a 'programming mode' (perhaps a pushbutton on the module, or simply only connecting one at a time), you could assign the CPLD an address. By assigning each CPLD a different address, you could add or remove modules at will, and only have to adjust the activity of the microcontroller (which I presume to be easier to adjust than the CPLD).
My suggestion for this project
If a 324-pin device will solve all your foreseeable use cases, then the single-PCB method should work fine. The multiple-slave-select method would allow you to program all the CPLDs simultaneously with a single programmer. Sorry, but this project as described doesn't seem like a candidate for daughtercards.
Event the very smallest FPGAs, and most CPLDs can do this. Each element of the shift register needs a single FF element, call that 1/4 of a slice, the xor for the tap logic will go into the LUT. Look at your datasheets, anything with > 300 slices should do it. You'll need extra logic to pre-load a value, control reset and sample the outputs, and probably clock enable logic as you would not want it free-running with the global clock while you read the final state.
Best Answer
If you are using Xilinx FPGAs, the LUTs can be configured as 32-bit shift registers (SRL32), each with one adjustable tap. What I would recommend is using 6 of these 32-bit shift registers as three 63-bit registers in parallel, one fixed at 63 bits and the other two for the variable taps. With a bit of additional logic it should be possible to implement this with fewer registers, though there could be some disruption when changing the tap selection under certain conditions.
If you aren't using Xilinx FPGAs, then it might be advisable to look at the programming manuals to figure out what sort of shift register features are supported. It is possible to make variable length shift registers with large MUXes, though this could consume a lot of logic resources. Dual-port RAMs are another option, especially for longer shift registers or for multiple parallel shift registers with identical taps. Depending on the architectural features of the FPGAs and your design constraints, one option may make more sense than the others.
Another consideration is constraints on how the shift register taps are changed. If you need to change the taps on-the-fly without disturbing the contents of the shift registers, this could limit what architectures you can use and some of the optimizations you might be able to make.