I've read numerous posts advising that trace length must be kept short on a high speed board to reduce the effects of impedance. If my traces are routed through a pin header that has an IDC ribbon cable "to nowhere" does that have an effect to manage too? If so, how do I manage it?
Longer explanation
I want to integrate some high-speed logic with my Raspberry Pi 4 via its GPIO header. I plan to make a PCB containing the logic in question and it will sit atop the Raspberry Pi, using a female socket header that connects to the Pi's (male, 40-pin) GPIO header. My target application speed is 20--25 MHz. I'm using 74LVC-series logic (IC propagation time 3.7--6.5ns, edge rise/fall time <= 2.5ns per datasheet) and is powered from the 3v3 pin of the Raspberry Pi; power consumption should be ~50 mA (well within spec for the Pi 4's regulator). On the Pi side, I would be reading a 16-bit bus of signals through the GPIO header.
I plan to put all this in a case, and I would also like to have the GPIO pins available outside the case so I could attach other devices as needed later. (The logic on my PCB has chip select pins that would set them to high-Z in this case.) So I am planning to add another 40-pin header on the top of the custom PCB - mapped 1:1 to the Pi's - that I could then connect to a ribbon cable that would bring those pins to the case edge.
Here's a side-view / cutaway shot of what I'm planning (not to scale):
When the PCB logic is active, there wouldn't be anything connected to the external GPIO pins; [16 pins on] the ribbon cable would just be implicitly being driven by the PCB's logic as it takes over them.
Does this plan create challenges for the performance of my application? I have a vague idea that the cable constitutes a radiating antenna in this case, but since it's up and away from the board, will that affect the integrity of the signal path and/or add capacitive loading that prevents it from operating at the target speed? (This is a hobby project, not for mass production, so I'm not too worried about radiated EMI -- the device doesn't need to pass an interference certification test.)
And, would it matter if the extra header is "inside" or "outside" the signal path? Consider the following overhead sketch of the PCB:
If header 1 is to the Pi, and the traces go "through" the pins of header 2 (which connects to the IDC cable) and then to the onboard logic, is that different from the case where the Pi connects to header 2 and the IDC cable is on the "outboard" header 1 connection? (Note that per figure 1, the Pi-side connector will be on the bottom layer of the board, although with THT mounting that is a somewhat moot distinction.)
The system is simple enough that everything could be routed on a 2 layer board. But I am leaning toward a 4-layer board of (signal--ground--power--signal) for speed purposes -- necessary, or overkill here? I am fairly confident the high speed data signals can all be routed on the top layer, the bottom signal plane is only for the underside connector, and infrequently-changing signals like chip-select and other control lines.
Assuming that, yes, the cable is an issue... is there any way I could electrically isolate or decouple the two "sides" of the system using passive components? Preferably without sourcing current unnecessarily from the Pi's GPIO pins? (I cannot use a buffer w/ chip select like 74LVC245 between the ribbon cable and the main connection because each of the Pi's GPIO pins would need to be independently selectable as input or output to retain its original capability.)
I'm new at PCB design and grateful for the advice. Thank you in advance!
