MSEL[4] and return current cap...
Added by Austin Franklin about 1 year ago
The dev board has MSEL[3:0] with 1000pF caps, and MSEL4 does not have a cap, unless I missed it. In the design guide:
7.4 Return Currents
The module’s edge connector has GND connections distributed along the interface pinout to maintain
signal integrity. These connections should be connected to a solid ground plane to create an efficient
return current path. For some additional return current paths to help with the switching edges, 1000pF
caps have been included on specific nets. These are on the module and should be added to the I/O board
as well similar to the Dev Board. The return current caps are helpful on the following signals:
RTC_PSW
USB_FAULTn
MSEL[0-4]
BOOTSEL1
Should MSEL4 have a return current cap or not?
Replies (3)
RE: MSEL[4] and return current cap... - Added by David Rice about 1 year ago
The intention of the caps to ground on these signals is to allow some high frequency ground currents from the SOM I/O's going off-SOM to have additional paths to ground, since the MSEL signals (and other listed signals) are essentially DC and not switching. If you are running a fully loaded FPGA running at max speed on all I/O's, these additional caps MIGHT provide some help. They are more insurance policy than anything. Certainly leaving off the cap on a few of these signals should have minimal impact.
Dave Rice
RE: MSEL[4] and return current cap... - Added by Austin Franklin about 1 year ago
RE: MSEL[4] and return current cap... - Added by David Rice about 1 year ago
Sorry if I wasn't clear. The caps are optional. They might help in a worst case situation, but for most designs they are overkill. Leaving one off won't be a problem. If you are doing a design and have the option to put them on, it wouldn't hurt to put them on.
The bottom line is that we have never seen a case where these caps matter, but they were included in the design originally, just in case. There's no analytical way to tell if they are needed or not. There are lots of ground connections on the SOM, so the additional ground current paths that these caps might provide are probably unnecessary in any conceivable design. Like most engineers, our designers tend to err on the side of over-design, and it's not worth the cost to try to test if something like these caps are actually needed.
That's not a very clear answer, but there isn't a clear answer. I would be completely comfortable, based on my 42+ years of experience, leaving these caps off. I hope that helps.
Dave
