Series termination inside busses - net-tying single bus-member possible?

Good day, I am currently trying my best wrapping my head around how hierarchical busses are meant to be in KiCad (V8).

I attached you a broken down version of the project which contains only the top sheet and both ends of the SPI bus (MCU and two identical ADC).

I am routing a SPI bus containing MISO, MOSI and SCLK (aliased as SPI in the top sheet) and two CS lines from a MCU to two identical schematic sheets (each representing one ADC).
I would like to use the intermediate sheet “sensors” as a place to add series termination resistors to my signals, so this is where I broke out my bus into its members to add series termination for SCLK and MOSI.

That appeared to work, but on the MISO line, I would like to use one termination resistor per output driver. At the moment, I have them directly inside the ADC schematics (R15 and R27), leaving me with the question about how to connect the broken-out MISO line now. I cannot simply connect /Temperature Sensing/SPI.MISO to /SAMD21/SERCOM0.MISO as far as I know.

Is there a way to place a net-tie without any associated footprint so that KiCad just knows that U1 Pin 14 connects to R15 Pin 1 and R27 Pin 1?

After running some LTSpice simulations, it appears to me that it would not even matter if I connect both together after terminating each one or if I place a termination resistor after merging both into one single signal line.

But I am still curious if and how this would be achievable. Thank you in advance for sharing your knowledge.

Best regards,

Nettying single member of a (123.7 KB)

If I simply connect them with a wire:

… then ERC generates two warnings. the first (Both MISO and SPI.MISO are attached … is expected. KiCad defaults to issuing a warning, because it does not know whether this is intended or a mistake.

The second: ** … is graphically connected to bus … but is not a member of that bus** is a bit more convoluted. When the wire gets renamed, KiCad gets a bit confused about whether it’s still in the bus or not. So using a net tie is probably still the easiest / most reliable solution.

Implementation of the net tie has changed a bit in KiCad V8, but I have not used them yet and am not sure about the details. A footprint for a net tie is just a bit of copper It does not add any cost to the PCB. In general, net ties are also used at very specific locations, and defining them as a footprint does give a logical connection where the nets get connected.

Thank you for your fast and helpful response, Paul. I went for the net tie approach and wholeheartedly have to agree with your statement of being an easy and reliable solution.

1 Like