profile/missing-pullup

Remedy ¶

Fit a pull-up from the signal to its rail, sized for the speed the bus runs at and the capacitance it carries.

What it means ¶

The profile marks some signals as needing a pull-up, such as a chip-select or an open-drain line. The rule reports such a signal when its net reaches no power rail through a resistor, so no pull-up sits in its path.

Why engineers want it ¶

An open-drain or chip-select line with no pull-up floats between drives. At power-up, before any driver takes it, it can sit at an undefined level and select or clock the device spuriously. The pull-up holds it inactive.

How it is checked ¶

It runs the bounded walk that the built-in i2c-pull-up uses, by calling it directly rather than approximating it. The walk goes from the signal net out through resistors, for up to three crossings, looking for a rail. A rail is a legal DESTINATION and never a transit node, and ground is never crossed at all. A resistor to ground is a pull-down, and counting it would pass exactly the line that this requirement exists to catch.

The finding and the pass both carry the path. A pass says “SPI_CS reaches rail +3V3 through R1” and hands the viewer the resistor and the rail as clickable entities; a failure says no rail is reachable within the hop limit, and names the limit, because “no pull-up” and “a pull-up four hops away” are different situations, and a bare message cannot tell them apart.

Of the profile requirements, only this one compiles to Go rather than to a query, and the previous paragraph shows why. Datalog could state a reachability question but could not name what the walk crossed. So the profile route proved a pass with the net’s own name, while the built-in proved one with the route (agni issue 516). Calling the same function makes the two reports identical rather than merely similar.

So the same datasheet credit applies here too. If a part’s datasheet binds an internal pull-up to a pin, the pin passes, citing the row (agni issue 1298). The MNT Reform’s SD lines are pulled up inside its TXS0108E level translator this way.

For software readers ¶

A pull-up works like a default value, and without it the line has no defined idle state, the hardware analogue of reading an uninitialised variable. The rule checks that the default is wired in.