net.bias

What it is ¶

net.bias(net, level) yields one row per net a bias resistor holds at a rail: high when the resistor reaches a supply, low when it reaches ground. A net with no bias resistor produces no row, and so does a net held by a divider, so not net.bias(?n, ?_) reads as “unbiased”, a genuinely different state from “biased the other way”.

For hardware engineers ¶

A bias resistor decides what a line does when nothing is actively driving it. A reset line that must sit released between drives needs a pull-up; an enable that must default off needs a pull-down. The resistor is what makes the resting state a design decision rather than an accident.

Getting it backwards is a classic bring-up failure: a pull-down on an active-low reset holds the part in reset from the moment power comes up, and the board looks dead. One resistor to the wrong net, invisible until someone meters the pin.

Query this to see what every line rests at, or join it against a declared intent to find lines that rest at the wrong level.

Two clauses, and the second is the one that gets forgotten ¶

The bias resistor commonly sits directly between the net and its rail. It does not have to. It can reach the rail through further passives, a filter or a second resistor, and a check that only looked for the direct arrangement would silently report those nets as unbiased.

Both forms count here. profiles.pullupRule learned this the hard way (WS3-108): its walk-based form could not enter a wide rail at all, so a direct clause had to be added beside it. Keeping both in one predicate is what stops the next consumer reimplementing half of it.

A divider reports neither ¶

A net with both a pull-up and a pull-down sits at an intermediate level. It is not held at either rail, so it yields no row rather than an arbitrary one. A caller asking “is this held asserted” gets the honest answer instead of a coin flip.

Go projector ¶

netBiasFacts in stdlib/relations/facts.go calls check.NetBias, which lives in core/check/guards.go beside the other Model-level predicates. The Go rules and any datalog query therefore read ONE definition, and this relation projects it rather than reimplementing it.

Datalog ¶

Every biased net and which way:

net.bias(?n, ?level) => ?n, ?level

Lines that rest low, the ones an active-low reset must not be among:

net.bias(?n, "low") => ?n

Unbiased nets that a part still drives, which is where an undeclared internal pull is doing the work:

component-on-net(?r, ?n), not net.bias(?n, ?_) => ?n, ?r