i2c-pull-up-split-rail

An I2C net is pulled up to two different rails.

Remedy

Pull the bus to one rail. Where the bus genuinely crosses supply domains, that is a level translator’s job rather than two pull-ups, and the translator’s own pull-ups belong one per side of it.

What it means

An I2C net reaches TWO different rails through its pull-up resistors. One part pulls the bus to one supply and another pulls it to a second.

Why engineers want it

Two pull-ups to two rails tie those rails together through the bus. Whenever one supply is up and the other is not, current flows from the live rail through its pull-up, along the bus, and back through the other pull-up into a rail that is meant to be off. That rail gets partially fed by the bus, which is a sequencing fault: it can hold a supply above its off threshold, back-feed a regulator, or bring a device up in a state its reset never sees.

It also leaves the bus idling at whichever rail wins, which may exceed the input rating of a device sitting on the lower supply.

The failure mode is the awkward one. On the bench every supply comes up at once and nothing is observed; the fault appears as a board that fails on one particular power-up order, or after a brown-out, which is the hardest kind of report to act on.

Impact

A supply is fed through a signal bus during sequencing, and the bus idles at a level one side may not tolerate. Neither shows up in a functional test that powers everything simultaneously.

Fires versus fine 

  fires                                 fine

  +3V3      +1V8                        +1V8
   |         |                           |
  R1        R2                          R1
   |         |                           |
   +----+----+                           |
        |                                |
       SDA                              SDA
                                        (a bus that crosses domains
                                         gets a level translator, and
                                         one pull-up per side of it)

Scope

  • Only nets matching the I2C naming convention at a token boundary (SDA / SCL).
  • A net whose pull-ups all land on ONE rail is i2c-redundant-pull-up, not this. The two conditions are disjoint, so a bus is reported once.
  • Rail identity is the rail NET, so two rails that happen to carry the same voltage are still two rails. That is deliberate: the sequencing question is about which supply, not which voltage.
  • What counts as a rail comes from the naming lexicon, so a project whose supplies the built-in vocabulary does not recognise reports nothing here. Declare them in conventions.yaml; the rail-not-classified rule is the tripwire that says when this is happening.
  • Severity is warning, because a bus deliberately pulled to one domain with a translator elsewhere on the net can look like this to a netlist.

The query structure

Enumerate I2C nets; walk out through resistors up to three crossings, collecting every distinct resistor whose far side is a rail, with the rail each landed on; fire when the set of rails has more than one member.

For software readers

Two connection pools onto one socket, each configured against a different endpoint. Nothing is wrong with either pool’s own configuration, and the bug only appears when one endpoint is down: traffic takes the path through the other one. Reachability tells you the socket is connected; only counting the pools and comparing their endpoints tells you it is connected to two things.