intent/property-strap

Remedy ¶

Move the strap resistor to the rail that latches the declared level. The board boots either way, configured as something other than what was intended.

What it checks ¶

A net the design intent declares as a boot/config strap must not be biased to the opposite level from the one it is meant to latch. Declared high with a pull-down, or declared low with a pull-up, means the part will come up configured the other way from what the design intended.

It is not property-reset-polarity under another name ¶

The two read the same evidence and ask inverted questions, which is worth being precise about because a reviewer who assumes they are the same check will misread both.

reset-polarity’s value is the level that asserts reset. Bias toward it is the defect, bias away from it is correct, so the rule can only ever catch one of the two ways a reset line goes wrong.

strap’s value is the level the pin should latch. Bias toward it is correct and bias away from it is the defect, in either direction. So this rule catches both mistakes, not one.

Read this before binding a review item to it ¶

A pass means “no contradiction found”, not “strap confirmed”, the same caveat property-reset-polarity carries and for the same cause.

Strap pins almost always carry an internal pull, and the standard datasheet instruction is to fit an external resistor only for the non-default state. So a design declaring the default level with no resistor on that net is correct and extremely common. Firing on absent bias would report a non-defect on the majority of real straps, so the rule stays silent there.

It checks resistance only against a band you declare. “Is this strap resistor an appropriate value” (strong enough to hold against leakage, weak enough not to fight an active driver) is now checked, but only where the declaration states min_ohms / max_ohms. There is no built-in band and there deliberately will not be one: a CMOS input with nanoamp leakage is happy past 100k, while a strap an active driver has to override wants a few hundred ohms, so any universal range the engine invented would flag correct boards. The person declaring the strap holds the datasheet; the band is theirs to state.

Declare neither bound and the value half simply does not run, and the direction half still does.

It does not identify strap pins. The declaration names the nets. On a design whose strap nets are not named for their function, which is usual, the pins have to be read off the part’s datasheet pinout and written into the declaration by hand. Nothing here can infer them.

For hardware engineers ¶

A strap is a resistor that parks a pin at a known level so the part can sample it at reset and pick a configuration: boot source, bus width, clock ratio, or its own address on a shared bus.

Getting one backwards does not look like a wiring fault. The board powers up, the part runs, and it runs in the wrong mode. It boots from the wrong flash, or it answers on an address another device on the same bus is already using, and then two parts drive that bus against each other and the whole segment is unreliable in a way that looks intermittent rather than wired wrong.

What counts as bias ¶

A resistor on the declared net whose other end reaches a power rail (pull-up, latches high) or a ground net (pull-down, latches low). The far end may reach its rail through further passives.

A net with both, a divider, reports neither. Some parts use a divider for a tri-level strap, and the level it selects depends on the ratio of the two resistances rather than on either one, so the rule declines to guess rather than reporting a direction it does not know. The value check declines there for the same reason: neither resistor is “the” strap pull, so judging one would name an arbitrary part.

Declaring it ¶

net_properties:
  # direction only
  - {net: BOOT_MODE0, property: strap, value: high}
  # direction plus an acceptable pull band, in ohms
  - {net: PHYAD1, property: strap, value: low, min_ohms: 1000, max_ohms: 100000}
  # one-sided: bound the weak end only
  - {net: BOOT_CFG2, property: strap, value: high, max_ohms: 47000}

The value is the intended latched level and is required: without it the rule has nothing to contradict, so an omitted or misspelled level is rejected at load rather than becoming a rule that silently never fires.

min_ohms and max_ohms are optional and independent, so a one-sided band checks one side only. Both are rejected at load on a property kind that has no resistance to bound, and a min_ohms above its max_ohms is rejected too: a band nothing can satisfy would compile to a check that can never pass, which is a declaration error rather than a design finding.

The value check runs only when a bound is declared AND the pull resistor’s value reads as a resistance. A resistor with no value, an unparseable one, or one stamped in another unit is skipped. Unreadable is not the same as acceptable, and firing on a number the engine could not read would report a defect on no evidence.

Fixing a finding ¶

Either the strap resistor goes to the wrong net, or the declared configuration is wrong. Check the part’s datasheet strap table before moving the resistor. If the declaration is the thing that is wrong, moving the resistor creates the bug the rule was reporting.