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The wishbone specification refers to signals being qualified by other signals:
The strobe output [STB_O] indicates a valid data transfer cycle. It is used to qualify various other signals on the interface such as [SEL_O()].
What does this mean exactly? I can only assume that they meant that [SEL_O()] may be invalid while [STB_O] is not asserted.
I have not been able to find any definition for this term online. I would appreciate any references to textbooks or other reliable sources.
I can only assume that they meant that [SEL_O()] may be invalid while [STB_O] is not asserted.
Your assumption is correct. Signals such as SEL_O() should not be considered to have valid values while STB_O is low; any changes on these signal should be ignored.
I don't know of any official documentation that strictly defines the term "qualify" in this context. It is generally understood to have this meaning.
You are correct in your assumption.
If a signal qualifies another, it means that it indicates the validity of the other signal.
So in otherwords you should only perform an action on the value of SEL_O if STB_O is asserted. When STB_O is deasserted, the SEL_O signal has no actionable meaning.
What does it mean for a digital logic signal to qualify another signal?
For once (and perhaps just this once), it's same as in everyday language :)*
A qualified statement is a statement that is true or correct only when the qualification is met.
For example, "Sprockets will be made only after Widgets have been delivered". Without the delivery, sprockets will not be made.
And the signal equivalent: "SPROCKET_MAKE is qualified by WIDGETS_DELIVERED". You can make SPROCKET_MAKE active, but no sprocket making will take place until WIDGETS_DELIVERED is active too.
*A lot of technical jargon takes everyday words much farther than their original meaning - say the word theory as used in science vs in casual language.
