What is the difference between an AND-type and an ORtype isolation cell (clamp value)?
From PDVerse Low-Power Physical Design Mentor Guide · pdVerse Mentor Guide
Definition
An AND-type isolation cell clamps its output to logic-0 when isolation is active (an AND gate forces a 0 through one input); an OR-type isolation cell clamps its output to logic-1 when isolation is active (an OR gate forces a 1 through one input).
Mentor Explanation
There are two clamp behaviors worth knowing cold: AND-type clamps to 0, OR-type clamps to 1. Mechanically it's simple Boolean logic — an AND gate with one input tied to the isolation-enable signal (activelow sense) forces the output low regardless of the data input, and an OR gate with one input tied to the enable (active-high sense) forces the output high regardless of data. In implementation, this choice is exposed directly as a strategy option: the '-clamp_value' argument of set_isolation takes 0, 1, or latch, where 0 maps to an AND gate and 1 maps to an OR gate.
Example
If the downstream logic treats an inactive request line as '0 means no request,' you'd choose an AND-type (clamp-0) isolation cell so the request line reads safely inactive whenever the source block is off.
Why It Matters
I like to check that a candidate ties the clamp value to actual downstream semantics, not just 'isolation cells clamp to something' Shutdown Domain Driver cell ISO clamp=0 power_ctrl Active Domain Receiving cell — picking the wrong clamp value can leave a design functionally broken even though isolation is technically present.
Command
set_isolation ISO1 -domain PD1 -clamp_value 0 (AND-gate clamp) or clamp_value 1 (OR-gate clamp)Common Beginner Mistake
Choosing an OR-type (clamp-to-1) isolation cell on a signal that downstream logic interprets as active-high 'enable' would inadvertently leave that function permanently enabled during shutdown — exactly backwards from the intent.
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