A Boolean function has many implementations. They differ in area (how many gates and how big), delay (how many levels the slowest path crosses) and power (how much switches).
Good RTL gives the tools a structure that is easy to optimize. Bad structure shows up in reasoning about depth.
AND-ing eight signals with 2-input gates can be arranged two ways:
For n inputs a balanced tree is ⌈log₂ n⌉ levels deep and a chain is n − 1. The gate count is the same, but the delay is very different.
For plain AND/OR/XOR of equal-arrival inputs, synthesis tools rebalance the tree automatically. Writing a & b & c & d & e & f & g & h is fine.
Structure starts to matter for arithmetic carries, priority logic and inputs that arrive late. There the tool cannot freely reorder.
Lever | Effect on timing | Effect on area |
|---|---|---|
Fewer logic levels | Faster | Often larger (duplicated logic) |
Fewer literals | Usually faster | Smaller |
Higher fan-out | Slower (more load) | Needs buffers |
Sharing a sub-expression | Slightly slower (fan-out) | Smaller |
Duplicating logic | Faster (lower fan-out) | Larger |
There is rarely a free lunch: speed usually costs area and the reverse.
The constraints given to the tool (clock period, area goals) tell it which side to favour.
Consider y = a·b + a′·c. When b = c = 1 the output should stay at 1 no matter what a does.
But when a falls, the AND term a·b turns off one gate delay before a′·c turns on (the inverter adds a delay). So y can briefly drop to 0: a static-1 hazard.
In a synchronous design this glitch is harmless to logic values only if it settles with setup and hold margin before the sampling edge. That is why synchronous design is so forgiving. It still costs switching power on every transition.
It becomes a real bug when the signal drives something that reacts to any pulse:
An asynchronous reset.
A clock gate.
A latch enable.
An asynchronous FIFO pointer.
Each of those pins needs its own treatment. Prefer a synchronous enable for data paths, a dedicated clock-gating cell for clocks, and a synchronised release for asynchronous resets. Latch enables need a stable level through the open window.
The chain is 15 gates deep; the balanced tree is ⌈log₂16⌉ = 4. For a pure AND of equal-arrival inputs the synthesis tool restructures to a balanced tree on its own, so one long & expression is fine. Structure matters when inputs arrive at different times or when order is semantically meaningful (priority logic, carry chains).
The consensus term is logically redundant, so removing it saves area without changing the steady-state function. But that term is what covers the transition between the two remaining terms; deleting it creates the static hazard. Keep it only where the output must be glitch-free (asynchronous or clock-related logic), and prefer registering such signals instead.