Structuring and Buffering · synthesis

Part 2 · Logic Optimisation and Mapping · Intermediate

5topics
5lessons

Large fanouts slow drivers and break slew and capacitance limits. Synthesis repairs them with buffer trees, gate sizing, cloning and restructuring, trading area for rules that must hold.

Term	Meaning
Fanout	The number of sinks one driver feeds. Large fanout means large load.
Slew (transition)	How fast a signal edge rises or falls. Heavy loads stretch edges out.
Capacitance load	Wire plus pin capacitance the driver must charge. It sets delay and slew.
Buffer tree	Staged buffers that split one large fanout into small, legal steps.
Cloning	Duplicating a driver so each copy feeds fewer sinks.

Four repairs that fix an overloaded driver.

One overloaded driver is repaired by trees, sizing, cloning or restructuring until the rules hold.

Lesson	What it covers
Fanout, Slew and Capacitance Rules	Max fanout, transition and capacitance limits with a worked load sum.
Delay of a Driver with Load	A delay-table delay model with EXAMPLE numbers and a runnable check.
Buffer Trees	Stage counts and buffer area for a fanout-64 EXAMPLE tree.
Sizing, Cloning and Restructuring	Gate sizing, driver cloning and depth restructuring with exact delay math.
Fanout Constraints in Practice	How max fanout, transition and capacitance constraints steer synthesis.

Check yourself A net drives 64 sinks through 3 stages of branch-4 buffers. How many buffers does the tree hold?

21 buffers: 16 in the first stage, 4 in the second and 1 in the last. Each stage divides the remaining load by 4.

Up nextFanout, Slew and Capacitance Rules

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Term

Meaning

Fanout

The number of sinks one driver feeds. Large fanout means large load.

Slew (transition)

How fast a signal edge rises or falls. Heavy loads stretch edges out.

Capacitance load

Wire plus pin capacitance the driver must charge. It sets delay and slew.

Buffer tree

Staged buffers that split one large fanout into small, legal steps.

Cloning

Duplicating a driver so each copy feeds fewer sinks.

Lesson

What it covers

Fanout, Slew and Capacitance Rules

Max fanout, transition and capacitance limits with a worked load sum.

Delay of a Driver with Load

A delay-table delay model with EXAMPLE numbers and a runnable check.

Buffer Trees

Stage counts and buffer area for a fanout-64 EXAMPLE tree.

Sizing, Cloning and Restructuring

Gate sizing, driver cloning and depth restructuring with exact delay math.

Fanout Constraints in Practice

How max fanout, transition and capacitance constraints steer synthesis.