Area Against Delay · synthesis

Part 2 · Logic Optimisation and Mapping · Intermediate

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Two objectives

Area mapping minimises the sum of cell areas. Delay mapping minimises arrival times against the required times, path by path.

Mode	Cost	Decision
Area	Sum of cell areas	Cheapest cover that implements the function.
Delay	Worst arrival against required time	Fastest cover on each failing path, cheapest elsewhere.

The two modes disagree when the cheapest cover is too slow for the required time. The next section works one such case.

A worked choice

Take f = NOT ((a AND b) OR c) again, with EXAMPLE delays in delay units. Cover C is cheapest; cover A answers whether it is fastest.

Cover	Cells	Area	Delay	Slack at required 5
C	AOI21	3	6	-1, fails
A	AND2 + NOR2	5	5	0, meets
B	NAND2 + INV + NAND2 + INV	6	5	0, meets

Diagram

f = NOT ((a AND b) OR c); EXAMPLE delays in delay units: INV 1, NAND2 2, NOR2 2, AND2 3, AOI21 6
cover C: area = 3; arrival = 6; slack = 5 - 6 = -1 (fails)
cover A: area = 3 + 2 = 5; arrival = 3 + 2 = 5; slack = 5 - 5 = 0 (meets)
cover B: area = 2 + 1 + 2 + 1 = 6; arrival = 2 + 2 + 1 = 5; slack = 5 - 5 = 0 (meets)
area choice = cover C; delay choice at required 5 = cover A

Slack is required time minus arrival time. Area mode picks C; delay mode at required 5 pays two extra area units for A and meets timing.

Common mistakes

Check yourself At a required time of 6, which covers meet timing and which one does area mode pick?

All three meet: C with slack 0, A and B with slack 1. Area mode picks C at area 3.

Check yourself Load doubles on the output of cover A. What may the mapper do, and what does it cost?

It may swap in a higher-drive variant of the same function where the library provides one. The swap costs extra area for the same logic.

Related topics

Up nextInverters and Banned Cells

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Two objectives

Area mapping minimises the sum of cell areas. Delay mapping minimises arrival times against the required times, path by path.

Mode	Cost	Decision
Area	Sum of cell areas	Cheapest cover that implements the function.
Delay	Worst arrival against required time	Fastest cover on each failing path, cheapest elsewhere.

The two modes disagree when the cheapest cover is too slow for the required time. The next section works one such case.

A worked choice

Take f = NOT ((a AND b) OR c) again, with EXAMPLE delays in delay units. Cover C is cheapest; cover A answers whether it is fastest.

Cover	Cells	Area	Delay	Slack at required 5
C	AOI21	3	6	-1, fails
A	AND2 + NOR2	5	5	0, meets
B	NAND2 + INV + NAND2 + INV	6	5	0, meets

Diagram

f = NOT ((a AND b) OR c); EXAMPLE delays in delay units: INV 1, NAND2 2, NOR2 2, AND2 3, AOI21 6
cover C: area = 3; arrival = 6; slack = 5 - 6 = -1 (fails)
cover A: area = 3 + 2 = 5; arrival = 3 + 2 = 5; slack = 5 - 5 = 0 (meets)
cover B: area = 2 + 1 + 2 + 1 = 6; arrival = 2 + 2 + 1 = 5; slack = 5 - 5 = 0 (meets)
area choice = cover C; delay choice at required 5 = cover A

Slack is required time minus arrival time. Area mode picks C; delay mode at required 5 pays two extra area units for A and meets timing.

Common mistakes

Check yourself At a required time of 6, which covers meet timing and which one does area mode pick?

All three meet: C with slack 0, A and B with slack 1. Area mode picks C at area 3.

Check yourself Load doubles on the output of cover A. What may the mapper do, and what does it cost?

It may swap in a higher-drive variant of the same function where the library provides one. The swap costs extra area for the same logic.