Logical effort
Encyclopedia
The method of logical effort, a term coined by Ivan Sutherland
and Bob Sproull
in 1991, is a straightforward technique used to estimate delay
in a CMOS
circuit. Used properly, it can aid in selection of gates for a given function (including the number of stages necessary) and sizing gates to achieve the minimum delay possible for a circuit.
(Some authors prefer define the basic delay unit as the fanout of 4 delay—the delay of one inverter driving 4 identical inverters).
The absolute delay is then simply defined as the product of the normalized delay of the gate, d, and τ:
In a typical 600-nm process τ is about 50 ps. For a 250-nm process, τ is about 20 ps. In modern 45 nm processes the delay is approximately 4 to 5 ps.
The normalized delay in a logic gate can be expressed as a summation of two primary factors: parasitic delay, p (which is an intrinsic delay of the gate and can be found by considering the gate driving no load), and stage effort, f (which is dependent on the load as described below). Consequently,
The stage effort is divided into two components: a logical effort, g, which is the ratio of the input capacitance of a given gate to that of an inverter capable of delivering the same output current (and hence is a constant for a particular class of gate and can be described as capturing the intrinsic properties of the gate), and an electrical effort, h, which is the ratio of the input capacitance of the load to that of the gate. The stage effort is then simply:
Combining these equations yields a basic equation that models the normalised delay through a single logic gate:
In other words, the pFET of the inverter is designed with twice the width (and therefore twice the capacitance) as the nFET of the inverter,
in order to get roughly the same pFET resistance as nFET resistance, in order to get roughly equal pull-up current and pull-down current.
Choose sizes for all transistors such that the output drive of the gate is equal to the output drive of an inverter built from a size-2 PMOS and a size-1 NMOS.
The output drive of a gate is equal to the minimum – over all possible combinations of inputs – of the output drive of the gate for that input.
The output drive of a gate for a given input is equal to the drive at its output node.
The drive at a node is equal to the sum of the drives of all transistors which are enabled and whose source or drain is in contact with the node in question. A PMOS transistor is enabled when its gate voltage is 0. An NMOS transistor is enabled when its gate voltage is 1.
Once sizes have been chosen, the logical effort of the output of the gate is the sum of the widths of all transistors whose source or drain is in contact with the output node. The logical effort of each input to the gate is the sum of the widths of all transistors whose gate is in contact with that input node.
The logical effort of the entire gate is the ratio of its output logical effort to the sum of its input logical efforts.
The path effort is expressed in terms of the path logical effort G (the product of the individual logical efforts of the gates), and the path electrical effort H (the ratio of the load of the path to its input capacitance).
For paths where each gate drives only one additional gate (i.e. the next gate in the path),
However, for circuits that branch, an additional branching effort, b, needs to be taken in to account; it is the ratio of total capacitance being driven by the gate to the capacitance on the path of interest:
This yields a path branching effort B which is the product of the individual stage branching efforts; the total path effort is then
It can be seen that b = 1 for gates driving only one additional gate, fixing B = 1 and causing the formula to reduce to the earlier non-branching version.
where N is the number of stages in the circuit.
The ideal value for N can be found by calculating the best stage effort:
Or
where 
Then
By definition, the logical effort g of an inverter is 1. If the inverter drives an equivalent inverter, the electrical effort h is also 1.
The parasitic delay p of an inverter is also 1 (this can be found by considering the Elmore delay
model of the inverter).
Therefore the total normalised delay of an inverter driving an equivalent inverter is
For larger gates, the logical effort is as follows:
The normalised parasitic delay of NAND and NOR gates is equal to the number of inputs.
Therefore, the normalised delay of a two-input NAND gate driving an identical copy of itself (such that the electrical effort is 1) is
and for a two-input NOR gate, the delay is
Ivan Sutherland
Ivan Edward Sutherland is an American computer scientist and Internet pioneer. He received the Turing Award from the Association for Computing Machinery in 1988 for the invention of Sketchpad, an early predecessor to the sort of graphical user interface that has become ubiquitous in personal...
and Bob Sproull
Bob Sproull
Dr Robert F. Sproull is an American computer scientist, who works for Oracle Corporation where he is director of Oracle Labs in Burlington, Massachusetts .- Biography :...
in 1991, is a straightforward technique used to estimate delay
Delay calculation
Delay calculation is the term used in integrated circuit design for the calculation of the gate delay of a single logic gate and the wires attached to it. By contrast, static timing analysis computes the delays of entire paths, using delay calculation to determine the delay of each gate and...
in a CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
circuit. Used properly, it can aid in selection of gates for a given function (including the number of stages necessary) and sizing gates to achieve the minimum delay possible for a circuit.
Derivation of delay in a logic gate
Delay is expressed in terms of a basic delay unit, τ = 3RC, the delay of an inverter driving an identical inverter with no parasitic capacitance; the unitless number associated with this is known as the normalized delay.(Some authors prefer define the basic delay unit as the fanout of 4 delay—the delay of one inverter driving 4 identical inverters).
The absolute delay is then simply defined as the product of the normalized delay of the gate, d, and τ:
In a typical 600-nm process τ is about 50 ps. For a 250-nm process, τ is about 20 ps. In modern 45 nm processes the delay is approximately 4 to 5 ps.
The normalized delay in a logic gate can be expressed as a summation of two primary factors: parasitic delay, p (which is an intrinsic delay of the gate and can be found by considering the gate driving no load), and stage effort, f (which is dependent on the load as described below). Consequently,
The stage effort is divided into two components: a logical effort, g, which is the ratio of the input capacitance of a given gate to that of an inverter capable of delivering the same output current (and hence is a constant for a particular class of gate and can be described as capturing the intrinsic properties of the gate), and an electrical effort, h, which is the ratio of the input capacitance of the load to that of the gate. The stage effort is then simply:
Combining these equations yields a basic equation that models the normalised delay through a single logic gate:
Procedure for calculating the logical effort of a single stage
CMOS inverters along the critical path are typically designed with a gamma equal to 2.In other words, the pFET of the inverter is designed with twice the width (and therefore twice the capacitance) as the nFET of the inverter,
in order to get roughly the same pFET resistance as nFET resistance, in order to get roughly equal pull-up current and pull-down current.
Choose sizes for all transistors such that the output drive of the gate is equal to the output drive of an inverter built from a size-2 PMOS and a size-1 NMOS.
The output drive of a gate is equal to the minimum – over all possible combinations of inputs – of the output drive of the gate for that input.
The output drive of a gate for a given input is equal to the drive at its output node.
The drive at a node is equal to the sum of the drives of all transistors which are enabled and whose source or drain is in contact with the node in question. A PMOS transistor is enabled when its gate voltage is 0. An NMOS transistor is enabled when its gate voltage is 1.
Once sizes have been chosen, the logical effort of the output of the gate is the sum of the widths of all transistors whose source or drain is in contact with the output node. The logical effort of each input to the gate is the sum of the widths of all transistors whose gate is in contact with that input node.
The logical effort of the entire gate is the ratio of its output logical effort to the sum of its input logical efforts.
Multistage logic networks
A major advantage of the method of logical effort is that it can quickly be extended to circuits composed of multiple stages. The total normalised path delay D can be expressed in terms of an overall path effort, F, and the path parasitic delay P (which is the sum of the individual parasitic delays):The path effort is expressed in terms of the path logical effort G (the product of the individual logical efforts of the gates), and the path electrical effort H (the ratio of the load of the path to its input capacitance).
For paths where each gate drives only one additional gate (i.e. the next gate in the path),
However, for circuits that branch, an additional branching effort, b, needs to be taken in to account; it is the ratio of total capacitance being driven by the gate to the capacitance on the path of interest:
This yields a path branching effort B which is the product of the individual stage branching efforts; the total path effort is then
It can be seen that b = 1 for gates driving only one additional gate, fixing B = 1 and causing the formula to reduce to the earlier non-branching version.
Minimum delay
It can be shown that in multistage logic networks, the minimum possible delay along a particular path can be achieved by designing the circuit such that the stage logical efforts are equal. For a given combination of gates and a known load, B, G, and H are all fixed causing F to be fixed; hence the individual gates should be sized such that the individual stage efforts arewhere N is the number of stages in the circuit.
The ideal value for N can be found by calculating the best stage effort:
Or
where Then
Delay in an inverter
The parasitic delay p of an inverter is also 1 (this can be found by considering the Elmore delay
Elmore delay
Elmore delay is a simple approximation to the delay through an RC network in an electronic system. It is often used in applications such as logic synthesis, delay calculation, static timing analysis, placement and routing, since it is simple to compute and is reasonably accurate...
model of the inverter).
Therefore the total normalised delay of an inverter driving an equivalent inverter is
Delay in NAND and NOR gates
The logical effort of a two-input NAND gate is calculated to be g = 4/3 because a NAND gate with input capacitance 4 can drive the same current as the inverter can, with input capacitance 3. Similarly, the logical effort of a two-input NOR gate can be found to be g = 5/3. Due to the lower logical effort, NAND gates are typically preferred to NOR gates.For larger gates, the logical effort is as follows:
| Number of Inputs | ||||||
|---|---|---|---|---|---|---|
| Gate type | 1 | 2 | 3 | 4 | 5 | n |
| Inverter | 1 | N/A | N/A | N/A | N/A | N/A |
| NAND | N/A |  |
 |
 |
 |
 |
| NOR | N/A |  |
 |
 |
 |
 |
The normalised parasitic delay of NAND and NOR gates is equal to the number of inputs.
Therefore, the normalised delay of a two-input NAND gate driving an identical copy of itself (such that the electrical effort is 1) is
and for a two-input NOR gate, the delay is