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V22.0436 - Prof. Grishman

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Lecture 3: Logic Design -- Simulation; combinational
Circuits:
propagation delay

(finish lecture 2 material: how many
types of gates do we need?)

#### Circuit simulation

- old-fashioned approach: breadboarding ... build and test
- difficult even for systems using off-the-shelf parts

- importance of simulating designs before they are built

- simulation becomes much more important for VLSI

- tooling up to build a new VLSI circuit is very expensive
(low cost is obtained by making a large number of a single design) -
hard to debug a circuit once it is built
- distributing a chip (processor) with a bug can cause huge
problems

- special purpose simulators for logic simulation
- two approaches to circuit entry:

- specialized languages: VHDL, Verilog
- graphical entry of circuits

- we will use a graphical entry approach: Logisim

#### Propagation delay *(note: not covered in text)*

- delay of individual transistor -- how fast it can switch --
determined by physical factors (e.g., size)
- speed of transistor determines speed of gate
- the
*propagation delay* (speed) of a combinatorial circuit
is the
length of time from the moment when all input signals are stable until
the moment when all outputs have stabilized
- propagation delay of a combinatorial circuit can be determined as
longest
path (in number of gates) from any input to any output

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Fan-in *(note: not covered in text)*

- sum-of-products form suggests any combinatorial function can be
computed
in 3 gate delays (one delay for inverters, one for ANDs, one for OR)
- but gates are limited in their fan-in (number of inputs a gate
has)
- so, for example, if fan-in is
*f*, it takes log (base *f*)
n
gate delays to OR or AND together n inputs

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Ripple carry adder **(B-28 to 30; B-45)**

- simplest n-bit binary adder connects together n full adders,
feeding Cout
of bit k into Cin of bit k+1 (where low order bit is bit 0)
- delay is approximately n * delay(Cin,Cout) of full adder

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Representing signed numbers (text, section 3.2)

- negative numbers generally represented in two's complement
- computing the two's complement: flipping each bit and
adding 1
- doing subtraction by adding the two's complement