FPGA 台湾3.ppt

Combinational Functions and Circuits
Chapter 4
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Overview
Functions and functional blocks
Rudimentary logic functions
Decoding
Encoding
Selecting
binational Functions Using:
Decoders and OR gates
Multiplexers (and inverter)
ROMs
PLAs
PALs
Lookup Tables
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Functions and Functional Blocks
The functions considered are those found to be very useful in design
Corresponding to each of the functions is binational circuit implementation called a functional block.
In the past, many functional blocks were implemented as SSI, MSI, and LSI circuits.
Today, they are often simply parts within a VLSI circuits.
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Rudimentary Logic Functions
Functions of a single variable X
Can be used on theinputs to functionalblocks to implementother than the block’sintended function
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Multiple-bit Rudimentary Functions
Multi-bit Examples:
A wide line is used to representa bus which is a vector signal
In (b) of the example, F = (F3, F2, F1, F0) is a bus.
The bus can be split into individual bits as shown in (b)
Sets of bits can be split from the bus as shown in (c)for bits 2 and 1 of F.
The sets of bits need not be continuous as shown in (d) for bits 3, 1, and 0 of F.
F
(d)
0
F
3
1
F
2
F
1
A
F
0
(a)
0
1
A
1
2
3
4
F
0
(b)
4
2:1
F(2:1)
2
F
(c)
4
3,1:0
F(3), F(1:0)
3
A
A
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Enabling Function
Enabling permits an input signal to pass through to an output
Disabling blocks an input signal from passing through to an output, replacing it with a fixed value
The value on the output when it is disable can be Hi-Z (as for three-state buffers and transmission gates), 0 , or 1
When disabled, 0 output
When disabled, 1 output
See Enabling App in text
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Decod