SN74AS882A Datasheet, PDF (8/9 Pages) Texas Instruments – 32-BIT LOOK-AHEAD CARRY GENERATORS

SN54AS882A, SN74AS882A
ą
32ĆBIT LOOKĆAHEAD CARRY GENERATORS
ąą
SDAS235 − D2661, DECEMBER 1982 − REVISED NOVEMBER 1985
TYPICAL APPLICATION DATA
The application given in Figure 1 illustrates how the ’AS882A can implement look-ahead carry for a 32-bit ALU
(in this case, the popular ’AS881A) with a single package. Typical carry times shown are derived using the
standard Advanced Schottky load circuit.
A
B
LSB
6 ns
Cn Cn−4
’AS881A
A
B
’AS881A
Cn
Cn−4
6 ns
A
B
Cn Cn−4
’AS881A
A
B
6 ns
Cn
Cn−4
’AS881A
A
B
’AS881A
6 ns
Cn Cn−4
6 ns
A
B
’AS881A
Cn Cn−4
6 ns
A
B
6 ns
Cn
Cn−4
’AS881A
A
B
MSB
6 ns
Cn
Cn−4
’AS881A
F
P0 G0
Cn
F
F
F
F
P1 G1 Cn _{+ 8}
P2 G2
’AS882A
P3 G3 Cn+16
6 ns
P4 G4
F
F
P5 G5 Cn _{+ 24} P6 G6
7 ns
F
P7 G7
Cn _{+ 32}
Figure 1
Likewise, Figure 2 illustrates the same 32-bit ALU using two ’AS882s. This shows the worst-case delay from
LSB to MSB to be 19 ns as opposed to 25 ns in Figure 1.
LSB A
B
4
4
A
B
4
4
A
B
4
4
A
B
4
4
A
B
4
A
B
4
4
A
B
4
A
B MSB
4
4
6 ns
Cn
’AS881A
’AS881A
Cn
’AS881A
6 ns
6 ns
Cn
’AS881A
’AS881A
Cn
’AS881A
Cn
6 ns
’AS881A
Cn
’AS881A
Cn
6 ns
F
F
F
F
F
F
F
F
P0 G0
Cn
LH
P1 G1
6 ns
P2 G2
P3 G3 Cn _{+ 16} P4 G4
’AS882A
P5 G5 Cn _{+ 24} P6 G6
P7 G7
Cn _{+ 32}
P0 G0
Cn
Cn
P1 G1 Cn _{+ 8}
P2 G2
P3 G3 Cn _{+ 16}
’AS882A
7 ns
P4 G4
P5 G5 Cn _{+ 24}
6 ns
P6 G6
Figure 2
P7 G7
Cn _{+ 32}
8
• POST OFFICE BOX 655303 DALLAS, TEXAS 75265
• POST OFFICE BOX 1443 HOUSTON, TEXAS 77001