XOR3 and XNOR3 cells made from two XOR2 or XNR2 cells

	gate count 1390 number of cells 536 number of library cells 109 number of used cells 40 max fanin 17 max input capacitance 192 max internal fanout 34 critical path 0fF 2282 critical path 6fF 2886
	xor3
	xnr3

As we have seen, frequently the XOR and XNOR gates are grouped together so that two of them could be replaced by a single 3-input XOR or XNOR gate.

Previously a single stage 3-XOR gate was tried, but proved to be slow and, especially, large. Now 3-XOR and 3-XNOR gates are made by joining two 2-input gates into one. The area is no more than the separate gates, and can be less. The delay is reduced because the 6fF wireload capacitance on the input of the second gate is no longer present.
The schematic for the xor3v1x1 below shows it is basically made up of two xor2v0x1 cells.

The a and b inputs pass through two gates, but the c input is faster since it passes through the second gate only.

Since the 3-XOR and 3-XNOR gates are now efficient implementations of the function, the cell delay used by BOOG is set equal to the 2-AND gate at 400ps. Now 29 xor3 cells and 10 xnr3 cells are chosen by BOOG, although after replacing non-critical outputs using these gates with an equivalent netlist made up of 2-XOR and 2-XNOR gates having a lower input pin capacitance, there remain 23 xor3 and 10 xnr3 cells.

Although none of the 3-input XOR and XNOR gates appears on the critical path, four of them have been buffered up to an x1 or x2 drive strength. This implies that they can appear on the critical path, and need to be sized up to remove them. This is why, even if they are not on the critical path, their presence improves the netlist performance.

               fanout         -- delay--
    x 1          17                  192
 1  aoi22v0x2     1  b1->z     279    87
 2  oai21v0x2     4  a2->z     415   136
 3  iv1v0x4       1  a->z      464    49
 4  oai21v0x2     4  a2->z     603   139
 5  iv1v0x4       1  a->z      652    49
 6  oai21v0x2     4  a2->z     792   140
 7  iv1v0x4       1  a->z      841    49
 8  oai21v0x2     4  a2->z     980   139
 9  iv1v0x4       1  a->z     1029    49
10  oai21v0x2     4  a2->z    1167   138
11  iv1v0x3       1  a->z     1222    55
12  oai21v0x2     4  a2->z    1372   150
13  iv1v0x3       1  a->z     1428    56
14  oai21v0x2     5  a2->z    1598   170
15  xoon21v0x2    1  b->z     1700   102
16  cgi2v0x2      4  c->z     1835   135
17  iv1v0x2       1  a->z     1890    55
18  nd2av0x2      1  a->z     2000   110
19  oai21v0x2     3  b->z     2125   125
20  xor2v0x2      1  b->z     2216    91
21  cgi2v0x2      2  c->z     2312    96
22  iv1v0x4       1  a->z     2361    49
23  cgi2v0x2      2  c->z     2450    89
24  an2v0x2       2  b->z     2569   119
25  an2v0x2       2  b->z     2715   146
26  xaon21v0x2    0  a1->z    2886   171
    r 15