XOR3 cell made with single inverting gate

	gate count 1400 number of cells 562 number of library cells 104 number of used cells 38 max fanin 17 max input capacitance 188 max internal fanout 34 critical path 0fF 2302 critical path 6fF 2931
	xor3

Frequently the XOR gates are grouped together so that two of them could be replaced by a single 3-input XOR gate. The advantages of this are:

• The wireload capacitance between the two gates is removed.
• The area can be smaller than two separate gates.
• The transistor sizes of the first gate can be sized with more precision.
• There are fewer cell connections (4 instead of 6) which reduces routing congestion.

In this experiment a single stage 3-XOR gate has been added to the library. This type of gate is slow compared to making a gate from two regular 2-XOR gates. The Prop delay is similar, but the Ramp delay is about twice for the same value of input capacitance. In addition, a 3-XOR made from two 2-XOR gates has one input with a faster input passing only through the final gate. The physical size is nearly 50% larger than two separate 2-XOR gates. The schematic used is shown below.

It is theoretically possible to minimise the number of transistors, but this increases the amount of internal cell connections beyond the limits of the vsclib cell height.

The 3-XOR timing in the cell used by BOOG is set to 1500ps in order to discourage the cell use except where a real advantage is seen. This is 3X the Prop delay of regular non-inverting gates. Despite this, eight of these gates are chosen, none of them being in the critical path. Three of these drive non-critical outputs and are replaced by an x2 drive strength equivalent netlist by a script, leaving five xor3v0x05 cells.

Circuit speed is slightly slower because the inputs of some of the xor3v0x05 gates load the critical path and have a higher pin capacitance than the equivalent 2-input XOR gates.

Area is more because a 3-XOR gate is bigger than two separate 2-XOR gates. The cell count has increased because there are fewer xooi21 and xoon21 cells, replaced by separate 2-NOR and 2-XOR or 2-XNOR combinations.