Here is a different take on the FET Switch Adder. This one relies on a 2:1 74AUC2G53 FET Switch. (Thanks to Dr Jefyll for suggesting this part). This configuration requires an additional gate, but capacitance on the carry-chain is lower — AUC parts have lower intrinsic capacitance to begin with, and the carry chain now connects to one pin on the switch rather than two, as follows:

Here is the test circuit:

I took the opportunity to extend the carry chain to better simulate a 16-bit incrementer. This circuit also includes four AND gates in series to simulate carry lookahead feeding the final four bits of the adder. Here is they layout of the test board:

74AUC08 ICs are only available in a VQFN package, so I thought I would experiment with that in passing. Honestly, the footprint (bottom center on the board) looks about the same size as the other VSSOP packages, and the big center pad makes routing harder. 

Incidentally, the good folks at PCBWay have very kindly offered to support this project with PCB manufacturing. Many thanks to them for that! I used them for all my prior boards, so I’m happy to continue to do so. For now, these little test boards are quite straight forward. I’m sure I will welcome having a contact to talk to when we get to the more demanding impedance controlled boards.

To configure the board for the test, jumpers R2, R4, R5, R7, R10, R13 and R15 were fitted. In this setup, the oscillation of the carry chain includes the switch-time of the first FET Switch, so it accurately reflects the transit time as it would be used in the Adder. 

I ran the test at various operating voltages to see what would happen. The normal operating voltage for AUC logic is 2.5V, the Recommended Maximum is 2.7V and Absolute Maximum is 3.6V. Once again we measure pin 11 of the 74LVC163 counter which is a divide by 16 function. We are looking for a 6.5ns tpd to the output carry in order to meet the target. Here are the results:

• @2.5V, 4.25MHz * 16 = 68MHz. 1000/68 = 14.7 / 2 = 7.35ns tpd 
• @2.7V, 4.65MHz * 16 = 74.4MHz. 1000/74.4 = 13.4 / 2 = 6.72 ns tpd
• @2.8V, 4.87MHz * 16 = 76.9MHz. 1000/76.9 = 13 / 2 = 6.5ns tpd
• @3.3V, 5.45MHz * 16 = 87.2MHz. 1000/87.2 = 11.47 /2 = 5.73ns tpd

I then had a chance to do some surgery ... 

This is to double up the driver at the input of the carry-chain, as Dr. Jeffyl suggested. To do so I stacked another SOT23 gate on top of the existing driver on the board. (I didn’t have another AND gate, so I used an XOR gate and tied one of the inputs to GND with a little patch cable. It’s a mess but it did the job). 

The rationale here is that AUC logic has relatively weak drive: 9mA as compared to 24mA for LVC. Doubling up the drivers will add a tiny bit of capacitance on the input, but the reduced tpd though the FET switches should more than compensate for that and tpd overall should drop. At least that’s the theory. 

Now, recall that we are looking 6.5ns or less here. We measure the frequency of oscillation divided by 16 and calculate the tpd through the 8-bit adder at various voltage levels. Here are the results:

• @2.5V, 5MHz x 16 = 80MHz —> 6.25ns
• @2.7V, 5.54MHz x 16 = 88.64MHz —> 5.64ns
• @2.8V, 5.65MHz x 16 = 90.4MHz —> 5.5ns
• @3.3V, 6.14MHz x 16 = 98.24MHz —> 5.08ns

The additional drive has done it, and we even have a reasonable safety margin. ttlworks’ FET Switch Adder as enhanced by Dr. Jefyll is a winner! I then fired up the test at 2.5V with a NC7SV08 in place of the 74AUC1G08 in the 8-bit adder carry-chain., Here is what I got:

• @ 2.5V, 4.94MHz ==> 6.33ns

Bingo! it's confirmed. NC7SV logic is a nice choice to drive the carry chain. It can be used conveniently for all the AND gates along the carry chain to provide the additional drive when needed. There is also an NC7SV74 flip-flop available which will do nicely for the ALU input Carry.