We frequently have customers come to us with unusual problems. We had a customer that had an old 5 volt ASIC that they needed more of. This is not a problem. But they wanted to include a driver chip that had a +/- 5 volt output. And that was a problem. The challenge is how to get 10 volt operation out of available processes that are set up for 5 volts or less.
Many of our 68XX microcontrollers use an 0.6u EEPROM process. This process uses special high voltage transistors that are rated at ~18 volts. We needed 10 volt operation, and chose to create a gate array that used these high voltage transistors everywhere. This gave us the process, but we still had several design issues.
We were a little worried about speed, since we had to more than double our "L's" in order to support the higher voltage. However the loss of speed due to the increase in "L" was offset by the higher voltages, and so performance was not a problem. It turned out that our biggest problem was the inputs. Normally, inputs use a ratioed inverter. A TTL input is a ratioed inverter that is set to have a threshold of 1.4 volts, which is exactly in the middle of the TTL range.
In this case, we were working off of a +/- 5 volt supply. As far as the chip was concerned, ground was the -5 volt supply, and the new TTL input thresholds were between 5.8 and 7.0 volts. This is an impossibly tight target for a ratioed inverter. Instead, we made a simple P-channel comparator, and use an internal resistor string between +5 and ground to establish the threshold. The rest of the chip was straightforward digital logic.
This chip was a good example of using a current process in a legitimate but unusual way to achieve superior performance to meet the customers' goals.