We can probe the process parameters (the doping levels, which determine the speed of the transistors in PulseNet) by examining the ring oscillator output. When Pulsenet has power, a chain of twenty-nine inverters will start to oscillate. The frequency of this oscillation is a measure of the process parameters, as shown in the table below. All simulations were done with a supply voltage of 2.5 V.

Output frequency	Process Parameters
1.64 MHz	TTTT (normal)
1.53 MHz	SSTT (slow transistors)
1.74 MHz	FFTT (fast transistors)
1.84 MHz	TTTF (10% overvoltage)

Each inverter consists of a 40x4 NMOS and a 40x4 PMOS transistor (that’s 40 lambda in the slow direction). The signal is buffered by a 3-level inverter fan-out of four chain (minimum channel lengths) on its way to the output pad.

A shot of the test setup is shown below. It works! It works!

We measured the ring oscillator frequency as a function of supply voltage (when the chip was not being clocked, and was therefore cool). Here’s what we found:

Voltage	Frequency
2.400 V	1.633 MHz
2.500 V	1.748 MHz
2.600 V	1.828 MHz

So it looks like we’re in the FFTT process corner (fast transistors). This is a good thing.