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.