847 IEEE Ring Oscillator PUF
847 : IEEE Ring Oscillator PUF
- Author: Rajkamal
- Description: 8-pair RO-PUF: LFSR-driven toggle FFs, 8x12-bit counter pairs, 3-state FSM
- GitHub repository
- Open in 3D viewer
- Clock: 50000000 Hz
How it works
This project implements a Ring Oscillator Physical Unclonable Function (RO-PUF) on the sky130 process node.
A 32-bit LFSR (Linear Feedback Shift Register) with taps at positions 32, 22, 2, and 1 runs continuously. Its outputs feed 16 toggle flip-flops, each driven by a unique combination of LFSR tap XOR values. Because each toggle FF sees a structurally different Boolean function of the LFSR, the synthesiser cannot merge them — they behave like independent oscillators whose frequencies are set by silicon manufacturing variation.
Eight 12-bit counter pairs accumulate the toggle values of adjacent FF pairs over a fixed 1000-cycle window. A three-state FSM (IDLE → RUN → DONE) manages the measurement cycle cleanly, ensuring the done pulse fires for exactly one clock cycle with no restart race. After each window, counters in each pair are compared; the PUF response bit is the XOR of all eight comparison results, scrambled by the LFSR and a running 8-bit CRC.
How to test
- Set clock to 50 MHz.
- Assert
rst_nlow for at least 5 cycles, then release high. - The FSM starts automatically. Wait for
uo_out[1]to pulse HIGH (exactly 1 clock cycle). This takes approximately 1002 cycles (~20 µs at 50 MHz). - Read the PUF response from
uo_out[0]on the same cycle thatuo_out[1]is HIGH. - The FSM restarts immediately —
uo_out[1]will pulse again after another ~1002 cycles. uo_out[7:2]outputs scrambled counter MSBs for debug/characterisation.- To re-seed the LFSR with a different sequence, pulse
rst_nlow and release.
External hardware
None required.
IO
| # | Input | Output | Bidirectional |
|---|---|---|---|
| 0 | Unused | PUF response bit (XOR of 8 comparisons, LFSR scrambled) | |
| 1 | Unused | Measurement done flag (1-cycle pulse every ~1002 clocks) | |
| 2 | Unused | Debug MSB 0 | |
| 3 | Unused | Debug MSB 1 | |
| 4 | Unused | Debug MSB 2 | |
| 5 | Unused | Debug MSB 3 | |
| 6 | Unused | Debug MSB 4 | |
| 7 | Unused | Debug MSB 5 |