522 Current-Starved Ring Oscillator Based True Random Number Generator :: Quicker, easier and cheaper to make your own chip!

8-Oscillator True Random Number Generator

in SKY130A for Quantum-Safe Optical Communications

Rakha Naufal | Tiny Tapeout | SKY130A | 1x2 Analog Tile

1. Overview

This design implements a True Random Number Generator (TRNG) using eight current-starved ring oscillators whose outputs are combined through a chain of seven XOR gates and sampled by a TSPC D flip-flop. Thermal noise and process variation cause each ring oscillator to run at a slightly different frequency, creating phase jitter that accumulates over time into genuine entropy. The sampled output is a non-deterministic bitstream suitable for cryptographic key generation and secure optical communications applications.

2. Architecture

Block	Count	Description
Current-Starved Ring VCO	8x	5-stage ring oscillator, intentionally mismatched sizing
Output Buffer	8x	4-stage inverter chain, squares up VCO output
CMOS XOR Gate	7x	Full custom 8-transistor static CMOS XOR
TSPC D Flip-Flop	1x	11-transistor true single-phase clock DFF

Signal flow:

VCO1-8 → 8x Buffer → XOR chain (7 stages) → TSPC DFF → FOUT

3. Pin Assignment

Analog Pin	Signal	Direction	Description
UA[0]	VCTRL	Input	VCO control voltage (nominal 1.2V)
UA[1]	XORCHAIN	Output	Raw XOR chain output (debug)
UA[2]	FOUT	Output	Sampled random bit output
UA[3]	CLK	Input	Sampling clock (recommended 1 MHz)

4. Key Specifications

Parameter	Value	Condition
Technology	SKY130A 1.8V	—
Supply Voltage	1.8V	—
VCO Control Voltage	1.2V (nominal)	—
Nominal VCO Frequency	278 MHz	TT / 27°C
KVCO	~1 GHz/V	At operating point
Tuning Range	0 – 630 MHz	VCTRL: 0 – 1.8V
Power Consumption	157 µW	TT / 27°C / VCTRL=1.2V
Recommended Sampling Rate	1 MHz	—
Random Bit Output Rate	1 Mbit/s	At 1 MHz CLK
Tile Size	1x2 Tiny Tapeout Analog	~160 x 225 µm

5. PVT Corner Analysis

VCO frequency measured at VCTRL = 1.2V across all corners:

Corner	Temperature	Period (ns)	Frequency (MHz)
TT (Typical)	27°C	3.60	278
FF (Fast-Fast)	27°C	2.49	402
SS (Slow-Slow)	27°C	6.49	154
TT (Typical)	85°C	3.97	252
FF (Fast-Fast)	-40°C	2.51	398
SS (Slow-Slow)	85°C	6.59	152

The VCO operates across a 2.6x frequency range from worst-case slow (152 MHz) to worst-case fast (402 MHz). The TRNG remains functional across all corners as long as CLK_SAMPLE is set well below the VCO frequency.

6. Monte Carlo Analysis

2500 Monte Carlo runs at TT corner, 27°C, VCTRL = 1.2V using SKY130A tt_mm mismatch models:

Parameter	Value
Number of runs	2500
Mean frequency	274.39 MHz
Standard deviation	23.83 MHz
Minimum	196.12 MHz
Maximum	350.32 MHz
3σ range	202.91 – 345.88 MHz
3σ variation	±26.1%

monte_carlo The ±26.1% frequency variation across 2500 runs is typical for current-starved ring oscillators in 130nm and confirms that process mismatch between VCO instances will naturally generate entropy without additional tuning.

7. Simulation Results

7.1 Pre-Layout Functional Simulation

The TRNG was verified at TT/27°C with VCTRL = 1.2V and CLK = 1 MHz. FOUT shows a clean random bitstream switching between 0V and 1.8V at the sampling clock rate with no visible periodicity.

Key observations:

Rail-to-rail output (0V / 1.8V) confirmed
Random switching pattern with no visible periodicity
Clean edges at CLK rising edges
No metastability observed at 1 MHz sampling rate

fout_waveform

8. Testing After Tapeout

Step	Action	Expected Result
1	Apply 1.2V DC to UA[0] (VCTRL)	VCOs begin oscillating
2	Apply 1 MHz square wave to UA[3] (CLK)	DFF begins sampling
3	Read UA[2] (FOUT) with oscilloscope or logic analyser	Random bit sequence visible
4	Collect 1 million bits at 1 Mbit/s	Takes ~1 second
5	Run NIST SP800-22 statistical test suite	Expect pass on all 15 tests

9. Photonics & Security Application

True random number generation is a critical primitive in quantum-safe optical communications. Specific applications include:

Quantum Key Distribution (QKD): Random basis selection and key sifting
Optical encryption: Session key generation for WDM channel encryption
LiDAR anti-spoofing: Random pulse timing to prevent replay attacks
Secure coherent optical links: Nonce generation for authenticated protocols

The use of multiple ring oscillators exploits the inherent unpredictability of thermal noise in CMOS — the same noise source that limits phase noise in optical clock recovery circuits — making this TRNG directly relevant to photonic integrated circuit security.

10. Design Summary

Metric	Result
Total custom transistors	~300 (fully custom, no standard cells)
VCO count	8
XOR gates	7 (full custom CMOS)
DFF	1 (TSPC topology)
Nominal output frequency	278 MHz (TT/27°C)
Power	157 µW
DRC status	Clean (0 errors)
Tiny Tapeout precheck	All checks passed ✅
Pre-layout simulation	Functional TRNG output verified at 1 MHz

`ua`	PCB Pin	Internal index	Description
0	B0	6	VCTRL
1	B1	7	XORCHAIN
2	B3	9	FOUT
3	B2	8	CLK

522 Current-Starved Ring Oscillator Based True Random Number Generator