Large-scale deployments of resource-constrained embedded devices require lightweight, self-contained hardware roots of trust that avoid long-term secret storage. Physically Unclonable Functions (PUFs) enable secure key extraction from intrinsic hardware variations without storing keys in non-volatile memory (NVM). SRAM PUFs leverage existing Static Random Access Memory (SRAM), but face reliability issues due to environmental noise. Existing solutions rely on complex error correction codes with NVM-stored helper data, or extensive SRAM measurements to pre-select stable cells. Eliminating NVM storage for helper data mitigates information leakage risks and manufacturing costs, offering a crucial benefit for resource-constrained devices lacking NVM. This paper presents a novel approach for stabilizing SRAM PUFs without NVM-stored helper data, using a simple decoder and few SRAM measurements, at the cost of increased SRAM size. We generate consistent On-Demand Helper Data (ODHD) temporarily stored in volatile memory and validate our method experimentally on real hardware. ODHD exposes a flexible trade-off between enrollment consistency, regeneration reliability, and SRAM size by varying a single enrollment threshold, a degree of freedom absent in the fixed Dark Bit approach. At the threshold-free operating point, ODHD achieves a key error rate of 8.2% for a 16-bit output, more than six times lower than Dark Bit’s 51.5%, using only 6 bytes of SRAM per 16-bit output, when ≈ 500 power cycles are used at every key derivation.

Swarm robotics focuses on designing and coordinating large groups of relatively simple robots to perform tasks in a decentralised and collective manner. The swarm provides a resilient and flexible solution for many applications. However, contemporary swarm robots have a significant power problem in that secondary (i.e. rechargeable) batteries are slow to charge and offer lifetimes of only a few years, increasing maintenance costs and pollution due to battery replacement. We imagine a different future, wherein battery-free robots powered by supercapacitors can be recharged in seconds, offer long-life autonomous operation and can rapidly pass charge between one another using trophallaxis. In pursuit of this vision, we contribute the CapBot, a battery-free swarm robot equipped with Mecanum wheels, a Cortex M4F application processor and Bluetooth Low Energy networking. The CapBot fully recharges in 16 s, offers 51 min of autonomous operation at top speed, and can transfer up to 50 % of its available charge to a peer via trophallaxis in under 20 s. The CapBot is fully open source and all software and hardware source is available online.

Bluetooth Low Energy and IEEE 802.15.4 are commonly used wireless communication protocols in IoT applications. Both operate in the 2.4 GHz ISM band, where their coexistence can lead to interference. We study the bit error rate (BER) under controlled timed interferences within the physical payload, varying interference power and frequency offsets. Measurements with a co-located software-defined-radio antenna are captured and shown to validate the experimental setup. IEEE 802.15.4 proves more robust to BLE interference than vice versa, consistent with results in literature. Interference at lower relative frequencies is found to have a greater impact on BER. Controlled interference causes persistent bit errors after it ends, suggesting receiver desynchronisation.

Accurately determining the time constant of a circuit enables IoT nodes to easily read out resistive or capacitive sensors. However, power and cost constraints lead to hardware limitations that complicate such measurements, including ADC noise, sampling clock jitter, poor voltage control over temperature and process, and a low-power microprocessor without a fast multiplier or floating point support. This work discusses estimating the time constant of a decaying exponential’s ADC samples using a simple weighted linear regression and describes the on-chip implementation of the regression on a low-cost, low-power microprocessor. Experimental results with an imperfect ADC show that time constants over more than two orders of magnitude can be accurately estimated within 5% of the nominal value with a mean standard error of about 1% of the nominal value.

SRAM Physically Unclonable Functions (PUFs) derive secret keys from start-up values for inherent security benefits but suffer from reliability issues due to bit flipping. We introduce the Threshold-based Majority Voting Scheme (TMVS), a lightweight method that eliminates noise and mitigates bias in SRAM PUFs while retaining the simplicity of majority voting decoders used by repetition codes, without the significant entropy loss that repetition codes incur under biased responses. TMVS runs entirely in software, requires no cell-level bit-error rate qualification or SRAM redesign, and avoids the complex decoders of heavy error correcting codes. We derive closed-form expressions for decoding-error probability and expected memory, validate them on experimental data, and present a security analysis that provides exact formulas for min-entropy and secrecy leakage due to helper data and bias, identifying conditions under which TMVS achieves zero secrecy leakage. On a large public dataset, TMVS shows near-zero cross-chip secrecy leakage and preserves average conditional min-entropy above 1 bit despite biased, spatially correlated SRAM statistics. Compared with prior work, TMVS offers the smallest decoding complexity at the cost of a larger PUF size. In a representative configuration, TMVS generates a 128-bit key with failure probability 9.15 · 10−6and zero secrecy leakage at a bit-flip probability of 10%, requiring only ∼ 248k clock cycles on a 32-bit ARM Cortex-M0. These results show that TMVS is practical and implementation-friendly for resource-constrained, low-power devices.

Bit errors in wireless communication predominantly stem from external interference as well as from multipath fading and attenuation. In order to tackle these harmful influences, the IEEE 802.15.4 standard includes time slotted channel hopping. We have collected packets containing bit errors from 200,000 packets generated in two different testbeds. We show that the channels used in IEEE 802.15.4 exhibit different error patterns typical for either external interference or multi-path fading and attenuation. These insights allow to detect, classify and quantify the presence of these phenomena. Furthermore, practical use cases for exploiting the knowledge on error patterns on a per-channel basis are presented. We propose to choose Forward Error Correction on a per channel basis and provide reference values in terms of code error correcting capability required to recover from 50% of the occurred packet errors on certain channels.

In this work we propose and demonstrate a protocol for a time synchronized channel hopping mesh network for wireless transceivers that use exclusively imprecise and inaccurate on-chip oscillators. This protocol is built on an IEEE 802.15.4 physical layer radio that enables interoperability with protocols such as 6TiSCH or Thread. A calibration-bootstrapped multi-hop mesh network is demonstrated with a single crystal-enabled node acting as the root. The protocol is designed to create a multi-hop mesh while compensating noisy and drifting oscillators and timers. With a 4 s synchronization period, an experimental implementation of the network maintains, in the worst case, 1.8 ms 3σ absolute time synchronization and 820 µs 3σ hop-to-hop synchronization across four hops, under ambient environmental conditions. The resistance to environmental variation is tested by varying one node's supply voltage. With time and frequency feedback from received packets, the node maintains this synchronization with a supply variation of 2.5 mV/s, which is equivalent to a temperature variation of 10°C/min with a packet rate of 0.5 Hz.

A growing range of networked embedded devices are moving away from batteries and towards super-capacitor charge storage. However, mobile robots remain largely dependent upon batteries with slow recharge cycles and limited lifetimes. In this demonstration paper, we introduce a novel battery-free platform for swarm robotics which features: 24 minutes of operation running at its top speed of 1.24 km/h, a carrying capacity of over 2.5kg, full recharge cycles of under 12 seconds and rapid peer-to-peer charge transfer or trophallaxis in the field. This is supported by an nRF52840 Cortex-M4F equipped with BLE/ANT/802.15.4 transceiver. Notably, while the autonomy of FreeBots is limited compared to battery-powered robots, their operational vs charging duty-cycle is significantly higher at over 99%.

The single-chip micro-mote (SC$\mu \text{M}$) is a$2\times 3$mm2single-chip crystal-free mote-on-chip. SC$\mu \text{M}$implements the IEEE802.15.4 and BLE standards and can communicate with off-the-shelf radios compliant to those standards. SC$\mu \text{M}$exclusively uses on-chip oscillators, including a 2.4-GHz LC oscillator to synthesize the communication frequency, and a 2-MHz RC oscillator to clock the chip rate. The challenge is that the LC oscillator drifts at 2100 ppm over a temperature range of 45 °C, far from the 40-ppm maximum drift mandated by the IEEE802.15.4 standard. While one-shot calibration is possible, any temperature change causes IEEE802.15.4 communication to fail. This article describes a continuous calibration approach for SC$\mu \text{M}$to adapt the tuning of its oscillators as the temperature changes. Experimental results show that it allows SC$\mu \text{M}$to keep communicating with an IEEE802.15.4 radio even under the extreme condition of using a hair dryer to heat up the chip at 3 °C/min. Under these conditions, the drift of the LC oscillator stays within the ±40-ppm limit over 94% of the time. Similarly, the drift of the 2-MHz RC oscillator stays within ±1000 ppm limit 99.98% of the time.

The single-chip micro mote (SC$\mu \text{M}$) is a crystal-free single-chip mote that brings us one step closer to the Smart Dust vision, in particular, as it can communicate with off-the-shelf IEEE802.15.4 and Bluetooth low energy devices. However, before it can be part of such networks, the crystal-free SC$\mu \text{M}$chip needs to be able to accurately tune its communication frequency to synchronize to the network. This is a challenge since its onboard RC and LC-based resonating circuits have a drift rate that can be three orders of magnitude worse than crystal-based oscillators typically used in today’s radios. This article introduces QuickCal, a solution that allows an SC$\mu \text{M}$chip to self-calibrate against off-the-shelf devices dedicated to assisting with its calibration. We show that an SC$\mu \text{M}$chip can self-calibrate against this QuickCal Box in fewer than 3 min. We further validate that once it has self-calibrated, an SC$\mu \text{M}$chip can reliably communicate with off-the-shelf IEEE802.15.4 devices. Finally, we demonstrate a heterogeneous network—composed of an SC$\mu \text{M}$chip and an OpenMote device—implementing a full 6TiSCH Industrial IoT protocol stack, which uses time synchronization and channel hopping. This is the first time that a crystal-free radio is participating in a channel-hopping-enabled TSCH network.

The Internet-of-Things (IoT) promises one trillion wireless sensors in the next 10 to 15 years. To enable this scale, we present the experimental results on the time-keeping ability of crystal-free radios to lower the cost of the wireless sensors. We propose a network referenced frequency lock loop and demonstrate a one sigma mean frequency accuracy of 47 p/min, post calibration. We also extend this concept to calibrate the on-chip RF local oscillator by exploiting the network time synchronization for frequency calibration. This timing accuracy is sufficient to create a scalable wireless mesh network up to 10 hops deep with a 1 ms guard time interval. The impact of time synchronization on the average power consumption of the wireless sensor node is negligible and especially true for environments with mobility and high data traffic. We envision these emerging microsystems to be embedded into everyday objects and discuss the tradeoff of mobility with the average power consumption.

Ring oscillators have area and power advantages over LC tanks, but conventional wisdom is that rings must be locked to a high-Q external reference to be useful in RF communications. In this paper we explore performance of a 2.4GHz receiver incorporating only a free-running ring as a local oscillator. Using a simple technique to compensate for frequency error, we find that a minimum-size ring fabricated in 65nm CMOS and consuming only 105μW is able to demodulate 75% of received 802.15.4 packets and, if the FSK tone deviation is doubled from 802.15.4 spec, packet receive rate exceeds 99.8%.

The recent proliferation of easy to use electronic components and toolkits has introduced a large number of novices to designing and building electronic projects. Nevertheless, debugging circuits remains a difficult and time-consuming task. This paper presents a novel debugging tool for electronic design projects, the Toastboard, that aims to reduce debugging time by improving upon the standard paradigm of point-wise circuit measurements. Ubiquitous instrumentation allows for immediate visualization of an entire breadboard's state, meaning users can diagnose problems based on a wealth of data instead of having to form a single hypothesis and plan before taking a measurement. Basic connectivity information is displayed visually on the circuit itself and quantitative data is displayed on the accompanying web interface. Software-based testing functions further lower the expertise threshold for efficient debugging by diagnosing classes of circuit errors automatically. In an informal study, participants found the detailed, pervasive, and context-rich data from our tool helpful and potentially time-saving.