Feature: Hardware design
not complete the requirements. “Real time” still needs an update interval. “Home use” still needs an operating range, enclosure assumption and accuracy target. Requirements should therefore be labelled by origin: customer requirement, engineering assumption or derived value. If an assumed 200pF bus capacitance is not identified as provisional, it can quietly become a design fact and survive into production without measurement.
Define operating states before choosing parts An electronic product doesn’t have one current consumption or one interface configuration. It has operating states. For this sensor, useful states include ship mode, boot, standby, measurement, display update, USB-powered operation, charging, programming and fault-safe operation. Each state should define:
• the enabled rails, clocks and peripherals; • its entry and exit conditions; • retained data and permitted wake sources; • maximum duration or response latency; • expected and maximum current; • GPIO levels when external devices are unpowered; • other states with which it may overlap. The final point is easily missed. Adding every maximum
load produces an unrealistic power supply, but treating loads independently can be equally unsafe. An OLED update might coincide with a sensor conversion, processor wake-up or charger transition. If the combined event exceeds the rail or battery impedance budget, the architecture must support it or firmware must prevent the overlap. That scheduling rule is
part of the electrical design even though it is implemented in software. State modelling also exposes what “off” means. A processor
in sleep mode may draw only microamps, while I²C pull-ups, the OLED module, regulator feedback network, charger or supervisor dominate battery current. Ship mode must specify current at the battery terminals rather than quoting only the microcontroller’s lowest datasheet figure.
Calculate energy, transients and charging separately Average energy determines service life, but peak current determines whether the product operates reliably. Both calculations are needed before the battery, regulator and decoupling choices are fixed. For periodic loads, average current can be estimated from:
Iavg = Ibase + Σ (Ik − Ibase)tk/T Subtracting the base current prevents it being counted
twice during an event. As an illustrative budget, consider 10µA at standby, a 0.5mA sensor conversion lasting 10ms every minute, 2mA of processor activity lasting 20ms every minute and a 10mA OLED update lasting 100ms every minute. The estimated average is approximately 27.4µA. These are requirement-development values, not measurements from the generated project. If only 700mAh of a nominal 1000mAh cell is allocated
after allowing for discharge limits and ageing, the ideal calculation is about 2.9 years. This is an architecture estimate,
Figure 2: Power implementation: (a) USB-C input protection and BQ24074 battery charging; (b) TPS63031 buck-boost conversion for the 3.3V system rail
www.electronicsworld.co.uk October 2026 17
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