The engineering preliminary design
A sensor that wakes up for a tenth of a second per minute at 20 mA and sleeps at 5 μA lasts almost six years on two AA batteries. If it draws 50 μA when asleep, it lasts less than three: the sleep mode decides the battery life.
01What a preliminary design contains
The preliminary design is the document that shows, before building, that the proposed solution will meet the specification. It is not the detailed design —the complete schematics and the board come later—, but the architecture decisions justified with calculations. A typical preliminary design includes:
| Part | What it says |
|---|---|
| Descriptive report | The problem, the user, the scope and what is left out |
| Specification | The quantitative and verifiable requirements |
| Block diagram | The functions, the interfaces between them and the signals that connect them |
| Selection of main components | The microcontroller, the sensor, the radio, the power supply, with the discarded alternatives and why |
| Sizing budgets | Energy, measurement error, radio link, dissipation, response times |
| Test plan | How each point of the specification will be verified |
| Preliminary costs and schedule | The priced bill of materials and the stages |
02The sizing budgets
A budget distributes a limited resource among the blocks and verifies that it is enough with a margin. The error budget adds up the uncertainties of the sensor, signal conditioning and converter, as seen in Measurements; the link budget adds up powers, gains and losses down to the receiver sensitivity; the thermal budget verifies junction temperatures with the dissipation of each component, and feeds the reliability estimate. In a battery-powered device, the most important is the energy budget: with active and sleep duty cycles, the average current is
A node that wakes up every T seconds, measures and transmits for ta at current Ia, and sleeps the rest of the time at Is. The curve is the battery life in years as a function of the period, on a logarithmic scale; the yellow dot is the chosen design.
03The test plan
Each point of the specification needs a test that says how it will be measured, with what instrument, under what conditions and what the acceptance criterion is. Writing it into the preliminary design forces the specification to be verifiable and anticipates the instruments that will be needed. The degree’s laboratory tests —from Electronic Measurements I and II— are the basis; those for regulatory compliance, if the product requires them, are usually done in an external laboratory and should be budgeted. The manufacturing technology was defined in construction technology.
04In the laboratory
Draw the block diagram of the final project with the interfaces, the supply voltages and the signals between blocks.
With a sense resistor and the oscilloscope, measure the current profile of a microcontroller that sleeps and wakes up, and compare it with the budget.
Build a table with each specification, the test that verifies it and the instrument required.
05Common mistakes
- Taking the sleep current from the microcontroller datasheet alone. Regulators, dividers, sensors and pull-ups also consume.
- Using the full nominal capacity of the battery. At low temperature or with current peaks considerably less is usable.
- Promising battery lives of decades. Battery self-discharge sets a limit of a few years to a decade.
- Leaving the test plan for the end. If a specification cannot be tested, it has to be rewritten now.
06Self-assessment
What average current does the lab node have with its initial values?
About 38.3 μA.
What battery life does that give with two AA cells?
Almost 6 years: \( 0.8 \cdot 2500 / 0.0383 \approx 52\,000 \) hours.
And with a CR2032 coin cell?
A little over half a year.
What is the difference between a preliminary design and a detailed design?
The preliminary design fixes and justifies the architecture with calculations; the detailed design produces the complete schematics, board and software.
07Further reading
- Karl T. Ulrich and Steven D. Eppinger. Product Design and Development. McGraw-Hill. (Spanish edition: “Diseño y desarrollo de productos”.) Product architecture and system-level design.
- Elecia White. Making Embedded Systems. 2nd ed., O'Reilly, 2024. Embedded systems architecture and power reduction.
- Paul Horowitz and Winfield Hill. The Art of Electronics. 3rd ed., Cambridge University Press, 2015. Low-power circuit design and component selection.