Design Methodology · Study deck
Accelerometer Datasheets: Selection Evidence
A parcel tag must catch a hard drop but ignore road shake.
Blueprint Bina is your guide for this deck.
After studying this chapter
Learning objectives
You will be able to:
- Extract application-relevant accelerometer specifications from a manufacturer datasheet.
- Translate range, resolution, data rate, power, interface, FIFO, interrupt, package, and environmental rows into design decisions.
- Build a requirement-to-evidence record for an accelerometer selection.
- Identify what the datasheet proves, what firmware must configure, and what bench tests must still verify.
Major section
Start With the Acceleration Claim
None matters until the team links it to the product claim and a setting it can test.
- Bandwidth means the span of changes a sensor path can follow.: It is not the same as sample rate.
- A default value is not a reviewed choice.
- A similar part name may have different limits.
Major section
Start With the Acceleration Claim (continued)
Firmware is the built-in software that controls a device.: Record the exact range, rate, filter, interrupt, and power settings that firmware writes.
- The tag must preserve the event meaning and show when it cannot.
- Reopen the table after a part, mount, case, or code change.
- One bench set cannot copy every real parcel.
Major section
ADXL345 Datasheet Claim Filter
The ADXL345 datasheet is useful when it is read against one product claim at a time.
- A drop or shock flag cares about range, sampling, saturation, mounting, and what the part can actually observe.
- None of those features is a release claim until the board and firmware prove the chosen configuration.
Major section
ADXL345 Datasheet Claim Filter (continued)
The rows separate supply, temperature, shock, and ESD limits; each needs a matching application claim and board check.
- For the equipment-tag example, the first pass is not "use every ADXL345 feature." The first pass is to decide which product statements the part must support.
- Orientation detection needs a gravity reference, axis convention, offset tolerance, and calibration plan.
- Motion detection needs a data rate, threshold, duration, interrupt route, host wake sequence, and false-positive test.
Major section
ADXL345 Datasheet Claim Filter (continued)
Reading the datasheet through those claims keeps the design from treating an impressive feature list as proof.
- Selection claim:: The range, resolution, interface, package, and power modes fit the motion and board constraints.
- Firmware claim:: Register settings for data format, bandwidth/rate, power mode, FIFO, and interrupts match the datasheet assumptions.
- Bench claim:: Identity read, axis orientation, clipping, current, FIFO behavior, interrupt timing, and calibration are observed on the assembled board.
Major section
Convert Rows to Behavior
Activity/inactivity may support wake-on-motion.
- Data-ready may support fixed-rate sampling.
- FIFO watermark may let the MCU sleep between bursts.
- Free-fall or tap features may be useful for an interaction or handling claim, but only if thresholds, duration settings, interrupt mapping, and false-positive behavior are reviewed.
Major section
Convert Rows to Behavior (continued)
The firmware settings should be saved beside the bench log, not hidden inside a source file.
- SPI can be attractive when bus traffic, timing, or device addressing makes I2C awkward.
- I2C can save pins when timing and bus sharing are acceptable.
- A narrow range gives more useful low-motion detail but can clip handling shocks.
Major section
Convert Rows to Behavior (continued)
A low-power mode can reduce current but may change noise and timing behavior.
- Either way, the design needs sensor supply, I/O supply, pullups or chip-select behavior, logic levels, bus speed, driver support, and a traceable device-ID read.
- For a battery tag, the practitioner decision is usually a trade between missed events and excess wakeups.
- A high output data rate can catch short events but costs host reads and may fill the FIFO sooner.
Major section
Counts Need Product Meaning
The sensor produces signed digital samples whose meaning depends on range and resolution settings.
- Raw accelerometer counts are affected by range, data format, offset, noise, bandwidth, axis orientation, mounting, temperature, host timing, and filtering.
- A static gravity check can catch axis sign and orientation errors before those errors become application logic.
- Good bench evidence separates those layers.
Major section
Counts Need Product Meaning (continued)
The under-the-hood risk is that several conversions sit between the MEMS structure and the release decision.
- A full-resolution setting may preserve scale across ranges, but firmware still needs to parse the data format correctly and prove that the selected range does not clip the real motion.
- The product-level question is usually not "does the chip measure acceleration?" It is whether the assembled product can distinguish useful motion from noise, vibration, transport handling, user interaction, and fault states.
- A bus trace checks that multi-byte reads do not mix old and new samples.
Major section
What This Chapter Adds
The lists change from broad capabilities to electrical details, implementation instructions, and quality evidence.
- Claim Start from use The important rows depend on the application: tilt, step detection, asset motion, shock flag, or low-power wake.
Major section
Case Study Brief
The device uses a small MCU with a 3.3 V rail and can wake from an interrupt pin.
- The accelerometer can measure gravity on multiple axes with enough resolution for the chosen angle threshold.
- The part can sample at a suitable output data rate and trigger activity or inactivity events.
- The sensor and MCU can spend most time in low-power states and wake only when useful.
Major section
Extract the Critical Rows
For the ADXL345 example, the first extraction pass should produce a small selection table, not a copied datasheet.
- Full-resolution mode supports a scale factor of about 4 mg per LSB across ranges.
- Sensor supply and I/O supply have separate allowed ranges.
- SPI and I2C access are supported.
Deck summary
Key takeaways
None matters until the team links it to the product claim and a setting it can test.
- Firmware is the built-in software that controls a device.: Record the exact range, rate, filter, interrupt, and power settings that firmware writes.
- The ADXL345 datasheet is useful when it is read against one product claim at a time.
- The rows separate supply, temperature, shock, and ESD limits; each needs a matching application claim and board check.
- Reading the datasheet through those claims keeps the design from treating an impressive feature list as proof.
Retrieval practice
Recall check 1 of 2
Blueprint Bina says: answer from memory, then check your reasoning.
Q1A battery tag selects an accelerometer because it supports motion interrupts. What must still support the release claim?
Show answer
Answer: C The chapter says listed features become evidence only after the chosen configuration is proved.
Retrieval practice
Recall check 2 of 2
Blueprint Bina says: answer from memory, then check your reasoning.
Q2A tag uses a FIFO watermark to let its MCU sleep between bursts. What should its configuration record include?
Show answer
Answer: B The chapter connects the selected source and configuration to bench observations and host actions.
Print reference
Answers
Answer key.
- C · The chapter says listed features become evidence only after the chosen configuration is proved.
- B · The chapter connects the selected source and configuration to bench observations and host actions.