Inside the BME280

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About This MicroSim

This is the payoff diagram for Chapters 6, 7 and 8. Each transduction mechanism was taught on its own; here all three sit in one package, sharing one ADC and one bus interface, and you can see the path that connects them.

Click any of the seven blocks for what it does: the silicon diaphragm over a sealed cavity, the polymer humidity film, the silicon diode whose forward voltage falls about 2 mV per degree, the shared ADC, the factory calibration registers unique to your individual chip, the compensation block, and the I2C/SPI interface at 0x76.

Trace a reading follows one measurement end to end with real numbers at every stage, computed with the actual Bosch compensation formulas and the datasheet's example calibration constants:

1. Air at 1013.2 hPa presses on the diaphragm
2. Piezoresistive elements change resistance as it flexes
3. The ADC samples: 407,962 counts
4. Raw counts mean nothing yet. Read dig_P1 through dig_P9
5. The formula needs t_fine = 109568, derived from the temperature reading of 21.4 °C
6. Compensated: 101,320 Pa
7. Divide by 100 and store it with its unit: 1013.2 hPa

Then turn on "Skip the temperature read." The red dashed path along the bottom of the diagram lights up as STALE, and a plot appears showing reported pressure against room temperature. With the fault off it is dead flat. With it on it is a steep straight line: the same air at 1013.2 hPa reads 1047.5 hPa in a cold room and 985.0 hPa in a warm one, a swing of over 60 hPa with no weather involved at all.

That is a real bug, it is common, and the symptom looks exactly like a failing sensor. The pressure element is temperature-sensitive by design; the compensation formula is what removes that dependence, and it cannot do its job without a fresh temperature reading.

The Actual size overlay draws the chip at true scale next to a grain of rice, because students consistently imagine something much bigger than 2.5 mm.

How to Use

• Click each block in turn. The connected signal paths light up with it.
• Run Trace a reading and step through all seven stages.
• Turn on Skip the temperature read, then drag the ambient temperature slider and watch the reported pressure move.
• Turn the fault off and drag the slider again. Nothing budges. That is the compensation working.
• Press Actual size and look at how small it is.

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Lesson Plan

Grade Level

6-12

Duration

20-25 minutes

Bloom's Taxonomy Level

Understand (L2)

Prerequisites

• Chapter 4 on analog-to-digital conversion
• Chapters 6, 7 and 8 on temperature, pressure and humidity sensing
• Chapter 12 on I2C addressing

Activities

1. Name the mechanisms (5 min): Click the three sensing elements and write one sentence each connecting them back to their own chapter.
2. Follow the numbers (8 min): Run the trace and record the value at every stage. At which stage does the reading first have a unit attached to it?
3. Diagnose the drift (8 min): With the fault on, record the reported pressure at 5, 15, 25 and 35 °C. Plot it. Then explain to someone who thinks their sensor is broken what is actually wrong.

Assessment

• Summarises how three transduction mechanisms coexist in one package.
• Explains why the temperature element is required even by a pressure-only program.
• Identifies calibration registers as chip-specific rather than model-specific.

References

1. Bosch BME280 datasheet - the compensation formulas and calibration registers this sim implements.
2. Wikipedia: Piezoresistive effect - the pressure element's transduction mechanism.
3. Wikipedia: Hygrometer - the capacitive polymer film used for humidity.
4. Wikipedia: Silicon bandgap temperature sensor - why a diode makes a usable thermometer.