HTU21D Arduino: 7 Timeout, CRC and Reading Checks
An HTU21D Arduino sketch should distinguish a measurement from a library error code. Begin with the documented breakout and library, verify the electrical setup, and build the failure path before connecting the result to a display or automatic controller.
Inspect wiring while power is disconnected
Use the exact breakout pin labels for supply, ground, SDA and SCL. The referenced SparkFun board runs at 3.3 V. Arduino models differ in I2C pin locations and logic levels, so map the selected host rather than blindly copying A4 and A5 from an Uno drawing.
Check pull-ups before adding a second device
The reference breakout includes 4.7 kΩ pull-ups. Other connected boards may add more resistors or pull to a different rail. Parallel resistors change the effective load; an unwanted 5 V pull-up can also create an electrical problem. Use a compatible bus or proper level shifting, not a blanket assumption that all I2C devices coexist safely.
Install and identify the intended library
The cited example uses the SparkFun HTU21D library and methods such as readHumidity() and readTemperature(). Record the installed version and open its bundled example. A similarly named library may use different initialization, return types or integrity checks, so do not combine fragments from unrelated APIs.
Separate error values from physical quantities
In the referenced implementation, 998 indicates a timeout and 999 indicates a CRC failure. Check these outcomes before formatting or averaging results. Do not convert either to an apparently valid degree value, clip it into a permitted range, or send it to a control algorithm as a real sample.
Respect conversion and resolution choices
Higher resolution can require a longer conversion. Begin with the documented defaults and a modest logging interval. If settings change, use the library’s supported configuration routine and timing. Repeatedly requesting values faster than conversions complete does not create a higher-bandwidth humidity instrument.
Log enough evidence to diagnose a failure
Store successful-sample time, humidity, temperature, communication status and a consecutive-error counter. Keep unit labels explicit. If a read fails, show an unavailable state or a clearly aged previous value. Avoid smoothing error markers into valid data because the resulting average can look deceptively plausible.
Newest in the store
Test recovery without wetting or overheating the device
Use an incorrect configuration in software to confirm that the sketch reports failure, then restore the correct setup. Power down before reconnecting hardware. Re-run initialization after a normal power cycle. Do not spray water or apply high heat as a first functional test; a communication and room-air workflow is sufficient to expose many integration mistakes.
Frequently asked questions
Why does the example print 998 or 999?
For the referenced library these are error markers, not measurements. Inspect the installed library to confirm the convention.
Should I reject every unusual humidity value as a bad sensor?
First inspect transfer status, CRC handling, placement and units. A plausibility check is useful, but it cannot replace the communication checks.
Availability and illustration note
No matching Epro product was returned by the catalog search on 19 October 2026. This learning guide does not assert stock, a selling price or compatibility of an unidentified substitute. The illustration is an original conceptual signal-flow diagram, not a product photograph or complete wiring schematic.
Related Epro reading
For a first bench setup, review breadboard connections and continuity checks and I2C voltage-level translation. These guides do not replace the exact module schematic.
Technical references and limitations
Source: SparkFun HTU21D hookup guide and library reference. Electrical limits apply to the named part, not automatically to every carrier board. This is a documentation-based learning workflow, not a claim of independent hardware testing, traceable calibration or safety certification.