Interfacing ADXL345 IMU / Accelerometer with Arduino Uno

ADXL345 practical reference

A compact, low-power accelerometer used for tilt sensing, tap/double-tap detection, and freefall detection in wearable projects.

Key specifications

• Type: 3-axis MEMS digital accelerometer
• Interface: I2C (up to 400kHz) or SPI (up to 5MHz), selectable via the CS pin
• Measurement range: Selectable ±2g / ±4g / ±8g / ±16g, set via register
• Resolution: 10-bit standard, up to 13-bit in full-resolution mode (~3.9 mg/LSB at full resolution)
• Output data rate: Programmable, 0.1 Hz to 3200 Hz
• Supply voltage: 2.0V–3.6V (breakout modules commonly accept 3.3-5V via onboard regulation)
• Current draw: ~23 µA standby, ~140 µA in measurement mode (typical)
• Built-in features: Tap/double-tap detection, free-fall detection, activity/inactivity detection, 32-sample FIFO buffer
• I2C address: 0x53 default, 0x1D if the ALT ADDRESS pin is pulled HIGH

Pinout and connection notes

CS pin determines the bus mode — this trips people up because it's not obviously a "mode select" pin from the silkscreen. Tie CS to VCC for I2C; if you leave it floating on some breakout revisions, the chip can default to SPI mode and I2C reads will silently fail.

SDO doubles as the I2C address pin — this is the same physical pin used for SPI data output, repurposed in I2C mode as an address selector. Floating or grounding it gives 0x53; pulling it to VCC gives 0x1D. Useful for running two ADXL345s on one I2C bus without a multiplexer.

Full-resolution mode matters for the ±g range you pick — in 10-bit mode, selecting a wider range (±16g) trades away resolution at low-g movements. If fine tilt sensitivity matters, use full-resolution mode, which keeps ~3.9mg/LSB sensitivity regardless of the selected range.

Interrupts save you from polling — tap/double-tap/free-fall detection is computed on-chip; wiring INT1 or INT2 to a GPIO interrupt is far more responsive and power-efficient than continuously polling axis registers and computing thresholds in software.

• VCC: Power, 3.3–5V DC depending on breakout
• GND: Ground
• CS: Chip select — tie HIGH (to VCC) to force I2C mode; drive LOW as part of SPI transactions to use SPI mode
• INT1: Programmable interrupt output 1 (e.g. tap, free-fall, activity events)
• INT2: Programmable interrupt output 2 — can route different event sources than INT1
• SDO / ALT ADDRESS: In I2C mode: pull HIGH for address 0x1D, leave LOW/floating for default 0x53. In SPI mode: serial data output
• SDA: I2C data line (SPI: MOSI/SDI depending on breakout labeling)
• SCL: I2C clock line (SPI: SCK)

Integration checklist

1. Confirm the exact part number and breakout-board revision.
2. Verify supply voltage, current demand, logic levels, and signal direction against the datasheet.
3. Check controller pin assignments and bus address or interface mode against the example code.
4. Power up with external loads disconnected, then test signals before full integration.
5. If readings or communication fail, check power and ground first, then pin mapping, interface settings, and library initialization.

Troubleshooting and reproducibility

Record the board used, wiring, library/version, supply arrangement, and measured behavior. Use the selected board-specific wiring example as the source of truth. Never assume visually similar modules share identical voltage tolerance or pin order.