Sensing Low-g Acceleration Using the ADXL345 Digital Accelerometer Connected to the ADuC7024 Precision Analog Microcontroller
The ADXL345 is well suited for mobile device applications. It measures the static acceleration of gravity in tilt-sensing appli-cations, as well as dynamic acceleration resulting from motion or shock. Its high resolution (4 mg/LSB) enables measurement of inclination changes of about 0.25°. Using a digital output accelerometer such as the ADXL345 eliminates the need for analog-to-digital conversion, reducing system cost and real estate. Additionally, the ADXL345 includes a variety of built-in features. Activity/inactivity detection, tap/double-tap detection, and free-fall detection are all done internally with no need for the host processor to perform any calculations. A built-in 32-stage FIFO memory buffer reduces the burden on the host processor, allowing algorithm simplification and power savings. Additional system level power savings can be implemented using the built-in activity/inactivity detection and by using the ADXL345 as a “motion switch” to turn the whole system off when no activity is felt and on when activity is sensed again.
The ADXL345 communicates via I2C or SPI interface. The circuits described in this document demonstrate how to implement communication via these protocols.
Figure 2. ADXL345 and ADuC7024 in I2C Configuration (Simplified Schematic: Decoupling and All Connections Not Shown)
Both schematics are simplified, but required connections (supplies, ground connections, etc.) are shown. In these schematics, the ADuC7024 is programmed via UART (connected to Pin 49 and Pin 50). SW2 and SW3 are Reset and Download buttons, respectively, for programming the microcontroller. SW1 is an on/off power switch.
For information on programming the ADuC7024, please see the ADuC7024 data sheet. Sample code for the I2C configuration can be found at CN0133_Source_Code.zip.
Figure 3. 3-Wire SPI Connection Diagram
The circuit described uses the ADuC7024 microcontroller. The same configuration can be applied with any SPI- or I2C-capable microcontroller, as outlined in the diagrams in Figure 4. The standard I2C and SPI connections are used. Pin functions for the two protocols are listed in Table 1.
Table 1. ADXL345 Pin Functionality in SPI and I2C Communication Modes
ADXL345 Pin Number
||(Connect to VDD for I2C)
||Alternate Address Select
||Serial Data Output
||Serial Data Input (SPI 4-Wire)/
Serial Data Input and Output (SPI 2-Wire)
||Serial Communications Clock
||Serial Communications Clock|