2 Oscillator Types

An increasing number of AVR devices contain both an ultra-low-power (ULP) internal oscillator and an oscillator module that functions only when an external 32.768 kHz crystal is connected to the device. The internal oscillator achieves extremely low power consumption, but its timing accuracy is not sufficient for some applications. A typical ULP oscillator may have a ±1% frequency tolerance. For some applications this is not an issue, but in other applications such as long-term timing, this can result in an error of 14 minutes per 24-hour day.

For applications that require precise timing, the external 32.768 kHz crystal oscillator circuit is provided. A crystal oscillator may have a frequency tolerance on the order of ±10ppm or less than one second of timing error per 24-hour day, so it is well suited for applications that require precise long-term timing. It does, however, have two disadvantages. First, an external crystal must be connected to the AVR device, adding cost and area to the printed circuit board. Second, the crystal oscillator has higher power consumption than the internal oscillator.