External oscillator (EC) is made outside of the microcontroller and connected to the pin OSC1 and OSC2 as figure 4.3. The external clock (EC) mode uses external oscillator as a clock source. The maximum frequency of this clock is limited to 20 MHz. Mainly crystal oscillator is used in this purpose.
In this mode the microcontroller starts operation immediately after the power is on. No time delay is required for frequency stabilization.
Figure: External Clock Oscillator Mode with Quartz Crystal
There are three modes as LP, XT and HS. Each modes are used on external oscillator as a clock source. The frequency is determined by quartz crystal or ceramic resonators which is connected to the OSC1 and OSC2 pins and depending on the features of the component in use. Any one mode is used on microcontroller circuits.
- LP (Low Power) mode - is used for low-frequency quartz crystal only. This mode is designed to drive only 32.768 kHz crystals usually embedded in quartz watches. It is easy to recognize them by small size and specific cylindrical shape. The current consumption is the least of the three modes.
- XT mode is used for intermediate-frequency quartz crystals up to 8 MHz. The current consumption is the medium of the three modes.
- HS (High Speed) mode - is used for high-frequency quartz crystals 4MHz or greater. The current consumption is the highest of the three modes.