Crystal Oscillator XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier that can be configured for use as an on-chip oscillator, as shown in Figure 2. Either a quartz crystal or a ceramic resonator may be used. To drive the device from an external clock source, XTAL2 should be left unconnected while XTAL1 is driven as shown in Figure 3. Figure 2. Oscillator Connections MAX 1 HC BUFFER HC C2 C1 XTAL2 XTAL1 GND Note: When using the MCU oscillator as a clock for an external device, an HC buffer should be connected as indicated in the figure. Figure 3. External Clock Drive Configuration 6 AT90S8515 0841G–09/01