C509-L_97 Datasheet, PDF (76/290 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

Reset / System Clock
C509-L
In this application the on-chip oscillator is used as a crystal-controlled, positive-reactance oscillator
(a more detailed schematic is given in figure 5-5). lt is operated in its fundamental response mode
as an inductive reactor in parallel resonance with a capacitor external to the chip. The crystal
specifications and capacitances are non-critical. In this circuit 20 pF can be used as single
capacitance at any frequency together with a good quality crystal. A ceramic resonator can be used
in place of the crystal in cost-critical applications. lt a ceramic resonator is used, C₁ and C₂ are
normally selected to be of somewhat higher values, typically 47 pF. We recommend consulting the
manufacturer of the ceramic resonator for value specifications of these capacitors.
Figure 5-5
On-Chip Oscillator Circuitry
To drive the C509-L with an external clock source, the external clock signal has to be applied to
XTAL2, as shown in figure 5-6. XTAL1 has to be left unconnected. A pullup resistor is suggested
(to increase the noise margin), but is optional if V_OH of the driving gate corresponds to the VI_H2
specification of XTAL2.
Figure 5-6
External Clock Source
Semiconductor Group
5-7
1997-10-01