Most of the C8051 devices include an external oscillator drive circuit to driver an external crystal, ceramic resonator, capacitor, or RC network.
For a crystal or ceramic resonator configuration, the crystal/resonator must be wired across the XTAL1 and XTAL2 pins as shown in the figure below. And a 10M ohm resistor must be wired across the XTAL2 and XTAL1 pins for the crystal/resonator configuration.
The 10 M ohm resistor between XTAL1 and XTAL2 is required to provide proper DC bias to the internal crystal driver. And the parallel impedance (10M ohm) decides the loop gain. Having a larger impedance is usually desired because it will increase the loop gain, and hence reduce start up time and increase negative impedance. So it shouldn't try to reduce this 10M ohm impedance.
With regard to the unstable or unable to start up oscillation, please check the items below one by one.
1. Review the PCB layout:
Crystal oscillator circuits are quite sensitive to PCB layout.
AN203 provided a good starting point in the design and layout of a PCB, and the methods presented in this application note should be taken as suggestions.
2. Check the capacitors value connected to the crystal.
The capacitors shown in the external crystal configuration provide the load capacitance required by the crystal for correct oscillation. These capacitors are "in series" as seen by the crystal and "in parallel" with the stray capacitance of the XTAL1 and XTAL2 pins.
The desired load capacitance CL depends upon the crystal and the manufacturer. Refer to the crystal data sheet when completing these calculations below.
The equation for determining the load capacitance for two capacitors is as follows:
C is the capacitor value connected to the two crystal leads, in general, will connect a same capacitor on each lead. CS is the total stray capacitance of the PCB, and the stray capacitance for a typical layout where the crystal is as close as possible to the pins is 2-5 pF per pin.
For example, a tuning-fork crystal of 25 MHz has a recommended load capacitance of 12.5 pF. With a stray capacitance of 3 pF per pin (6 pF total), the 13 pF capacitors yield an equivalent capacitance of 12.5 pF across the crystal.
12.5 pF = C/2 + 6
C = 13 pF
3. Choose a reasonable driver level
Please refer to the KB below to choose a reasonable driver level.
The KB describe how to specify the XFCN to issue a reasonable crystal driver current considering the required Drive Level (DL) and Equivalent Series Resistance (ESR) of the crystal rather than only considering the frequency.
4. Measure the actual driver level on board
If the crystal still unstable or cannot start up after checking the items above, you should measure the actual driver level on board (Crystal vendor should be able to provide the test report). If the measured driver level is way larger than the maximum specification, the crystal will be over-driven and may result to the crystal circuit not working, and the crystal operational lifetime will be reduced.
The recommended method is adding a series resistance Rd to the crystal to reduce the driver level.