Clock Generation in Microcontroller

generation of microcontroller

Hi all,

What is the best choice of oscillator for CMOS microcontroller?
What are the criteria to choose between quartz oscillator (fundamental or overtone)
and internal oscillator? It is considered impractical to design fundamental mode quartz oscillator for frequencies above 25-30 MHz.
What is the upper frequency limit of overtone mode quartz oscillators?

Thanks,
Vardan.

for simple and low cost applications, ring oscillators with +/- 20% (with thermal compensation) can be generated; ofcourse the frequency can be very high (up to GHz) to very low (KHz) with freq dividers.
for more accurate ICs like uC, simple Xtal oscillator can be made by only one inverter(NOT Gate) and two external capacitor to form a colpits like oscillator and Xtal acts as a large/High Q inductor thus the frequency deviation with time (jitter) is very low also depending on the type of Xtal, thermal deviation and initial accuracy can be a few ppm!
a value of near 10MHz is very common in osc design, cos there is a trade off betwwen cost/volume/accuracy; i.e lower fres= larger Xtal/higher cost but for higher fres=lower accuracy/more cost and shock sensitive.
usually freq up to 100 MHz is generated using over-tone generation and beyond it integrated PLL is used to generate freq. up to 3GHz (P4).

BEST!