Datasheet

7764A–8051–11/07

1.3.3 Overtone crystals

Problem: Sometimes overtone crystals are used with a standard oscillator circuit where a spe-

cific one is required to make the crystal to oscillate on its third overtone.

Consequence: an overtone crystal used with a standard oscillator circuit, oscillates on its funda-

mental frequency instead of oscillating on its third overtone frequency. The clock frequency seen

by the MCU is 1/3 of the expected frequency.

Action: check the type of the crystal you are using. If you really want to use an overtone crystal,

apply one of the following solutions.

1. A resistor of 3.9K between Xtal1 et Xtal2, a cap of 4.7 pF between Xtal1 and Vss, and a

cap of 15pF between Xtal2 and Vss

2. Read application note “How to use a third Overtone crystal with a 80C51 family micro-

controller” (doc397c496b2072e.pdf). Apply the following component values :

L1= 1.5 µH

C3=4.7nF

CP1=12pF

CP2=27pF

C2 : not needed

3. Same solution as number 2 except the LC network is, in this case, connected to Xtal1 :

L1= 1.5 µH

C3=4.7nF

CP1=10pF

CP2=10pF

Feedback

Resistor

XTAL1

XTAL2

GND GND

Microcontroller

4.7pF 15pF

To internal

clock circuitry

3.9K

Crystal

Overtone