AN1709 datasheet(28/38 Pages) STMICROELECTRONICS

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EMC DESIGN GUIDE FOR ST MICROCONTROLLERS

pins in order to minimize output distortion and start-up stabilization time. The loading capaci-

tance values must be adjusted according to the selected oscillator.

These oscillators are not stopped during the RESET phase to avoid the delay needed for the

the oscillator start-up.

Internal RC Oscillator

The internal RC oscillator is the most cost effective solution, with the drawback of lower fre-

quency accuracy. Its frequency is in the low single digit MHz range. In this mode, the two os-

cillator pins have to be tied to ground.

Process variations will also bring some differences from lots to lots (20 to 60%).

Some ST Microcontrollers (refer to product specification) embed a process compensation.

This feature is called “Trimmable internal RC”. A procedure during test operation analyzes the

process variation and calibrate the internal oscillator accordingly. This brings the internal RC

accuracy to 1%. This procedure can be also performed by the user:

Refer to AN1324 “Calibrating the RC oscillator of the ST7FLITE0 using the mains”

Table 11. ST7 Clock Sources

The ST multi-oscillator system is designed for flexibility and to allow the system designer to

find the best compromise between emission, accuracy and cost criteria.

Hardware Configuration

OSC1

OSC2

EXTERNAL

ST7

SOURCE

OSC1

OSC2

ST7

CEX

REX

OSC1

OSC2

LOAD

CAPACITORS

ST7

CL2

CL1

OSC1

OSC2

ST7

AN1709 Datasheet(PDF) 28 Page - STMicroelectronics