AN901 Datasheet, PDF (7/14 Pages) STMicroelectronics – Electromagnetic compatibility

EMC GUIDELINES FOR MICROCONTROLLER-BASED APPLICATIONS
3.3.3 Choice of Microcontroller
The use of a microcontroller with a high clock rate may cause dangerous EMI levels. This fea-
ture should not be used unless it is specifically needed for real-time application requirements.
If a high system-clock frequency is requested, certain microcontrollers (such as the STMicro-
electronics ST9 family) use an internal PLL to build a system clock frequency higher than the
oscillator frequency with an external resonator (EMI reduction). A hardware watchdog must
implemented in the microcontroller in order to meet EMC requirements.
Certain component suppliers, such as STMicroelectronics, have taken EMC requirements into
account when designing their products. It is best to use components designed with specific
EMC technical characteristics, rather than those with unknown EMC performance levels.
3.3.4 Unused Features
All microcontrollers are designed for a variety of applications and often a particular application
does not use 100% of the MCU resources.
To increase EMC performances, unused clocks or counters, as well as I/Os, should not be left
free, e.g. I/Os should be set to “0” or “1” and unused functions should be “frozen” or disabled.
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