AN901 Datasheet, PDF (5/14 Pages) STMicroelectronics – Electromagnetic compatibility

EMC GUIDELINES FOR MICROCONTROLLER-BASED APPLICATIONS
3.1.4 Oscillator
Almost all microcontrollers have an oscillator coupled to an external crystal or ceramic reso-
nator. On the PCB, the copper traces to pins EXTAL/XTAL/V_SS (for external capacitors) must
be kept as short as possible. These capacitors are included in certain resonators which further
shorten traces.
Since the RC option is potentially sensitive to spikes which can shorten clock periods, the res-
onator option is preferable.
3.1.5 Other Signals
When designing an application, the following areas should be closely studied to improve EMC
performances:
– noisy signals (clock...),
– sensitive signals (high impedance...).
In addition to:
– signals for which a temporary disturbance affects the running process permanently (the
case of interrupts and handshaking strobe signals, and not the case for LED commands).
A surrounding ground trace for these signal increases EMC performances, as well as a
shorter length and the absence nearby of noisy and sensitive traces (crosstalk effect).
For digital signals, the best possible electrical margin must be reached for the 2 logical states
and slow Schmitt triggers are recommended for eliminating parasitic states.
3.2 PROGRAMMING EMC-HARDENED SOFTWARE
3.2.1 Parallel Processes
With a programmable system, an obvious possible EMS weakness arises from an unique
process that relies on valid memorized data. At first, the unique process must be split into as
many parallel and independent processes as possible. This is particularly important for secu-
rity functions such as the watchdog, refresh routine and the initialization routine. Additionally,
such a split is useful for locating weaknesses during EMC debugging.
3.2.2 Watchdog
The watchdog is a circuit which must be updated within a maximum time slot. The best sys-
tems maintain the watchdog independent of the CPU (not built with a soft routine). For ex-
ample, STMicroelectronics ST62 microcontrollers have a watchdog integrated in the compo-
nent, and is able to run independently of the CPU.
The watchdog update routine must be treated as a critical process to reduce chances that the
watchdog is updated when the process is no longer in normal operation.
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