Recommendations AN2752
18/42 Doc ID 14651 Rev 3
6.5 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 permanently affects operation of the
application, for example, interrupts and handshaking strobe signals (but not LED
commands).
A surrounding V
SS
trace for such signals increases EMC performances, as does a shorter
length or absence of noisy and sensitive traces (crosstalk effect).
For digital signals, the best possible electrical margin must be reached for the 2 logical
states. Slow Schmitt triggers are recommended for eliminating parasitic states.
6.6 Unused I/Os and features
Microcontrollers are designed for a variety of applications, where often a particular
application does not use 100 % of the microcontroller resources.
To increase EMC performance, unused clocks, counters, or I/Os, should not be left free, for
example, I/Os should be set to ‘0’ or ‘1’ (pull-up or pull-down to the unused I/O pins) and
unused functions should be ‘frozen’ or disabled.
Alternatively, unused I/Os can be programmed as push-pull ‘low’ in order to keep them at a
defined level but not to use external components.
6.7 User options
STM8S devices have user option features that can be used for remapping or
enabling/disabling an automatic reset or low speed watchdog. For more details please refer
to the product datasheet.
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