AN2353 Datasheet, PDF (11/18 Pages) STMicroelectronics – Designing an application with the ST10F27x devices

AN2353
Filtering / decoupling
3.3
Filtering / EMC
As ST10F27x has an internal voltage regulator, +5V pins are not differentiated between
output-buffers and internal logic. This means that when EMC filters are used to isolate the
CPU from the supply, all ST10F27x supply pins should be isolated with the filter(s).
Example of filter
Filter placed on supply line of the CPU to remove the conducted noise from the supply line
of the module (as illustrated in Figure 5).
Figure 5. Filter based on discrete components
+5V plane
GND plane
ST10F269
L
C
L
V_DD pins
C
V_SS pins
Filter connected to supply line of the CPU
3.4
Note:
Unused general purpose pins
Unused pins may be an additional source of noise if not properly connected.
Used pins must NOT be left floating.
Two configurations are possible:
1. Unused pins configured as input (default after reset)
Connect the unused pin to V_SS (directly or via a pull-down resistor)
2. Unused pin configured as output (by user software)
The software sets the corresponding bit of the Direction Port Registers (DPPx) to
configure an unused pin as an output (set direction bit to 1) with level to V_SS (data bit to
0, default after reset). In this condition, the pad can be left unconnected on the board.
Avoiding floating pins in an application also ensures a good control on power consumption.
● Special handling for pins of Port 0 and Port 6
On the pins of Port 0 and Port 6, internal pull-ups are present (under Reset only). Our
recommendation is: to keep them open and configured by software as output (set direction
bit to 1) with output = 0 (data = 0).
● Special handling for pin P3.12 and BHE functionality
The pin P3.12 can be used as the BHE (Byte High Enable) signal for the external memory
interface. The BHE alternate function is automatically enabled when the start-up
configuration under Reset selects a 16-bit data bus.
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