AN2595 - Application note

Filtering / decoupling

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3.3

Filtering / EMC

As ST10F27xZx 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 ST10F27xZx 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

3.4

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)

2.

Unused pin configured as output (by user software)

The software sets the corresponding bit of the Direction Port Registers (DPPx) to

0, default after reset). In this condition, the pad can be left unconnected on the board.

Note:

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 startup

configuration under Reset selects a 16-bit data bus.

C

L

L

+5 V plane

GND plane

Filter connected to supply line of the CPU

ST10F269

C