AN1709 datasheet(23/38 Pages) STMICROELECTRONICS

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EMC DESIGN GUIDE FOR ST MICROCONTROLLERS

3.1.3.3 Internal Circuitry: Digital I/O pin

Figure 14. shows a schematic representation of an ST Microcontroller pin able to operate ei-

ther as an input or as an output is shown. The circuitry implements a standard input buffer and

a push-pull configuration for the output buffer. It is evident that although it is possible to disable

the output buffer when the input section is used, the MOS transistors of the buffer itself can still

affect the behaviour of the pin when exposed to illegal conditions. In fact, the P-channel tran-

sistor of the output buffer implements a direct diode to VDD (P-diffusion of the drain connected

to the pin and N-well connected to VDD), while the N-channel of the output buffer implements

a diode to VSS (P-substrate connected to VSS and N-diffusion of the drain connected to the

pin). In parallel to these diodes, dedicated circuitry is implemented to protect the logic from

ESD events (MOS, diodes and input series resistor).

The most important characteristic of these extra devices is that they must not disturb normal

operating modes, while acting during exposure to over limit conditions, avoiding permanent

damage to the logic circuitry.

According to the MCU used, some I/O pins can be programmed to work also as open-drain

outputs, by simply writing in the corresponding register of the I/O Port. The gate of the P-

channel of the output buffer is disabled: it is important to highlight that physically the P-channel

transistor is still present, so the diode to VDD works. In some applications it can occur that the

voltage applied to the pin is higher than the VDD value (supposing the external line is kept

high, while the Microcontroller power supply is turned off): this condition will inject current

through the diode, risking permanent damages to the device.

In any case, programming I/O pins as open-drain can help when several pins in the system

are tied to the same point: of course software must pay attention to program only one of them

as output at any time, to avoid output driver contentions; it is advisable to configure these pins

as output open-drain in order to reduce the risk of current contentions.

AN1709 Datenblatt(PDF) 23 Page - STMicroelectronics