ST92F120JR9T datasheet(13/320 Pages) STMICROELECTRONICS

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ST92F120 - GENERAL DESCRIPTION

1.2.4.3 Internal Circuitry: Digital I/O pin

In Figure 4 a schematic representation of an ST9

pin able to operate either as an input or as an out-

put 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 possi-

ble to disable the output buffer when the input sec-

tion is used, the MOS transistors of the buffer itself

can still affect the behaviour of the pin when ex-

posed to illegal conditions. In fact, the P-channel

transistor 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-diffu-

sion 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 oper-

ating modes, while acting during exposure to over

limit conditions, avoiding permanent damage to

the logic circuitry.

All I/O pins can generally 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 ST9 power supply is turned off): this con-

dition 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 at-

tention 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 conten-

tions.

Figure 5. Digital Input/Output - True Open Drain Output

OUTPUT

BUFFER

INPUT

BUFFER

ESDPROTECTION

CIRCUITRY

PORT CIRCUITRY

I/OCIRCUITRY

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