AN1709 datasheet(24/38 Pages) STMICROELECTRONICS

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EMC DESIGN GUIDE FOR ST MICROCONTROLLERS

Figure 15. Digital Input/Output - True Open Drain Output

In Figure 15. a true open-drain pin schematic is shown. In this case all paths to VDD are re-

moved (P-channel driver, ESD protection diode, internal weak pull-up) in order to allow the

system to turn off the power supply of the microcontroller and keep the voltage level at the pin

high without injecting current in the device. This is a typical condition which can occur when

several devices interface a serial bus: if one device is not involved in the communication, it can

be disabled by turning off its power supply to reduce the system current consumption.

When an illegal negative voltage level is applied to the Microcontroller I/O pins (both versions,

push-pull and true open-drain output) the clamp diode is always present and active (see ESD

protection circuitry and N-channel driver).

OUTPUT

BUFFER

INPUT

BUFFER

ESD PROTECTION

CIRCUITRY

PORT CIRCUITRY

I/O CIRCUITRY

AN1709 Datasheet(PDF) 24 Page - STMicroelectronics