Voltage on I/O pin without VDD !

[jps]

Posted on May 10, 2005 at 12:39

Voltage on I/O pin without VDD !

[jps]

Posted on May 10, 2005 at 09:10

Hi,

I'm using a ST7Lite09 to manage a battery backup system.

What happend if I get voltage on I/O pin (battery voltage measure for example) when the ST7 is not running (no voltage between VSS and VDD) ?

Is the ST7 supporting this case or is it mandatory to be supplied on VDD before applyng voltage on any I/O pin.

Thank you for your answers

Whitebull

[alok2]

Posted on May 10, 2005 at 11:43

I think this configuration is not recommended. As per Datasheet the Vin cannot be greater than Vdd+0.3.

But if you still connect, the ESD Diode inside the IO will form a path and it may start the micro for some time. This will cause all device current to flow through the diode and burn the device.

The configuration you are using can probably be used on true open drain pins.

[jps]

Posted on May 10, 2005 at 12:39

Thank you Alok for your answer,

I'm not sure that in my case I will burn the protective devices because the voltage applied can't be more than 5V1 (zener diodes clamping the inputs) and a resistor of 1K is in serial with the inputs (there are analog input PB0 and PB1). All other pins are protected from voltage injection without VDD.

In any case, this problem is not supposed to occur ! It's just in case my voltage regulator circuit (complicated because of 0 consumption for battery autonomy!) have failed !

Regards

Whitebull