Voltage on pins of powered off MCU

[_Mark81_]

Hello,
I have a battery-powered board which enables the power of an MCU only when requested.
With this MCU I want to read some analog values such as the battery voltage.

Of course I cannot route the voltage to the input pin directly. When in off state the internal ESD diodes will conduct.

I wonder the best way to handle this, with a simple and cheap circuit. I have 3-4 lines to manage...

Thanks!

 

[shihab_leo]

why do you think the diodes will conduct when VCC is disconnected?

 

[tpetar]

What µC is involved ?


Best regards,
Peter

 

[electroM]

Think of ADC.

 

[_Mark81_]

What µC is involved ?

I'm talking about a ATmega164A.

why do you think the diodes will conduct when VCC is disconnected?

Because Vcc is virtually connected to gnd (due to the power supply capacitors). The ESD diodes are placed between any pin and gnd and between any pin and vcc.
When Vcc is present, both diodes are reverse biased. But then Vcc = gnd any voltage over 0,6V on the I/O pin will forward bias the upper diode. In fact, I measure about 0,65V on those pins...

 

[FvM]

You are of cause correct about substrate ("ESD") diodes. A possible solution for battery powered systems is to set the processor into power down mode instead of remving the power supply. Otherwise some kind of analog switches can be used. It depends on the overall circuit, involved voltage levels and acceptable leakage current.

 

[_Mark81_]

You are of cause correct about substrate ("ESD") diodes. A possible solution for battery powered systems is to set the processor into power down mode instead of remving the power supply. Otherwise some kind of analog switches can be used. It depends on the overall circuit, involved voltage levels and acceptable leakage current.

Yep, the previous prototype was done in that way.
Now I need to completely shutdown the MCU to save even few uA. Before I was wrong: the MCU is not an AVR but a PIC24 (I have both on the board) but it doesn't matter.

Analog switches: the leakage and supply current should be ok (e.g. for standard CD4066 with all 4 inputs connected the total current should be less than a dozen of nA @ 25 °C). The input voltage is max 3.3V.

They come in very tiny package (eg. VQFN-14) and very cheap (0,12 euro @ 100+ pcs for 74HC4066BQ,115). 4-ch so I need just one. Anyway this one requires much more current than the CD4066... I will look for a good compromise.

 

[shihab_leo]

Because Vcc is virtually connected to gnd (due to the power supply capacitors).

sorry I neglected power supply and decoupling capacitors