voltage calculation - circuit analysis

[electronic_satya]

Hi,
Can somebody please explain the circuit for the voltage that appears at the microcontroller?
My specific doubts
1.If the frequency signal is not available what will be the voltage at the microcontroller input?
2.If the frequency signal is available for voltage ranges
a.From 0v to 5V what will be the voltage at the microcontroller input?
b.For > 5V what will be the voltage at the input?
3.Will microcontroller input will add any resistance to the circuit. Does that need to be considered?

I could only analyse the following things.

If the input signal is not available (frequency signal) then the Diode D1 is forward biased, the current signal has the path to route towards the frequency input, but since the input is open the current does not go through this path and routes towards the microcontroller. But I failed to figure out how the effective resistance would fall either parallel or series or both. Could somebody explain me this? From the data sheet of the zener diode I found that it Min Zener voltage is 4.8V and Max Zener voltage is 5.4V. When it will come into picture?

waiting for your reply,

thanks in advance,
regards,
satya

 

[ckshivaram]

Hi,

do you mean the crystal frequency??

if the crystal frequency is not there then also all port pins will be active high state at power on due to bootloader action.
if the crystal oscillator frequency is there then you can see around 2.3V at the pins where crystal is connected.

 

[electronic_satya]

i have attached the circuit which is part of the complete schematic. crystal frequency and other required items are available.

thanks and regards,
satya

 

[emresel]

In the schematics typical for the zener seems as 5.1V. It means as your voltage above the 5.1V mcu should see the 5.1V at its port. But should be checked if 10K is enough to drive D2 to zener region.