Strange behaviour - Oscillator, PIC and COMMON GROUND

Thread Starter


Joined Dec 13, 2011

Let me begin by explaining what I have: LC Oscillator , PIC , UART.

The LC Oscillator oscillates at a frequency (120 KHz in my case) --> This frequency is converted to a digital wave for PIC to read ---> PIC reads the rising edges of the digital wave in a fixed time ( Frequency) ---> This value is sent to my laptop.

Ok now to the strange part, previously I had no common ground b/w the LC oscillator ( which is on a breadboard right now) and my PIC ( on a PICDEM2 PLUS board). I use to read random values , much lower my current frequency (120Khz).

Today ( infact just now) I added a common ground b/w the LC oscillator ( on breadboard) and the PIC by connecting the Vss of the PIC to -ve end of the breadboard. The PIC is powered by 9V DC ( AC_DC regulator) and the breadboard is powered by 9V battery (Energizer).

So with the common ground I see '0' on my UART output and without a common ground I get 1500 -1800 values on my laptop!

Why is having a common ground giving me 0s ?

The input I see on PIC hasnt changed after common ground, its still 2.4 V with 120 Khz.....then is UART affected so badly by common ground?

Any idea as to why that is happening?


Joined Jul 5, 2008
They all need common ground Pic, bread board and UART all need to be tied to the ground

You say 9 volts your using regulator to bring this down to 5 volts right?

Only time I have seen this happen it was because of the UART not being grounded to the board with the pic

Thread Starter


Joined Dec 13, 2011
I do have a common ground, Uart is a part of the Microchip board so its om common grounds with the PIC and the circuit