The PIC is properly configured for input. And I also tried checking the PCLK directly on oscilloscope without connecting the PIC but it is still the same.
Tried connecting pull-down resistor, pull-up resistor, series resistor but still the same. Connecting high value of series resistance is actually decreasing the difference between the two levels and making them almost the same.
Low values of pull-down resistors are bringing both the levels down. I need to bring only the low level down and keep the high level same.
Also, how can I remove those spikes?