Crystal in 16F877 how to know it works or not

[fuyejun]

Hi

I used a 20MHZ crystal for my 16F877 , I test the chip by on and off a LED connect to PINA1, But I found that when I cut the line which connected 2 capacitors 22pF and ground. The LED also function well.
I checked the datasheet 16F877 do not have internal oscillator. This confused me a lot.
Then I want to found weather my crystal work. Can anyone told me how to measure it. I try many time and found it's not work.
I connect 2 prob of digital oscilloscope to 2 pin of crystal, but no signal. Then I try use 1 pin connect to crystal ,the other connect to ground also no signal.
I used Hi_tech compiler, do I need add __CONFIG(); to my code. I fund that this when I don't use this word, the pic16f877 also can work, ADC, or DAC or LCD display all can work.

 

[Tahmid]

Yes, you need to set config bits, but since this PIC doesn't have internal oscillator, external oscillator is set by default, but you should set it for HS. I think it's working. The PIC operates without those 2 capacitors. Even if you don't connect those two capacitors, the PIC will work. However, if you disconnect the crystal, the LED should not work.

 

[dayanpad]

Dear fuyejun,
I think you will help this

 #include <p16f877a.inc>

__CONFIG   _WDT_OFF   &   _HS_OSC

First connect your 4MHz OSC to PIC using oscillator XT and apply proper delay that you can see blink led. Then change your oscillator 20MHz changing __CONFIG bit I shown above using same delay and monitor led operation