crystal oscillator tester ckt

[mhtavallaie]

hello dear friends

I am working on a faulty board which has a 8085 microprocessor and 2 parrallel ports 8255 and 8155.

the crystal connected to 8085 (6.144Mhz) deos not oscillate.

can anybody help me to find the reason and is there a circuit I can Check the crystal?

rgds
hadi

 

[Nigel Goodwin]

For a start, you MUST use a x10 probe to check the oscillator, a scope with a x1 probe usually stops the oscillator when you apply it.

The next thing to check is the reset circuitry, a fairly common problem is the reset line been held in reset - this obviously prevents the chip oscillating.

 

[Russlk]

Crystals are quite reliable and don't go bad unless they are overdriven, overheated, dropped or broken. Since this circuit probably was working at one time, I would suspect other components.

 

[mhtavallaie]

crystal oscillator tester ckt

Dear sir

with many thanks for yr advise , as per my request do you have a seperate circuit by which the suspected crystal can be tested?

rgds
hadi

 

[Nigel Goodwin]

Re: crystal oscillator tester ckt

Dear sir

with many thanks for yr advise , as per my request do you have a seperate circuit by which the suspected crystal can be tested?

Just do a web search for 'crystal oscillator', there are many different designs you could use. However, even if it works in a particular circuit, it's no guarantee that it will work with the oscillator in your microcontroller!.

Easiest option (and cheapest?), assuming you've done everything in my previous post?, is to simply try replacing the crystal!.

 

[Russlk]

I have used this circuit, crystals from 1 mHz to 150 mHz will oscillate in it altho overtone crystals tend to oscillate at the base frequency. I used 2N918 transistor instead of 2N2222.