Crystal question. Help please.

[eurgenca]

Hello.
 I am struggling with one remote control transmitter. The problem is that cpu not running. So I check all supplies to cpu, reset go high and stay high when power is turned on but no oscillation on crystal. On one side of crystal I get 1.6V which is half of feeding of cpu. So I presume the crystal is dead (or not). And here is my question.
Original crystal is SMD 8 Mhz. Can I replace this crystal for test only, with much bigger through hole type or will not work on 1.6V?

Thanks, Seba.

[srb1954]

Hello.
 I am struggling with one remote control transmitter. The problem is that cpu not running. So I check all supplies to cpu, reset go high and stay high when power is turned on but no oscillation on crystal. On one side of crystal I get 1.6V which is half of feeding of cpu. So I presume the crystal is dead (or not). And here is my question.
Original crystal is SMD 8 Mhz. Can I replace this crystal for test only, with much bigger through hole type or will not work on 1.6V?

Thanks, Seba.

What CPU part number are you using? Was the 1.6V reading you got obtained by using a scope or a DVM ?

You have to be careful probing around crystal oscillators as the capacitive and/or resistive loading of a probe can often stop the oscillator.  Directly probing around the crystal oscillator circuitry of a CPU will generally require the use of a low-capacitance FET probe to avoid upsetting the oscillator.

[eurgenca]

Cpu is M430F1491 and I have regular scope probe. I was replace crystal with through hole but is the same problem, no oscillation and 1.6V on one side. So I don't know if this is good substitute or I have problem with CPU.

[srb1954]

Cpu is M430F1491 and I have regular scope probe. I was replace crystal with through hole but is the same problem, no oscillation and 1.6V on one side. So I don't know if this is good substitute or I have problem with CPU.

Try probing the crystal oscillator O/P with a 1k resistor in series with the scope probe tip. This will reduce the effect of the extra capacitive loading introduced by the scope probe and might allow the crystal oscillator to continue operating.

The oscillator in the M430 series devices is designed for very low operating power and will more sensitive than other CPUs to extra loading on the oscillator pins.

[eurgenca]

Hello.
I try to measure frequency with 1k resistor in serie but same result, nothing. I really don't have experience with quartz so can I replace smd crystal with bigger through hole or I must search for smd type.
For testing purpose only. Now I have inside THT cristal and I don't know if cpu is damaged or crystal is incompatible.

[srb1954]

Hello.
I try to measure frequency with 1k resistor in serie but same result, nothing. I really don't have experience with quartz so can I replace smd crystal with bigger through hole or I must search for smd type.
For testing purpose only. Now I have inside THT cristal and I don't know if cpu is damaged or crystal is incompatible.

You can use a THT crystal in place of an SMD type. Just be sure to keep the leads as short as possible and mount it close to the CPU.

As well as the marked frequency the crystal will have a specified load capacitance, which is primarily determined by external capacitors adjacent to the crystal oscillator pins on the MPU. Are there any markings on the crystal being replaced to indicate the load capacitance?  If not, you should chose a crystal specified for 12pF load capacitance and that would likely work well enough.

It is unlikely that the crystal is faulty as they are generally pretty reliable and don't fail unless they have been subject to a very hard knock which can fracture the quartz element.