M
Miguel Gimenez
I'm using a TI MSP430 microcontroller. It has a low power oscillator
suitable for 32768 Hz quartzs that is internally multiplied for CPU usage.
For a frecuency meter application, I'm feeding the 32768 signal to a timer
and counting transitions of a signal between two timer IRQs. The readings
were affected by a "jitter" that, after inspection, was caused by IRQs
firing with an uncertainty of 375 ns over 1 ms.
Looking with the oscilloscope the quartz signal, there are two frequencies:
the nominal (32768) and other about 2500 Hz above and 10 dB lower. Also
appears the difference between the two.
Other brands of quartz show the same behaviour. The quartz is guarded and
the traces are short (about 3 mm).
If I change the 32768 quartz by a 4 MHz one, the jitter dissapears (down to
3 ns) and the readings are OK, but the CPU draws a lot more power and the
lythium battery life is too short.
Please, tell me how can I eliminate this second frequency. I have read about
"spurious modes" in quartzs, but I don't know if this is the case, and it's
strange that various brands show the same behaviour.
Best regards
Miguel Giménez
suitable for 32768 Hz quartzs that is internally multiplied for CPU usage.
For a frecuency meter application, I'm feeding the 32768 signal to a timer
and counting transitions of a signal between two timer IRQs. The readings
were affected by a "jitter" that, after inspection, was caused by IRQs
firing with an uncertainty of 375 ns over 1 ms.
Looking with the oscilloscope the quartz signal, there are two frequencies:
the nominal (32768) and other about 2500 Hz above and 10 dB lower. Also
appears the difference between the two.
Other brands of quartz show the same behaviour. The quartz is guarded and
the traces are short (about 3 mm).
If I change the 32768 quartz by a 4 MHz one, the jitter dissapears (down to
3 ns) and the readings are OK, but the CPU draws a lot more power and the
lythium battery life is too short.
Please, tell me how can I eliminate this second frequency. I have read about
"spurious modes" in quartzs, but I don't know if this is the case, and it's
strange that various brands show the same behaviour.
Best regards
Miguel Giménez