I have a ipod dock with a clock some may know it.. it's the Xtreme Mac Luna.. anyways I've had it about a year now.. but one of the things I noticed is the time keeping on this thing sucks... I was late to work one day because of it.. In about a months time the time will be about 5 min off.. I set the clock about a week ago and it's already about 1 minute off now.
So what I saw inside when I opened it is a 10khz crystal and the 32.768 KHz crystal.
I don't have a oscope to test it but the simple thing to do is replace the crystal?
I was doing some reading online and I found this site about RTC selection
**broken link removed**
"Crystal Selection
There are two types of crystals used for RTCs: 6pF load and 12.5pF load watch crystals. Generally, an RTC that uses a 12.5pF crystal has a timekeeping current of 1.7X more than an RTC that uses a 6pF crystal (that is, 6pF RTC current = 300nA @2V whereas 12.5pF RTC current = 500nA@2V). Timekeeping current is that measured when there is no serial bus activity and the RTC is only using current to run its 32.768kHz oscillator and count real time. A standard, low-cost, 50ma-hr, lithium, backup battery powers an RTC with timekeeping current of less than 570nA for up to 10 years.
A 12.5pF load crystal oscillator is somewhat more stable and less susceptible to noise and PCB layout stray capacitance than a 6pF load crystal oscillator. This is partially due to the capacitance from each crystal pin to ground, internal to the RTC, which is 25pF per pin for the 12.5pF crystal RTC and 12pF for the 6pF crystal RTC.
There are also inventory issues for 32.768kHz watch crystals. Load crystals at 12.5pF are readily available through many distributors. In contrast, load crystals at 6pF are not as readily available and can require a minimum-quantity order to be purchased. Check with your local supplier for availability and price.
It's important to use the correct specified crystal with an RTC, because the wrong crystal can cause as much as a 100ppm error in the 32.768kHz oscillator frequency, which translates to a 4.3-minute error over a one-month period. In addition, the wrong crystal can cause excessive timekeeping current or failure of the oscillator to start properly."
hmm 4.3 minute error over a one month period.. sounds almost like my issue..
So what I saw inside when I opened it is a 10khz crystal and the 32.768 KHz crystal.
I don't have a oscope to test it but the simple thing to do is replace the crystal?
I was doing some reading online and I found this site about RTC selection
**broken link removed**
"Crystal Selection
There are two types of crystals used for RTCs: 6pF load and 12.5pF load watch crystals. Generally, an RTC that uses a 12.5pF crystal has a timekeeping current of 1.7X more than an RTC that uses a 6pF crystal (that is, 6pF RTC current = 300nA @2V whereas 12.5pF RTC current = 500nA@2V). Timekeeping current is that measured when there is no serial bus activity and the RTC is only using current to run its 32.768kHz oscillator and count real time. A standard, low-cost, 50ma-hr, lithium, backup battery powers an RTC with timekeeping current of less than 570nA for up to 10 years.
A 12.5pF load crystal oscillator is somewhat more stable and less susceptible to noise and PCB layout stray capacitance than a 6pF load crystal oscillator. This is partially due to the capacitance from each crystal pin to ground, internal to the RTC, which is 25pF per pin for the 12.5pF crystal RTC and 12pF for the 6pF crystal RTC.
There are also inventory issues for 32.768kHz watch crystals. Load crystals at 12.5pF are readily available through many distributors. In contrast, load crystals at 6pF are not as readily available and can require a minimum-quantity order to be purchased. Check with your local supplier for availability and price.
It's important to use the correct specified crystal with an RTC, because the wrong crystal can cause as much as a 100ppm error in the 32.768kHz oscillator frequency, which translates to a 4.3-minute error over a one-month period. In addition, the wrong crystal can cause excessive timekeeping current or failure of the oscillator to start properly."
hmm 4.3 minute error over a one month period.. sounds almost like my issue..
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