[ANSWERED] RTC: how important is the capacitance of the crystal?

[JimboZA]

Hi gang....

I went to get a DS1307 RTC and the required 32.768kHz 12.5pF crystal.

They didn't have a 1307, so I took a 1302 instead.... I know the interfacing is different and have found a library for it.

BUT.... only when I got home did I realise that the 1302 requires a 32.768kHz crystal of 6pF, but I had purchased one of 12.5pF.

Is that likely to be a problem? (Confession: I don't even know what it means for a crystal to have a capacitance, although I do know what a capacitor is...   )

TIA,

Jim

[CrossRoads]

DS1302 refers to this Application Note:
Application Note 58: Crystal Considerations for Dallas Real-Time Clocks
found here:
http://www.maximintegrated.com/app-notes/index.mvp/id/58

Table 2 & 3 list crystal suppliers - the vast majority are 12.5pF.
Only 1 is listed with CL of 6pF.

"Fast Clocks
The following are the most common scenarios that cause a crystal-based RTC to run fast.
Noise coupling into the crystal from adjacent signals. This problem has been extensively covered above. Noise coupling usually causes an RTC to be grossly inaccurate.
Wrong crystal. An RTC typically runs fast if a crystal with a specified load capacitance (CL) greater than the RTC-specified load capacitance is used. The severity of the inaccuracy is dependent on the value of the CL. For example, using a crystal with a CL of 12pF on an RTC designed with a 6pF CL causes the RTC to be about 3 to 4 minutes per month fast."

Can you live with 3-4 minutes fast/month?

[Riva]

The correct frequency crystal is important for an RTC as just a slight difference in frequency will effect the clocks accuracy.
Temperature will effect the crystals speed so you may get variable time drift throughout the seasons.
I bought a DS1307 module and it looses approximately 7 seconds per day so ever since then I use DS3231 instead. They are a bit more expensive but have internal oscillator and temperature compensation so maintain time a lot better (less than a second per day)

[JimboZA]

Can you live with 3-4 minutes fast/month?

Long as I'm not flying to Mars and hit the TurboBoost button at the wrong moment  

Thanks for that CR and Tc ... question answered

[pwillard]

According to Dallas' app note, it will cause the clock to run 3-4 minutes a month fast:

My personal experience confirms this.

[pwillard]

Oddly enough... my experience was identical... I ordered the right part as well.