RTC time lag

[Karthik rajagopal]

Hi all,
I was recently working with BQ32002 RTC IC from TI. Everything works fine except it lags by a seconds for every 24hrs. I had the same problem with ds1307 ic which had a lag of 5 to 6 seconds per day.
I am using an SMPS power supply to power my clock. So will the noise in the supply interfere with the oscillations of the tuning fork crystal?

I have done the following from my side:

1)Since the datasheet mentions an internal load capacitance, I didn't connect any external capacitors to the crystal. So, the oscillations must be pretty stable.

2) I have soldered them on a pcb instead of a general purpose board to prevent stray capacitance from affecting its oscillations.

What could be the possible reason for the lag and how to prevent that?

Thank in advance.

 

[Harald Kapp]

What could be the possible reason for the lag and how to prevent that?

The quality (accuracy) of the crystal is the main influencing factor. The BQ32002 can be adjusted to compensate the error of the quartz. See page 16 of the datasheet.

 

[Karthik rajagopal]

The quality (accuracy) of the crystal is the main influencing factor. The BQ32002 can be adjusted to compensate the error of the quartz. See page 16 of the datasheet.

Thank you for the reply.
I didn't notice the calibration register in the datasheet.
How do we do the calculation for the calibration needed?


So if I have S=1 and set CAL=1, then I must have an increase of 4 microsecond per minute?
Is my understanding correct?

 

[Harald Kapp]

How do we do the calculation for the calibration needed?

There is a calibration output, usually at 512 Hz (but can be set to 1 Hz). You attach a frequency meter to that output and adjust the register such that the 512 Hz are as precise as possible.
Or do it by math: 1 s /24h = 1s /(24 h × 60 m/h × 60 s/m) = 1 / 86400 = 1.57 × 10^-6 = 1.57 ppm
Your clock is 1.57 ppm slow, therefore you need to run the clock faster by that amount.Since you cannot set the calibration to 1.57 ppm, use +2ppm as the nearest value.

Alternatively use a higher precision quartz.

 

[Karthik rajagopal]

There is a calibration output, usually at 512 Hz (but can be set to 1 Hz). You attach a frequency meter to that output and adjust the register such that the 512 Hz are as precise as possible.
Or do it by math: 1 s /24h = 1s /(24 h × 60 m/h × 60 s/m) = 1 / 86400 = 1.57 × 10^-6 = 1.57 ppm
Your clock is 1.57 ppm slow, therefore you need to run the clock faster by that amount.Since you cannot set the calibration to 1.57 ppm, use +2ppm as the nearest value.

Alternatively use a higher precision quartz.

Thank you so much for the reply.
Finally understood the conversion.