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RTC clock correlation

Todd Anderson78572
Guru 14640 points

All:

F552x family...

Since RTC is based on ACLK, there appears to be a correlation between ACLK output (P1.0 on F5529) and RTC output (P2.6 on F5529).

I was able to measure ACLK as 32766.7 Hz using XT1 crystal and

                                                           32970.4 Hz using REFO.

I then measured RTC output (set to 512 Hz) and measured 511.979 Hz with XT1 crystal supplying ACLK and

                                                                515.2 Hz with REF supplying ACLK.

I know this seems pretty obvious, but can someone confirm this?

One observation - the XT1 crystal appears to be a very stable frequency, where the REFO output as ACLK does have quite a bit of variation.

  • 0
    TI__Guru* 77991 points

    Hi Todd,

    Your results look as expected to me. The F552x device has the RTC_A module. Looking at the user's guide www.ti.com/lit/pdf/slau208 in the RTC_A registers section, you can see that this RTC is supplied either by ACLK or SMCLK (or RT1PS which is just a scaled version of either ACLK or SMCLK depending on your settings). So by default, RTC_A is supplied by ACLK so it is going to follow whatever clock source you've set for ACLK.

    For ACLK sources: XT1 should be very accurate and stable as long as you have a good crystal circuit design, as you saw. The REFO however, if you look in the datasheet, is less accurate than a typical 32kHz crystal. So I think your results look expected.

    As a note, if you know that your clock source tends to run slow or fast compared to ideal 32768Hz by a certain amount, you can use the RTC calibration registers to help compensate for this. You won't be able to see the calibration on the 512Hz output b/c it is applied later in the system than where that comes off - see the user's guide note:

    "

    The 512-Hz and 256-Hz output frequencies observed at the RTCCLK pin are not affected by changes in the calibration settings since these output frequencies are generated prior to the calibration logic. The 1-Hz output frequency is affected by changes in the calibration settings. Because the frequency change is small and infrequent over a very long time interval, it can be difficult to observe.

    "

    Hope this is clear.

    -Katie

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