System time jump problem and AM335x Advisory 1.0.30

The oscillator input buffer has a switching threshold voltage where the output will change states as the voltage applied to the input crosses the threshold.  The voltage applied to the oscillator input buffer is a slow changing sinusoidal signal when using a crystal circuit as the reference clock.  Noise coupled to this slow slew rate signal may cause the output of the oscillator input buffer to toggle several times as the signal slowly crosses the switching threshold.  If this occurs, the internal clock signal will think these unexpected toggles are real clock toggles.  However, these unexpected toggles are much higher in frequency than the real clock toggles.  These much faster toggles will violate internal circuit timing and in some cases may cause the circuits to perform unexpected operations.  For example, portions of the RTC counter circuit may see these fast toggles and respond properly while other portions of the RTC counter circuit do not see the fast toggles.  In such a case, the RTC counter may not function as expected and do very unpredictable things like counting backward.

If this happens, any internal circuits being clocked from the oscillator output may perform unexpected operations.  In most cases the DPLLs acts like a filter and circuits being clock from the DPLLs never see these unexpected toggles.  We saw one case of a DPPL performing an unexpected operation while debugging this issue, but as mention above any circuit being clocked from the oscillator could perform unexpected operations when excess noise is coupled to the oscillator input signal.

This issue is completely eliminated if you use a fast slew rate clock source like you get from an LVCMOS clock source.  It is also much harder to cause this issue on the Master oscillator vs the RTC oscillator because the slew rate of the 19.2 MHz - 26 MHz oscillator is much faster than the 32.768 KHz oscillator.

Regards,
Paul