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RTC on Am3359

Calum Mackinnon
Genius 4155 points


Hi,

My RTC is running so fast from the internal 32KHz oscillator? It appears running the RTC from the internal source makes it run about 3x as fast. Doesn't keep good time at all. Unfortunately I didn't include the external source in my design so I need this to work.

I've seen the Advisory 1.0.5 RTC: 32.768-kHZ Clock is Gating Off - But I'm not sure if this is causing my issue or not - any thoughts?

 

  • 0
    TI__Guru**** 393215 points
    Hi Callum,
     
    Advisory 1.0.5 deals with the case when "the internal 32.768-kHz clock stops when the VDD_CORE voltage domain drops below the recommended operating range or the PWRONRSTn input terminal is held low". This would rather cause the RTC to lose time, rather than speed up. Perhaps your case is linked with Peripheral PLL clock frequency (TRM section 20.3.2.2 RTC Clock and Reset Management). Your clock is probably PER_CLKOUTM2 / 5859.3752 (Table 20-56). You could check the PLL settings.
  • 0 in reply to Biser Gatchev-XID
    TI__Guru 63155 points

    AM335x has two internal 32kHz clock sources.

    CLK_32K_RTC is sourced by the RTC oscillator which can receive an external LVCMOS clock source or self-generate a clock when connected to the recommended crystal circuit.   This clock source is as accurate as the external components used to create or source the clcok.

    CLK_RC32K is an internal RC generated clock.  The frequency of this clock is not accurate since it is based on internal RC components that vary based on process, voltage , and temperature conditions.  The TRM mentions this clock ranges from 16kHz to 60kHz. 

    If you need an accurate clock you must use CLK_32K_RTC which requires external components connected to the RTC oscillator.

    Regards,
    Paul