AM335X External RTC crystal doesn't oscillate.

The oscillator can be enabled or disabled by using the OSC32K_GZ bit of the RTC_OSC_REG register and the source of the 32-KHz clock is selected using the SEL_32KCLK_SRC bit of the RTC_OSC_REG register.

I'm not sure what the EN_32KCLK bit of the RTC_OSC_REG register controls.  So I will try to determine its purpose tomorrow and let you know if it needs to be set high.

The register values you provided seem to indicate you have the oscillator enabled but not selected as the source.  You also have the internal feedback resistor enabled and you have an external feedback resistor shown in your schematic.  You should remove R383 or disable the internal feedback resistor.

Have you confirmed the oscillator is not oscillating using a high impedance scope probe or assume it is not oscillating because your RTC is not working as expected?  The oscillator may not operate properly if you connect a low impedance scope probe, less than 1M ohm, to the crystal circuit.

I also encourage you to read Advisory 1.0.30 in the AM335x Silicon Errata.

Regards,
Paul

I was told the EN_32KCLK bit of the RTC_OSC_REG register controls a clock gate that sits behind the clock multiplexer controlled by the SEL_32KCLK_SRC bit of the RTC_OSC_REG register.  So the EN_32KCLK bit should also be set high to enable the clock gate.

Regards,
Paul

Hi, Paul,

We found that when selecting external 32K crystal and enabling internal feedback, we have to remove the external 1M resistor to make the RTC working.

But if we set to use external 32K crystal and enable external feedback, no matter the external 1M resistor is removed or not, the RTC can't oscillate. Is possible that 1M resistor value is not suitable?

Thanks,

In my last post, I recommended you remove the external resistor and use the internal resistor.

Is there a reason you are trying to use an external resistor?

I will send the oscillator designer an email and ask them to confirm the expected external resistor value.

We would like to review the crystal data sheet.  Can you point me to it by sending the URL?

Regards,
Paul

Hi, Paul,

We can remove the external resistor and use the internal resister to make RTC working. This means the PCA BOM has to be updated. As we have strict hardware design change control, the managers and quality engineer will challenge why already intalled external resistor can't work, and if there is some potential issue.

So unless we see in some TI documentation saying that external resistor can't work, we will not close our design case. That why I'm continuing to ask you for help on this.

The crystal we are using is ABS13-32.768KHZ-T from Abracon.http://www.abracon.com/Resonators/abs13.pdf

Thanks,

The crystal they are using requires a load capacitance of 12.5pf.

The total load capacitance is C_L = [(C₃₁₂*C₃₁₃)/(C₃₁₂+C₃₁₃)] + C_shunt, where C_shunt is the crystal shunt capacitance (C₀) specified by the crystal manufacturer plus any mutual capacitance (C_pkg + C_PCB) seen across the AM335x RTC_XTALIN and RTC_XTALOUT signals.

The crystal data sheet does not specify C_shunt, but this should be verified because this oscillator expect a crystal with C_shunt less than 1.5pf.  I'm not expecting the C_shunt of the crystal to be greater than 1.5pf.  However, if so they should select another crystal with a lower C_shunt.

If we assume C_shunt of the crystal is 0.5pf, a PCB shunt capacitance of 0.5pf, with the AM335x shunt capacitance being less than 0.2pf, the 18pf values used for capacitors C312 and C313 would give a total load capacitance of 10.2pf.  This is less than the load capacitance of 12.5pf specified by the crystal.

They should determine the actual C_shunt of the crystal then size the external load capacitors such the total load capacitance is 12.5pf.

Regards,
Paul

Hi, Paul,

I changed the external load resistgor to 25pF, now the crystal can oscillate.

But I still have one question, in the datasheet of the AM335X, Rev.F, Table 4-5. OSC1 Crystal Circuit Requirements. It says the load capacitor is from 12pF to 24pF. But in the Beagle board and the AM335X EVM board, the load capacitor is 25pF, is this inconsistence negligeable?

Thanks,

Peng,

The maximum shunt capacitance for the AM335x 32.768kHz oscillator is 1.5pf. The AM335x ZCE package contributes 0.17pf of shunt capacitance. The crystal data sheets I found, shows a shunt capacitance range of 0.9 - 1.2 pf. If you use the worst case value, the PCB shunt capacitance would need to be less than 0.13pf (1.5 - 1.2 - 0.17). Assuming the total shunt capacitance was 1.5pf, the value of C1 and C2 would need to be 22pf [(12.5 - 1.5) * 2] to provide the correct load for the crystal.

You may need to select another crystal, with a lower shunt capacitance, if you cannot design your PCB to have less than 0.13pf of shunt capacitance.

Regards,
Paul