OMAP L138 RTC crystal requirement

Hi,

I would like to ask a question on the choice of OMAP L138 RTC crystal.

There could be two crystals used for OMAP L138. (1 ) one is for system PLL  (2) another is for RTC.

• For (1), §5.5 of L138 datasheet SPRS586C gives requirement on ESR and load capacitance.
• For (2), I found NO requirement on ESR and load capacitance in §5.31.1, nor anywhere in SPRUH77 OMAP-L138 C6-Integra DSP+ARM Processor Technical Reference Manual.

In forum post OMAP-L138 RTC Current TI employee tlee gave the 15 µA as the typical RTC current, which is also consistent with  http://processors.wiki.ti.com/index.php/OMAP-L137_Power_Consumption_Summary 's 0.02mW result. Because if we use 1.2V RTC power, and the current is 15µA, then power = 1.2V × 15µA = 18µW ≈ 0.02mW

I also found this typical RTC current seems to have strong relationship with the selection of capacitors.

Both boards used crystal with 65KΩ ESR.

1.2V / 65KΩ = 18.46µA > 15 µA (typical current)

I am not familiar with the physics of oscillator, but I read somewhere that there is also a current limiting resistor inside the oscillator, so perhaps

1.2V / (65KΩ + R_LIMIT) < 18.46µA ≈ 15 µA?

However, what happens if the crystal has larger ESR? I am looking at Abracon ABS06 whose dimensions are 2.0 x 1.2 x 0.6mm, the smallest available in the market. But its datasheet show max ESR = 90KΩ.

1.2V / 90KΩ < 13.3µA < 15µA

If also assuming R_LIMIT exist,

1.2V / (90KΩ+ R_LIMIT) < 13.3µA < 15µA

The actual current will be even smaller.

So for ABS06, the current is going to be smaller than typical 15µA current. Would this cause RTC to fail?

After all, what is the requirement on ESR and load capacitance for RTC crystal?

Zheng