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Original question:

RM48L952: Crystal circuit recommendations?

RM48L952: Crystal circuit recommendations - follow up

August Hultman
Prodigy 65 points

Part Number: RM48L952

Hi,

As the previous thread got old and locked and the issues remains, I started a new thread.

Here is the old one: e2e.ti.com/.../rm48l952-crystal-circuit-recommendations

We did have an error in the calculation initially, but it had very small impact on the result.

Measuring the RMS current through the crystal gave us the following results for different series resistances:

0Ω showed 4.3mA and 740µW.
1.3kΩ showed 3.1mA and 384µW.
3.3kΩ showed 2mA and 160µW.

The series resistance is placed on OSCOUT from the MCU:


Looking at ECLK output, we can see the following for different series resistances:

  • 0 Ohm: approx 45% duty cycle - 28 ns pulse
  • 5k: approx 38% duty cycle - 24 ns pulse
  • 18k: approx 9% duty cycle - 6 ns pulse
  • 33k: doesn't start
  • 75k: doesn't start

According to TI datasheet, 6ns is a limit for oscillator square wave input, but there are no limits for crystal operation.

Looking at ST devices datasheets, there are safety factor calculations for the crystal operation:

We are still looking for TI's recommendation on this topic. What is the limit for our device?

And, do you have any recommendations for specific crystals that are approved?

Best regards,

August Hultman

  • 0
    TI__Guru**** 199386 points

    Hi August,

    I agree with your calculation. The crystal ESR and the series resistor (R_Ser) affect the crystal start-up time. The higher the two values, the longer the crystal takes to start up. 

    The maximum drive level or power dissipation of 1mw specified in the ABLS Series crystal data sheet is usually the maximum the crystal can dissipate without damage or significant reduction in operating life. 

    For cases where the actual drive level or power dissipation exceeds the maximum drive level (>1mw) of the crystal, a series resistor (R_ser) can be installed to limit the current and reduce the power dissipated by the crystal. Bear in mind that R_ser reduces the circuit gain.

  • 0 in reply to QJ Wang
    Prodigy 65 points

    Hi,

    Yes, this is no new information so far.

    We need to get the crystal drive level down to around 100µW, according to discussion with Abracon, to achieve 20 year lifetime of the crystal.

    We need the following data from you:

    1. Transconductance (g_m) of the RM48L952 oscillator

    2. Acceptable gain margin for the RM48L952 oscillator

    Best regards,

    August Hultman

  • 0 in reply to August Hultman
    TI__Guru**** 199386 points

    Those information are not in the device datasheet. 

    Your calculation shows that the maximum power consumption is 740uw (when R_ser=0) which is below the maximum drive level of the crystal (1mW). This calculation is based on the maximum crystal ESR (40ohms) listed in crystal datasheet, and the typical power dissipation is 370mW (assume typical ESR=20ohms). 

    If a series resistor or damping resistor is required, it should be as small as possible because the size of the resistance affects start-up time: the smaller the damping resistor is, the faster the crystal starts up.

    The maximum drive level of RM48 crystal oscillator is 1mW. I could not find the info regarding the transconductance (g_m) and gain margin for the RM48L952 oscillator. 

  • 0 in reply to QJ Wang
    Prodigy 65 points

    Hi,

    Yes, it is below the maximum drive level, but we need to reach typical drive level (100µW) to achieve 20 year lifetime of the crystal. We have this written from Abracon.

    We know that series resistance affects start-up time but we need to find how much and where the limit is.

    As of now this MCU looks unusable with this crystal.

    If no-one at TI knows the g_m and gain margin of the oscillator in the RM48L952, maybe there is a list of acceptable crystals at least?

  • 0 in reply to August Hultman
    TI__Guru**** 199386 points

    No, we don't have crystal recommendation for Hercules MCUs.