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TM4C1294NCPDT: MOSC Crystal Worst-Case Drive Level Formula

Phil LaFayette10
Intellectual 605 points

Part Number: TM4C1294NCPDT

Hello,

I'm looking for information related to the formula TI uses to calculate the MOSC Crystal's Worst-Case Drive Level (WC DI)

The datasheet (page1840) states:

The table also includes three columns of Recommended Component Values. These values apply to system board components. C1 and C2 are the values in pico Farads of the load capacitors that should be put on each leg of the crystal pins to ensure oscillation at the correct frequency. Rs is the value in kΩ of a resistor that is placed in series with the crystal between the OSC1 pin and the crystal pin. Rs dissipates some of the power so the Max Dl crystal parameter is not exceeded. Only use the recommended C1 , C2 , and Rs values with the associated crystal part. The values in the table were used in the simulation to ensure crystal startup and to determine the worst case drive level (WC Dl). The value in the WC Dl column should not be greater than the Max Dl Crystal parameter. The WC Dl value can be used to determine if a crystal with similar parameter values but a lower Max Dl value is acceptable.

I'm looking at a crystal with a Max DI of 100µW, and according to the TM4C datasheet (page 1839);

Parameter Parameter Name Min Nom Max Unit
DL Oscillator output drive levelf - OSCPWR - mW

f. OSCPWR = (2 * pi * FP * CL * 2.5)2 * ESR / 2. An estimation of the typical power delivered to the crystal is based on the CL , FP and ESR parameters of the crystal in the circuit as calculated by the OSCPWR equation. Ensure that the value calculated for OSCPWR does not exceed the crystal's drive-level maximum.

When I use formula (f), plugging in values:

FP = 25000000 Hz

CL = 8.0e-12 Farads

ESR = 50 Ω

I get OSCPWR = 246.74µW, which is higher than the manufacturer's listed Max DI of 100µW.

So, I'd like to know what formula TI uses to determine either WC DI (Worst-Case Drive Level) or RS (Series Dissipation Resistor)

  • 0
    TI__Guru 72500 points
    The Rs value comes from the crystal vendor after doing a characterization of the crystal and our microcontroller. If you need to use a crystal that has not been characterized, you will need to request that characterization from the crystal vendor.