TMS320F28335: MCU CLK pin Capcitance

Anudeep PSV

Hello,

As per datasheet www.ti.com/.../tms320f28335-q1.pdf, page no : 176-177

It is recommended to use an oscillator having the typical specifications as shown below

We are planning to use a crystal oscillator with a maximum capacitive load driving capability of 15 pF for the external clock. Based on my understanding, the oscillator requires a minimum load capacitance (CL) of 12 pF. Using a 15 pF load is close to the datasheet’s recommended value.

Could you please confirm whether it is advisable to proceed with a 15 pF load, or if a higher value is recommended? Additionally, what margin should be considered to ensure reliable validation?

Thank you in advance

15 days ago

0 Matt Kukucka 15 days ago

TI__Genius 15280 points

Hello,

Please refer to the crystal vendor's datasheet for load capacitance specifications. The MCU datasheet provides typical recommendations for an external crystal and should not be taken as the true specification for the crystal.

Please also refer to the hardware design guidelines for this device: https://www.ti.com/lit/an/spraas1/spraas1.pdf

Best,
Matt

0 Anudeep PSV 14 days ago in reply to Matt Kukucka

Prodigy 10 points

Hello Matt,

Thank you for your response

Could you please share the Capacitance of XCLK pin?

0 Matt Kukucka 14 days ago in reply to Anudeep PSV

TI__Genius 15280 points

Hello,

You can use 2pF as a typical number for your calculations.

Best,
Matt

0 Anudeep PSV 13 days ago in reply to Matt Kukucka

Prodigy 10 points

Hello Matt,

I have TMS320F28335 in our previous designs (PCB). I would like to test that XCLKIN Pin capacitance, could you please suggest the test procedure to measure the value practically?

Thank you,
Anudeep

0 Matt Kukucka 13 days ago in reply to Anudeep PSV

TI__Genius 15280 points

Hello,

You can measure the input capacitance to the XCLKIN pin with an LCR meter. You can also test the capacitance by driving the input with a square wave voltage and then measuring the rise time of the square wave. Then use the C * dV/dt = I_cap formula to derive the capacitance.

Best,
Matt

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TMS320F28335: MCU CLK pin Capcitance