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TMS320F28377DZWTQ crystal drive level requirement

Fatih SADIC
Prodigy 10 points

Hi.

In the 64th page of TMS320F28377DZWTQ datasheet, there is a chapter about using crystal resonators for clock circuit.

5.7.3.4 Crystal Oscillator

When using a quartz crystal, it may be necessary to include a damping resistor (RD) in the crystal circuit to

prevent over-driving the crystal (drive level can be found in the crystal data sheet). In higher-frequency

applications (10 MHz or greater), RD is generally not required. If a damping resistor is required, RD should

be as small as possible because the size of the resistance affects start-up time (smaller RD = faster startup

time). It is recommended that the crystal manufacturer characterize the crystal with the application

board.

Table 5-17. Crystal Oscillator Electrical Characteristics

PARAMETER TEST CONDITIONS MIN TYP MAX UNIT

f = 20 MHz

Start-up time(1) ESR MAX = 50 Ω 2 ms CL1 = CL2 = 24 pF

C0 = 7 pF

Crystal drive level (DL) 1 mW

The crystal connection which is used in development board can be seen below. Is R28 resistor used as a damping resistor? When we checked the drive level of crystal, which is used in development board, we see that drive level is typical 10uW, max 100uW. But in F28377D datasheet drive level is given as 1mW.

 

We will use a crystal from same manufacturer as in the development board. But we will use 16MHz version, drive level values are the same.

 

  1. Can we face any problems if we use 10uW drive level crystal?
  2. If R28 resistor is used as a damping resistor, how is its value determined?

Thanks.

  • 0
    TI__Expert 7885 points
    Hi Fatih,

    If a crystal is over-driven it is possible that the aging will be accelerated. R28 is not a damping resistor. RD is shown in figure 5-8 in the datasheet.

    We have not seen any specific issues with using the crystal in the controlCARD schematic. If you are concerned about over-driving the crystal you can take current measurements to determine the drive strength and select an appropriate RD for the circuit (in most cases it is not necessary to add RD to the circuit).

    TXC (crystal on controlCARD) mentions on their site that in common circuits RD is not necessary. http://txccrystal.com/faq.html

    In any case, TI recommends that customers have the resonator/crystal vendor characterize the operation of their device with the application board and MCU. The resonator/crystal vendor has the equipment and expertise to tune the tank circuit. The vendor can also advise the customer regarding the proper tank component values that will produce proper start up and stability over the entire operating range.

    Best Regards,
    Adam Dunhoft