TMS320F28034: External crystal amplitude requirement

Hareesh,

Thank you.

I found it in the new datasheet.

F28004x and F2837x, X1 and X2 are different. They are different. Vddio 3.3V is needed on X1 and X2.

Br, Jordan

Another doubt,

In customer's application, one crystal was chosen, with thermal issue on the crystal.

In order to reduce the crystal temperature, they'd like to use the resistor Rd.

The doubts are:

1) Is it possible to use Rd (increase the resistance value) to reduce the crystal temperature?

2) If yes, when Rd is used, the signal on X1 will be as below. Different Rd, the middle level 1.75V is stable, but the high and low level will vary between 2.2V and 1.3V. Do we need to limit the high level below 1.8V? Or the amplitude should be within 1.8V?

Br, Jordan

Hi Jordan,

Rd is a damping resistor used to reduce the drive level from the electrical oscillator. If you are seeing the crystal getting hot, it does seem to indicate it's being over driven. However, keep in mind that adding Rd will increase the ESR of the circuit which will reduce the overall gain. You can already see in your scope shot that the gain is suffering because the pk-to-pk = 2.2 - 1.3 = 0.9V which is much less than 1.8V. Yes, you are correct, high level should not exceed 1.8V.

I think you should consider switching to a crystal with a higher drive level or use a crystal with a very low ESR so if you add Rd, it doesn't significantly affect the gain.