TMS320F28035: 20MHZ External Oscillator Problem

The crystal oscillator is "on" by default. i.e. you should see the oscillations right after power-up. Is it that the oscillations are absent upon power-up or is it that the oscillations are present, but upon switching to the XTAL OSC, you detect a missing-clock condition?

When it doesn’t work, do you see any oscillations in the X1/X2 pins? Note that the very act of scoping these pins could alter the dynamics of the circuit (unless you use a low-capacitance FET probe)

Could you please share the schematics/layout of the circuitry around the resonator?

Would you be able to put a number on the occurrence of failure? i.e. Oscillator doesn’t work x out of y attempts.

Hi Hareesh,

                    Thank you for your rapid response. As you requested this is my application data.

This is our 4 layer PCB top view using the TMS320F28035

A zoom of the Resonator

A view of the top Layer PCB

Screen capture of the Oscilloscope measuring X1 signal

The only way I managed to start with the resonator is first flashing the Micro with a code that uses the internal oscillator. And after that I flash again a new program that uses the external Xtal. But if I power down the board and power up again, it doesn’t start. When the board isn’t working the signal on the X1 is what I showed.

To configure the board I am using the code provided by TI in DSP2803x_SysCtrl.c.

I call XtalOscSel() to configure the external Xtal.

void XtalOscSel (void) {

     EALLOW;

     SysCtrlRegs.CLKCTL.bit.XTALOSCOFF = 0;     // Turn on XTALOSC

     DELAY_US(10000L);                         // 10mS delay to ensure crystal

                                               // oscillator is up and running.

                                               // Adjust as needed.

     SysCtrlRegs.CLKCTL.bit.XCLKINOFF = 1;     // Turn off XCLKIN

     SysCtrlRegs.CLKCTL.bit.OSCCLKSRC2SEL = 0; // Switch to external clock

     SysCtrlRegs.CLKCTL.bit.OSCCLKSRCSEL = 1;   // Switch from INTOSC1 to INTOSC2/ext clk

     SysCtrlRegs.CLKCTL.bit.WDCLKSRCSEL = 0;   // Clock Watchdog off of INTOSC1

     SysCtrlRegs.CLKCTL.bit.INTOSC2OFF = 1;     // Turn off INTOSC2

     SysCtrlRegs.CLKCTL.bit.INTOSC1OFF = 0;     // Leave INTOSC1 on

     EDIS;

}

After that, I call InitPll(DSP28_PLLCR,DSP28_DIVSEL); with

#define DSP28_DIVSEL   2

#define DSP28_PLLCR   6

This is all. Probably with some software Trick the problem could be solved.

Thank you for your help in advance

Regards

Fernando

The fact that it only fails sometimes tells me that s/w is unlikely to be the cause of the problem. You have not answered my question: Would you be able to put a number on the occurrence of failure? i.e. Oscillator doesn’t work x out of y attempts. Please answer the questions below carefully:

1. You are saying even when the system does not work, you still see the oscillations in X1/X2 pins. How did you determine it is "not working"?
2. What does your application code do?
3. Can you flash a simple code, like toggling a GPIO pin, and retry the experiment?
4. Do you see this issue on multiple boards?
5. Are you using the most recent version of c2000ware?