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TMS320F28335: F28335 stuck in LIMP MODE. Please help.

Peter Adamcik
Expert 1425 points

Part Number: TMS320F28335
Other Parts Discussed in Thread: C2000WARE

Hi Guys. I've made my own breakout board for my F28335 176PIN. My F28335 won't go out of LIMP Mode. I've checked the power rails 3.3V for I/O and 2.1V for CPU. 

I have a 20Mhz crystal connected to pins 104 X1 and 102 X2 the other side connected to 15pf ceramic capacitors to GND.

CCS9 downloads my Assembler code to RAM fine with no errors. I have connected a LCD to PORTB so I can read values of the internal F28335 registers. LCD is working fine as well.

I then realised that the F28335 is running very slowly. I read the PLLSTS register in Assembler ( I'm an Assembler guy ) and it tells me LIMP mode is on. I read the XCLKOUT pin 138 with my scope and it says 1.00Mhz speed.

I would have though connecting the 20Mhz crystal would make it run at 20Mhz / 4 = 5Mhz but no, it's only running at 1Mhz.

Can someone please let me know what I missed in setting up the F28335 properly.

Thanks guys, hope to hear from you soon.

Peter

  • 0
    TI__Guru* 95315 points

    Peter,

    I've checked the power rails 3.3V for I/O and 2.1V for CPU. 

    You cannot exceed 1.995v for Vdd. See page 32 of SPRS439O. This is not the reason for your issue, but wanted to highlight.

    I have a 20Mhz crystal connected to pins 104 X1 and 102 X2 the other side connected to 15pf ceramic capacitors to GND.

    Have you probed the X1/X2 pins to ensure the circuit is actually oscillating? You need to use an active FET probe (<2 pF capacitance), lest the very act of probing impact the oscillations. It is OK to use a normal probe for a gross check.

    CCS9 downloads my Assembler code to RAM fine with no errors. I have connected a LCD to PORTB so I can read values of the internal F28335 registers. LCD is working fine as well.

    OK, this tells me that the JTAG debug chain is alive and well.

    I then realised that the F28335 is running very slowly. I read the PLLSTS register in Assembler ( I'm an Assembler guy ) and it tells me LIMP mode is on. I read the XCLKOUT pin 138 with my scope and it says 1.00Mhz speed.

    Could you please run an example from C2000ware? I recommend:

    C:\ti\c2000\C2000Ware_3_03_00_00\device_support\f2833x\examples\gpio_toggle

    C:\ti\c2000\C2000Ware_3_03_00_00\device_support\f2833x\examples\cpu_timer

  • 0 in reply to Hareesh Janakiraman
    Expert 1425 points

    Hi Hareesh, Thank you for your reply. I fixed the voltage to 1.9V and now my F28335 is running at 2.50Mhz. I have a 20Mhz crystal across X1 and X2 Pins.

    So LIMP mode is now off. Thanks for your help, I totally appreciate it. Now to get some help on setting up the PLL to 150Mhz.

    Have a great day,

    Peter