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TMS320F28377S: hardware design quesiton of the external oscillator

Aki Li
Genius 9235 points
Part Number: TMS320F28377S
Other Parts Discussed in Thread: C2000WARE, TMDSCNCD28379D, LAUNCHXL-F28027F, LAUNCHXL-F28379D

My customer found an issue for the circuit of  external oscillator. They design the circuit based on the schematic of F28377S launchpad, and they found only when the 1Mohm resistor is removed, the oscillator can work.

(C:\ti\c2000\C2000Ware_1_00_05_00\boards\LaunchPads\LAUNCHXL_F28377S)

And I found the instruction for the oscillator in TRM, and the circuits shown are different from the one of launchpad, so could you give explanation for this? And why are there 3 kinds of circuits?

One more thing is that I can not find the hard design files for the control card of F28377D in the below file.

C:\ti\c2000\C2000Ware_1_00_05_00\boards\controlCARDs\TMDSCNCD28377D

  • 0
    Guru 30330 points

    Aki,

    1. It looks like you have found something though I am not sure what. I will need to dig deeper on why that circuit is the way it is. it appears like it might be an issue where the Rd resistor was incorrectly placed (and sized) and then it was carried forward in future designs without much thought. 
    2. There are many ways to supply an external clock to the MCU. There is a difference between using the internal Crystal oscillator (XTAL), which requires the external quartz crystal and components,. an external Resonator includes those load caps you need for the external quartz. and then you can use single ended clocks as well. Please refer to the "Crystal Oscillator Option" section of http://www.ti.com/lit/sprs698 
    3. The design files are in that directory, however, we recommend using the design files for TMDSCNCD28379D located here: C:\ti\c2000\C2000Ware_1_00_06_00\boards\controlCARDs\TMDSCNCD28379D

    I will reply on #1 tomorrow 

    Thanks,
    Mark

  • 0 in reply to Mark Labbato
    Guru 30330 points
    Aki,

    we have done some digging and this looks like an error on the LaunchPad. It was there on the LAUNCHXL-F28027F and has been there since. Only on the LAUNCHXL-F28377S and LAUNCHXL-F28379D did the XTAL become populated. Please follow the datasheet for proper XTAL configuration. Additionally, you can refer to the controlCards for another possible XTAL or Clock in Circuit if you need one.

    My suspicion is that this R7 was supposed to be the Rd resistance mentioned in the Oscillator parameters but it was placed incorrectly initially. It was not populated on the first two LaunchPads, and likely never actually tested until the '377S LaunchPad where it worked even thought it is incorrectly placed and sized. As we saw no issues with the LP, it carried forward to the '379D LaunchPad where it again worked. Based on this HW design, it is probably just luck that it worked, (we have not analyzed the performance of the XTAL), but it most likely would not be very robust for startup or under all conditions including different layouts.

    I will file a ticket against the LaunchPads with this information for a future fix if the boards are ever revised again, but it will at least be documented in the user's guides for reference.

    Thanks,
    Mark
  • 0 in reply to Mark Labbato
    TI__Genius 9235 points
    Mark,
    thank you!