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Buad rate confusion?

Helmut Forren
Mastermind 6805 points

I have new custom boards designed for DM6467T but stuffed with DM6467.  I got two boards working just fine several months ago.  I can't get working the third board from the same build.

I have programmed UBL and U-Boot to NAND flash, and I'm trying to connect the console at 115200,8,N,1.  On the oscilloscope, the bit timing is supposed to be 8.68us/bit.  However, I'm measuring 7.75us/bit.  I haven't analyzed the bit pattern, and I don't have a 129000 baud device, but the bit pattern looks like it's good data, just at the wrong speed.  I don't have the original working boards around any more, so I can't compare anything to them.  I can, however, compare to the DM6467T EVM that I have.  Indeed, I measure 8.6us/bit and my TeraTerm program sees the nice U-Boot output.

So how am I possibly getting 7.75us/bit?  First, I can't make sense out of the ratio between than and 8.68us.  It doesn't relate to any combination of 27 and 33, like the crystal MHz involved.

Note that I am using a TMS320DM6467ZTU7, the same processor I used last time.  In both cases, I am programming with dm6467t_495_ubl.bin and u-boot-dm6467t-evm.bin.  I realize that these are intended for the DM6467T, not the DM6467.  But it worked twice before.

I haven't looked at the U-Boot source code, and I haven't found a document detailing how the DaVinci UART works.  I do have decades of UART and RS-232 experience in general, however.

What can possibly be wrong to cause this funny baud rate?  Also, can someone give me a quick link to the correct source for this version of U-Boot?  Where's a spru*.pdf that covers the UART?

Thanks very much,

Helmut

  • 0
    Mastermind 6805 points

    RESOLVED.

    Here's how.  I found spruer6d.pdf about the DaVinci UART, from the DM6467T product page, of course. http://focus.ti.com/docs/prod/folders/print/tms320dm6467t.html

    I read that the UART uses the 24MHz AUX clock.  Immediately I considered the ratio of 27/24.  It suitably matches my erroneous 7.75us/bit to the correct 8.68us/bit.

    I put a scope on the board and found the AUX clock running at ...  27MHz.  How this one out of three boards in the same fab lot got the wrong speed crystal in this position, I don't know.  But I've ordered a replacement overnight and fully expect this correction to fix my baud rate problem.

    Obviously, knowing which clock fed the UART was key.  I just hadn't found that UART doc yet because I hadn't needed to even look for it!

    -Helmut