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DS90UB638-Q1: Back Channel Frequency Select

Barry W
Intellectual 840 points
Part Number: DS90UB638-Q1
Other Parts Discussed in Thread: DS90UB635-Q1,

I have a DS90UB638-Q1 / DS90UB635-Q1 system running.

I would like to test at a lower forward frequency.

Table 7-8 (Clocking Modes) of the 635 data sheet describes a Synchronous (Half-rate) mode.  I would like to test that mode.

I understand the first step is to configure the BC_FREQ_SELECT field in register 0x58 of the 638 device.

On power up, register 0x58 reads 0x1E, indicating a 50Mbps back channel was configured by the MODE Pin strapping.  This is expected.

I try to write 0x1D to change the mode to 25 Mbps back channel.

Register 0x58 always reads back 0x1E.

Am I missing some step in this configuration?

  • +1
    TI__Genius 17455 points

    Hi Barry, 

    To enable writes to RX port 0 registers, register 0x4C should be set to 0x01 prior to writing. Let me know if this is already being done. 

    Best,

    Zoe

  • 0 in reply to Zoe Bison
    Intellectual 840 points

    Hello, Zoe,

    I was not writing 0x4C.  Setting it to one fixed the issue.  

    I am able to now connect at half rate.

    It does appear to have extended our cable length capability.

    Thanks again for your excellent support.

    In the 638 data sheet, register 0x4C is mentioned above the bit field table for register 0x4D.  It could be useful to mention 0x4C above all the affected registers like 0x58.  I now see that 0x4C is set in some of the example code for other functions we have not tested yet.  

  • 0 in reply to Barry W
    TI__Genius 17455 points

    Hi Barry, 

    Glad this resolved the issue! We'll keep this note in mind for future revisions. To clarify, port specific registers are 0x4C-0x7F and 0xD0-0xDF as shown in the below table.

    Best,

    Zoe