AD1938 Clock

hi

I am design broadcasting equipment and am using the ad1938 for the audio of the system

i have an external clock (12.288Mhz crystal) connected to the MCLKI  but i would like to just use that for the ADC  and use the BCLK from another device for the DAC .

i tried this is standalone mode and the ADC and DAC were not in sync, and now am using a microcontroller and i still can seem to the the system with with the different clocks

could you please advice me on if it can be done in standalone mode or how should i configur the registers ?

thanks

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  • 0
    •  Analog Employees 
    on Aug 4, 2014 11:36 PM

    Hello Andrews,

    In reading your description of the problem of the registers being unresponsive, I think you may be accidentally placing the part into Standalone mode. In standalone mode many of the registers are shadow registers and are bypassed so that is the behavior you will see. Have a look at Table 11 in the datasheet. If CIN, CCLK and CLATCH are all low for a period of time then the part will go into Standalone mode. With a 24.576 MHz clock on the MCLKI pin then the amount of time is 2.67 ms. Double that if it were a 12.288 MHz clock. This is easily avoided since SPI messages are far shorter in duration than 2.67ms. Where this has been a problem is when the system controller may have the SPI port on a lower priority, drops the CLATCH line low then gets interrupted before the start of the message for more than 2.67ms. Then the CCLK, CIN and CLATCH are all low for too long. Another possible source for this problem is at startup when in reset. This is best handled by placing a pull-up on the CLATCH line as recommended in the datasheet. Another strategy is to keep the CCLK line high at the end of the message and only drop it just before the start of the message.

    The only way to get it out of STANDALONE mode is to reset the part.

    I think otherwise, you are close to getting this working as you desire.

    Thanks,

    Dave T

Reply
  • 0
    •  Analog Employees 
    on Aug 4, 2014 11:36 PM

    Hello Andrews,

    In reading your description of the problem of the registers being unresponsive, I think you may be accidentally placing the part into Standalone mode. In standalone mode many of the registers are shadow registers and are bypassed so that is the behavior you will see. Have a look at Table 11 in the datasheet. If CIN, CCLK and CLATCH are all low for a period of time then the part will go into Standalone mode. With a 24.576 MHz clock on the MCLKI pin then the amount of time is 2.67 ms. Double that if it were a 12.288 MHz clock. This is easily avoided since SPI messages are far shorter in duration than 2.67ms. Where this has been a problem is when the system controller may have the SPI port on a lower priority, drops the CLATCH line low then gets interrupted before the start of the message for more than 2.67ms. Then the CCLK, CIN and CLATCH are all low for too long. Another possible source for this problem is at startup when in reset. This is best handled by placing a pull-up on the CLATCH line as recommended in the datasheet. Another strategy is to keep the CCLK line high at the end of the message and only drop it just before the start of the message.

    The only way to get it out of STANDALONE mode is to reset the part.

    I think otherwise, you are close to getting this working as you desire.

    Thanks,

    Dave T

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