Interfacing ADAU1761 x2

Question

Hello, 
 I've been working with a pair of 1761 eval boards and have run into a mysterious clock sync problem. Every ~4 seconds I have a short amplitude reduction in the output signal. I'm hearing this as a very brief drop out or click in my headphones. Perhaps I've missed a register setting. The audio streams are otherwise working/sounding fine and I'm running them through a pair of the ADI beamforming algorithms. 
 Setup: 
 Passing a digital signal from the slave board to clock master. I've tried Left/Right Justified and TDM modes, following the configuration parameters from pg 42-43 in the ADAU1761 datasheet. 
 I've taken a look at this tutorial: https://ez.analog.com/message/5675#5675 
 Jumper configuration for my setup is consistent for clock sync and passing ADC to DAC pins. I've not had any luck setting MCLK to the I2S setting, both boards are set to OSC. 
 Any ideas? 
 Best, 
 John

JAronson · Accepted Answer

Yes I have both boards externally powered and successfully deploy the programs. The clock signal pulled from R47 is not valid - we looked at it with a logic analyzer and its all over the place. I got a 1781 board setup today and it's driving the clock from MCLK0 to the 1761 MCLK input just fine.)

ADIApproved · Answer

Welcome to the forums! 
 Well, if each board is running off of its own clock source (OSC) then that means the boards are asynchronous by definition. Since neither oscillator is running at exactly the same rate, you will have small drops when a sample is either skipped or doubled by one chip or the other. This is similar to the concept of a beat frequency when two tones are combined. Basically, if one oscillator is running at 12.288000 MHz and the other is running at 12.28800025 MHz (completely possible with the tolerance of some crystals), that means that there is a difference between the two clocks of about 0.25 Hz, meaning that once every 4 seconds, a sample will be missed. 
 In order to make both chips synchronized, you really need to have them both running on the same master clock signal. 
 You can either take the master clock oscillator signal from one board and run it to the next, or you can take the clock output from one board (a buffered copy of the PLL output) and run that to the next. However, the one thing you can't do is have each chip running off of a separate clock source. 
 I hope that helps!

Natan · Answer

You want to have both chips running off the same MCLK signal. On the board you have designated the slave make sure you have the MCLK source selected as I2S, this would be S5 in the middle position. I could be wrong about this but I don't think there is a header that will give you the raw MCLK running to your master to bring over to your slave so you will need to solder a wire for the next step. Solder a wire from the master oscillator (R47) and connect it to the EXT_MCLK input on your slave board (J6-10).

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Interfacing ADAU1761 x2