Clocking AD1938 (slave) with ADAU1701

Hello Mayuresh,

Can you give me a little more information? The parts are really flexible so this is possible but I need to know what is available to work with?

There is no CLKOUT pin on the 1701. 

What sampling frequency are you using for the serial port? What format, I2S or TDM?

Will you have a system controller that could talk to the AD1938 to set it up? If you do not then you will need to run it in standalone mode and that limits what you can do. 

Where is the 12.288MHz clock coming from? A clock oscillator module or a crystal?

We can discuss further once I know some of this info.

Dave T

Hi Dave,

Planing to run ADAU1701 with sampling frequesncy of 192kHz, I2S format, this I will set in SigmaStudio. 

Crystal 12.288 MHz SMT, 18 pF, is connected to ADAU1701 as per the EVAL-ADAU1701MINIZ datasheet.

LRCK, BCLK, SDATA from ADAU1701 will go in as input to AD1938.

BCLK = MP11 from ADAU1701
LRCK = MP10 from ADAU1701
SDATA0/1 from MP6,7 from ADAU1701

I am using this exact schematic from EVAL-ADAU1452REVBZ-UG-1662.pdf to connect to ADAU1701.

Here is the schematic : 

Will you have a system controller that could talk to the AD1938 to set it up? It seems no, as I don't see it presend in EVAL-ADAU1452REVBZ-UG-1662.pdf.

Let me know if this information is sufficient?

Thanks
-Mayuresh

Hello Mayuresh,

You will need to read the AD1938 datasheet as well to understand how that part works. It is very flexible so there is a lot to learn. On the ADAU1452 eval board we are running the codec in standalone mode with the ADC clocks as a master. In that mode the ADC is providing the clocks to the DSP and the DSP is providing the master clock to the codec. This way they are in sync. 

You will need to use the 1701 to drive the DAC section of the codec so you will not be connecting to the ABCLK or ALRCLK, you need to connect to the DBCLK and the DLRCLK. So that is one change required.

Positives and Negatives:

Since you are using a crystal you will not be able to "Y" that signal over to the codec. There is one positive thing, if you run at 192kHz fs and you are using a standard I2S signal, then the bit clock will be 64x fs which is 12.288MHz. So you could "Y" the BCLK output of the 1701 over to the MCLK input of the codec and that would work just fine. 

The negative is the codec cannot operate at 192kHz in standalone mode. You will HAVE to program the registers to something different when it powers up. Hence, this is why I asked if you had a system controller in your design. 

I did write up an App Note several years ago on how to use a low cost PIC microcontroller to boot up one of these codecs into a mode that is not what it does in standalone mode. 

AN-1365 (Rev. 0) (analog.com)

In the user guide of the EVAL-ADAU1452RevB board I did go into how to modify the board to allow SigmaStudio to configure the codec. You also can use the master comms port of the ADAU1452 to have he DSP program the codec when it starts up but the ADAU1701 does not have that capability. So you will have to use some controller to setup the codec to run at 192kHz. 

Dave T

Hi Dave,

Thank you for the reply. I am not well versed with Microchips & their programming. Is there any other device I can use instead of AD1938 to avoid this scenario? Basically, easily plug and play type. My whole idea of adding AD1938 is to use the digital outputs from ADAU1701 and make them available as analog out for connecting to external amp as sub-woofer. 

Other option I could guess is to use additional crystal to drive AD1938 and only provide SDATA inputs from ADAU1701. Will that work? I will go through AD1938 datasheet as you have suggested. 

Thanks
-Mayuresh

Hi Dave,

Will this solution (connecting PIN31 to MCLK) work? Because if this works then I can try AD1955 instead of AD1938. 

Found this at  Interfacing ADAU1701 with AD1955  

Thanks

-Mayuresh

Hello Mayuresh,

Well, this can work but it is just not recommended for reliability of the crystal to oscillate. It loads down a little and adds some capacitance to the crystal circuit and it can be very sensitive. Why don't you just use an oscillator module where all the work to make sure the crystal oscillate is done and it usually can drive a couple of destinations? 

I also want to point out that it seems to me that you are using this board to drive a stereo 2-way set of speakers and a mono subwoofer correct? So you will use the four amplifier outputs for the stereo tweeter and woofer and then develop a mono sub signal in the DSP and send it out of the serial port to a DAC and then to a subwoofer amp correct? 

If you are running the DSP at 192kHz then you will run out of available instructions very easily. There is no way you will be able to have much EQ and no limiters. Basically, a stereo 2-way crossover and some filters for the sub output. I have not put together a project like that to see how many instructions it uses but at 192kHz you have 1/4th the instructions you would have at 48kHz fs. 

You may want to think about getting two of these boards and using one for each channel? 

Dave T

Hi Dave,

Thank you for the reply.

So I am currently using TDA7388 amp, which has 4 inputs & 4 outputs and does not require any Crystal etc. DSP works fine on 192MHz. I could even go higher than that with limited EQ and Crossover and system worked fine for me. Output volumes drops as I go with higher sampling rates.  Ofcourse this is without using digital input-outputs.

Yesterday I was trying to connect 2 working DSP boards which I have now, and was trying to output digital data from one board to other. Didn't work for some reason. But I will continue to test that and find out where the issue is, by referring to your earlier responses on my other post.

For subwoofer, I am now planing use TAS5828, which has digital inputs without MCLK. So that may work fine for me as a subwoofer amp. And with this configuration, my requirement for matching clock/crystal with AMPs should be taken care of. Because both of these AMPs to not require such matching. 

I will check with the oscillator module suggestion.

Thanks.
-Mayuresh