Custom board based on the USB Audio 2.0 Reference Design

[Steve]

Hi Guys,

I am working on a custom board based on the USB Audio 2.0 Reference Design. Essentially, I am modifying the reference design to use my own DAC. As this is my 1st XMOS project, I am looking at keeping the XMOS portion largely untouched, while running my own DAC via I2S. Hopefully, this will increase my chance of getting it right the very first time.

I have been running through the documentation, and have the following questions:

1.  I am looking at using the L-series chips. Which one should I be using, with the goal of making minimal changes to the reference design's schematics and SW? XS1-L6A-64 or XS1-L8A-64? From the USB Audio Design Guide (3.18), I would need a 500MIPS device, with 6 cores. If so, will either chip do the job?

2.  I took a deeper look at the XS1-L8A-64, and notice that the pin layout is largely similar (with reference to the reference design schematics ->XS1-L1-128TQFP), with the following exceptions:

              XS1-L8A-64     XS1-L1-128TQFP
Pin 19     GND               PCU_VDD 
Pin 20     GND               PCU_WAKE
Pin 22     GND               PCU_VDDIO
Pin 24     GND               PCU_CLK

Are they any major issue if I choose to use the XS1-L8A-64? I would guess not, but need a confirmation from you guys.

3.  I am thinking of soldering some headers to the available test points on the reference design PCB, so that I can "tap" out the I2S and power lines to connect to my own DAC for a quick evaluation. I will be removing the onboard Cirrus Logic DAC prior to the evaluation, and will be keeping the connecting wires as short as I can ~5cm. However, the audio clock are running in the 10s and 20s Mhz, will there be any major issues?

Appreciate your valuable time on my above questions!

Thanks,
Steve

[infiniteimprobability]

Hi Steve,

This is 3 questions so should probably be split into seperate Q&A to fit the format - or move to the forum.
Anyhow, quick answers for you:

1) EIther will do the job. In fact you can't use the extra 2 cores on the L8A-64 in USB audio, so save your pennies and specify the L6A. Also, why not consider the U6A? It's loads more integrated

2) The XS1-L1 is the same as the XS1-L8A-64. Package-wise, you can choose whatever you want but you may find the TQFP48 gives you what you need, and again saves a few pennies.

3) It's not ideal however using flying leads on the bench is probably OK for eval. We've done it for connecting DAC eval kits to our boards with clock rates of 22/24MHz. Not nice, but you will probably be OK, especially as I2S runs slower than MCLK in most cases (eg. 12MHz for 192KHz).

[Steve]

Hi,Thanks for taking the time to reply. Appreciate it!Yes, the U series is definitely the way to go. The PCB area for the current USB IC can be free up for other stuff. By the way, I ran some wires to my external DAC and it is working fine. So for anyone that is thinking of doing the same, just remember to run as the wires as short as you can. You should be alright.Thanks,Steve