CPU recommendation

Technical discussions around xCORE processors (e.g. xcore-200 & xcore.ai).
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Post by Biquad »

thanks for the information.
Can you have a look if i have made the basic setup correctly. Specially the X0D port pin setups for the adc and dac before
i begin with the pcb.
Can we use an 24.5760Mhz crystal instead of the recommended 24M ?
I have inserted an header so that we have the option for a later use of the multisamplerate dac features.
For now the dac will use the same ADC clock lines mclk,lrclk,sclk
Big thanks!
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Post by mon2 »

Hi. A few comments...

1) Review the datasheet for your CPU to note the requirement QSPI port pins. This is very important and linked to the boot rom inside each XMOS CPU. You cannot alter the pinout for these QSPI pins unless you are rewriting the boot rom code. You must use the QSPI pins noted in the respective datasheet.


As drawn in the posted schematic, your custom PCB will not be able to boot from the attached QSPI flash memory device.

2) Since USB interface is active, the CPU clock must remain @ 24 Mhz. Not an audio developer but since you are requiring the use of the 24.5760 Mhz clock value as well then consider to attach the low cost Si5351A PLL (Si5351A-B-GT) from Silabs. This part uses a local crystal (about $ 0.06- $0.08 USD from Shenzhen) to drive the part (no local caps are required for the crystal but leave room for them, just in case - 0402 size caps are fine on each leg of the crystal). The part is available with factory pre-set clock values so you can request for CLK0 for example to be @ 24 Mhz and CLK1 to be 24.576 Mhz and then feed this 24.576 Mhz clock into a single bit port pin (must be a single bit port for XMOS to support clocking of your internal IP). A key point here is that the CPU clock MUST not be disturbed during boot and operation of your IP else the CPU will lock up.

Also connect the Si5351A to your I2C port pins and now using I2C code, you can alter the clock values to suit - how nice is that ? :) Again, during such playing, do not alter the 24 Mhz else the CPU will lock.


If you are 100% confident that you will never require alternate clock values then of course consider to slap into the design a fixed 24 Mhz SMD oscillator and also a 24.576 Mhz SMD oscillator - fed to any free single bit port pin and tailor your audio IP for this external clock use.

Another idea is to consider sourcing of the special pre-configured version of the Si5351A initiated by XMOS:

http://www.silabs.com/internal-apps-man ... dendum.pdf

The full p/n must be referenced and will supply you with the desired clocks of:

CLK0 = 49.152 Mhz (not clear if the audio IP can use this value directly but assuming yes else you can apply an external low cost flip flop to divide by 2 = 24.576 Mhz)
CLK1 = 24.00 Mhz

and still use I2C to tweak these values after boot time - ie. your code can easily change CLK0 to be 24.576 Mhz upon boot for your audio IP and then you will save on not requiring the external flip flop.

http://www.mouser.com/ProductDetail/Sil ... 7Ndxpy4w==

https://www.digikey.ca/product-detail/e ... SD&lang=en

3) The details of how your USB interface is mated is not shown and is very important. You MUST have VBUS inrush current protection. See Figure 15 in the datasheet. You can also consider the Diodes Inc. APA2822AK silicon device which offers additional features which are meant for this same purpose - to limit in rush current using soft start, etc. If the room permits, apply support for both ideas and pick one during live production. It will be a lower assembly cost to stuff a single silicon than multiple passive parts during production.

4) Why is VDDIOL (Pin 3) floating ? Needs to be connected.

5) The power supplies must be sequenced for XMOS devices. Post your power supply and reset supervisor schematic for a review.
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Post by Biquad »

Thank you very much for your comments.
I have made the decision to use an extra 24Mhz crystal for the xu208. thanks.
I don't use the VUSB Power thanks for the hint. I have a schottky in line of the Vbus and go to the xmos USB_VBUS via an 100k resistor.
I have read that vbus should be connected even when not in use.
I have checked the QSPI lines and have connected it to the correct port x0d01(spi_cs), x0d10(spi_clk). I hope its all fine.
Ups, have not considered the power sequencing. I have one designed based on the Xc-200-Audio eval board.
VDDIO Pin 3 ins now connected.

Best regards
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Post by AlexAdvice »

Maybe somebody knows, what is inside MOTU 24Ai/24Ao ?


They claimed:
- USB 2.0 (44.1 to 48 kHz): 64 in/out
- USB 2.0 (88.2 to 96 kHz): 32 in/out
- USB 2.0 (176.4 to 192 kHz): 24 in/out

I think it is not XMOS, because currently XMOS solutions provides maximum 10channel at 192/24, or new USB stack with double bandwidth exists?