Designing a simple xCore-200 Board

XCore Project reviews, ideas, videos and proposals.
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Post by bzyzny »

Still have some tweaking, polishing, and minor adjustments to make on my schematic, but I felt it was done enough to upload to a project page so I can start getting feedback. Here is the project page link: ... oard-kicad

Earlier in my research I thought I had found some info on how to use the USB port without a PHY transceiver, driving the data lines directly from the gpio (essentially like V-USB for AVR), however I've not been able to find this info again. Anyone know if this is possible? Future revisions of this project will use an XU-series chip with the USB PHY built in, but for this one I was hoping to be able to use low speed USB since the port is already there for power anyways. thanks.
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Post by mon2 »

HI bzyzny.

R11, R12 (AP3417C - U5) and R13, R14 (AP3417C - U6) are supposed to be in 'K' values, you have them in the schematic in ohms.

As a general rule, we prefer to purchase fixed switching regulators that are not dependent on the external resistor voltage dividers. The key reason being that if either of these external resistors are not soldered properly or wrong value then this could lead to killing all powered devices downstream. The board will then be scrap. When not avoidable, we prefer to use larger sized resistors for such tasks (ie. 0603 or 0805 footprint) and we still visually inspect the assembled boards before powering up. We have a very high quality assembly shop who has automated vision equipment to confirm the part placements and values during production.

Share the details on the L1 and L2 inductors as they are not all equal to be sure the proper footprint is placed on the PCB. Many such parts will derate their inductance as the current draw increases so be sure to check the respective datasheets to make a valid selection. We have good luck with Taiyo Yuden. For example, consider p/n NR4018T2R2M which is available through Digikey. ... ND/1008284

What are the details of Y1 ? Size ? Part number ? In assembly, you will pay for each part placement so need to consider if it makes sense to stuff the 6 parts to create a 24 Mhz oscillator or stuff a single SMD oscillator for the same task. With some creative shopping it may be less expensive to purchase the SMD oscillator. For example, you should be able to source a single SMD oscillator @ $.28 to $.30 USD each from WTL Crystals in Shenzhen China. We have used them quite often now with zero defects. Just a suggestion. From our last feedback from this supplier, the 3225 SMD footprint was the most popular to apply due to the cellphone markets. Smaller footprints are also available on demand at higher costs. 3v3 operation and 30 PPM. However, it is possible to source the 3225 (crystal) @ $0.08 to $0.12 USD each, varying with the PPM accuracy from the same vendor.

Be sure to review this article on how capacitance varies with voltage and size of the parts: ... vp/id/5527

before selecting the bulk caps in this design. We found that Samsung appears to be the most competitive and have consumed very high volumes with success for many years. Check Avnet (preferred) and Future Electronics (sometimes lower cost but need to offer the comp pricing) for competitive pricing.

It appears that you will not be using USB data pins in this design. If you do, must apply ESD protection devices (Socay, Shenzhen, China) to prevent ESD damage to the controller. We have some pending designs that will use the USB interface to power the end product and plan to permit the use of the ultra low cost power supplies from China that are AC to 5 volt out via USB. To prevent damage to the 5 volt power supply parts on your board, we have over voltage supply limiters in case the 5 volts in is not within spec. This is very possible to support using a part like the USB load switch (select the proper current for the respective switch for this project).

For the USB connector, review how the board locks work for the part to allow for the best physical mounting strength. It will not be desirable to have the end user rip this connector off the PCB during insertion. For our designs, we are planning to use the FCI 10118194-0001lf component which is available through Digikey, Mouser, etc. and is supported by a large and solid manufacturer. We did import some USB connector parts from China at half the cost but the quality was not up to our standard - fit was loose and had to toy with the mated connectors to have a solid connection. Garbage.

What are the details for P1, P2 and P3 ? Through hole (vs. SMD) are the best choice here for best mounting strength.

We have an excellent supplier for such parts and are the real manufacturer, not a trading company. They deliver most goods made to our spec within 5-7 business days. Costs cannot be matched by other suppliers.

For these metal stamped parts:

Contact: Jane
Address:E Building,YuSheng Industrial Park,,Beside ZhouShiRoad,JiuWei,Xixiang,Bao'an District,Shenzhen,China
Tel:+86-755-29975806 Fax:+86-755-22975992

You can decide on the amount of gold plating, length of the pin before and after the insulator, the thickness of the insulator, etc. This company has also produced our 2 mm pitch parts for our High Def PCIe host adapter and we switched away from Samtec to this vendor with success. Perfect quality. They also offer the keyed box header which is recommended for the XSYS connector.

You may not need the on/off switch but if you do, what are the details ? Details of the manual PB for reset (SW1) ?

Perhaps add a small (0603) LED with a current limit resistor to denote power ? Cannot use the 1v0 rail due to the low voltage for the LED. With a bit of thought, could use a few parts to enable the same LED only if the 1V0 rail is enabled. Perhaps another LED (green) on the project board to allow for the end user to create a quick blinky demo ? This LED could then be disabled to remove from the circuit.

Prices for standard RGB leds are falling in Asia. We have received offers for the SMD RGB leds in the $0.014 to $0.03 USD range.

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Post by bzyzny »

Hi Kumar, thanks for all the info, many helpful points. I'll start by saying that this initial version of my design is mainly to test if I can make a functional board (both design wise and physically assembling the board). Once I go through some iterations and feel confident that I know what I'm doing, I do plan to have a batch manufactured, if there is sufficient interest perhaps even a crowdfunding campaign. Your tips about suppliers and pick and place, etc will be helpful later but right now I'm focusing on at-home assembly for my initial prototype. Thank you for the component suggestions, that is very useful since there's thousands of options to wade through.

I have not yet selected a specific part # for some of the components, including the inductors, capacitors, resistors, buttons and connectors. That will be next on my to-do list, and I will update the schematic with those part details. Regarding the crystal oscillator, I have to admit I pretty much just copied that section from the explorerKIT hardware manual schematic. I couldn't find much info about the required oscillator specs in the xmos documentation, and am unclear why the unbuffered inverter is there. From what I gather, it somehow helps stabilize the clock signal and is needed for parallel crystal layout (the external crystal is in parallel with the internal MCU oscillator, right?). I had looked at the "all-in-one" smd oscillators and knew that is a better solution, but I'm uncertain exactly what specifications are required by the XL208. Referencing the explorerKit again, I noticed that the EPSON FA-238 oscillator used for the atheros ethernet phy is an SMD package which is also available for 24Mhz. Would that work for the MCU crystal?

The USB port is only for supplying power from a wall-wart/cellphone charger (the charger for my Galaxy S4 is rated for 5.3V and 2A). I'm aware that the usb and regulator circuitry is not ideally protected for ESD, over-voltage, and over-current. I would like to improve this as much as possible, especially for the next board version. For this initial prototype, I'm trying to get a working board with as little as possible, both to reduce cost and to simplifying troubleshooting. Assuming I use a quality USB charger, and I add an polymer fuse, would my present configuration be acceptable? It is similar to the schematic used in the starterKIT. As for the USB data lines, this is also something I've decided to save for the next board version (since the next version will use the XU208 with usb PHY). Unless it is possible to implement USB on the XL208 in software, without requiring a usb phy? Again, I've not yet selected the exact micro-usb port I will use, but thank you for suggesting the FCI 10118194-0001lf I will take a look at it.

The pinheaders (P1...P3) have not been selected yet, but I know better than to go for the cheapest ones I find. I've been wondering though, why is the XSYS connected usually bent 90 degrees, is it because of the orientation of the XTAG cable?

Well, I'll probably have more questions after I re-read your post and research your suggestions, but for now that's all I can think of. Thanks again Kumar, if anyone else wants to give their 2 cents please do!
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Post by LulaNord »

Hi...i am a new user here. I also faced the same issue and observed that The TQFP package sounds like a sane package to use for hand soldering. We have done this many times in the past early on at our company. Now we have SMT lines so the automation helps to crank out volumes once the design is solid.You will find that some SMD part sizes are more common, so hence lower costs. At this time of writing, believe that 0402 is the best bang for the dollar. You will need tweezers to place down such parts by hand. Otherwise, consider 0603 sized parts which will be slightly higher in cost.Bulk caps are more practical in 0603 and/or 0805 size for your power supply. Too costly to source the same in 0402 footprint.

printed circuit assembly
Last edited by LulaNord on Thu May 18, 2017 6:04 pm, edited 1 time in total.
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Post by bzyzny »

Thanks for the tip :) I had put this project on hold for a while but hope to get back to it soon.