Chris -
1) does your external crystal based oscillator continue to oscillate on the non-working boards ? If not, can you share the details of your external clock source for a review ? Without a working clock, this design will not function and may be the root cause.
What is the external crystal value ? If known, which vendor, part number and the value of the loading caps ? Is your design featuring an external gate for this oscillator ?
In the datasheet, the recommended value is 24 Mhz - is that not oscillating ?
Update comment - forgot that you are using USB so you are obligated to use 12 or 24 Mhz as the value for the external clock source. On this note, IF you should consider to apply an external oscillator of 12 or 24 Mhz value, please review 18.10 of the datasheet which notes that the external oscillator must be of a 1V8 value (1.98V is the MAX but since 1V8 is the power rail for the XMOS device, consider to use a 1V8 oscillator. These are not so common but certainly available from offshore vendors. When penny pinching on BOM and assembly fees, you must weigh how many parts must be placed to operate the external crystal oscillator vs. a single device for the same purpose (ie. 3 parts vs 1 part). Varying with the placement costs, it may be wiser to use an external clock oscillator as a single part.
The caps used on crystal based oscillators can be a bear to get right so often it is easier to just source a full SMD oscillator. We can share the details of some very solid crystal and oscillator suppliers.
For crystals, you should be paying about $ 0.08 to $ 0.10 USD each (30 PPM).
For SMD oscillators, you should be paying about $ 0.30 to $ 0.50 USD each (30 to 50 PPM).
Contacts:
We have met with WTL at last year's HK trade fair and they have a very extensive line of devices. Recently started to consume their parts with success.
Lori Sales I Shenzhen WTL Electronic Technology Co., Ltd
T: +86-755-8267 7582 ext: 801 M: +86 135 3042 0550
:
wtl001@wtlcrystals.com:wtl-delly:WTL-crystals
W :
http://www.wtlcrystals.com Alibaba:
http://wtlcrystals.en.alibaba.com
A: Room 505,5th Floor,No 6 factory, Zhong Chu Building,4th Ba Gua Road, Ba Gua Ling, Futian District, Shenzhen 518029,China
Our long time supplier is - used on 500k-1M+ boards to date. No issues on QC but pricing is higher till we raised the concern. Private labelled parts (laser marked) for our company. Have visited their Shenzhen operation a number of years ago.
Foina Yeung
-------------------------------
TEL: 86-186 8242 0160
e-mail:
foinayeung@hotmail.com
skype: foina-chinafronter
F.O.B. Shenzhen, China.
If you want a really solid product, consider silicon oscillators or MEMS oscillators which would allow for a pure solid state design in that your board could be hit against a wall without concern of damage to the crystal. Best to review the specs of such parts. We have not yet moved to this technology but may do so for some military accounts working on drone designs.
We have used 5x7 size for ages but they are pigs on the PCB relative to the current designs. For that reason we are now using 3225 = 3.2 x 2.5 mm sized parts. With the some negotiations it is possible to get your company name laser marked on the parts (room and font size permitting).
2) moving forward, you noted the use of JTAG to reflash the target flash device. If you apply the same method on a known good board - does the same procedure and format of the flash allow your working board to remain working ? Just concerned about the serial data stream being applied using this method. It is vital the JTAG method be solid else the XMOS does not have working firmware. Perhaps you can code a small routine to blink a local LED -> test this code on any XMOS devkit -> then apply onto a working custom XMOS board you are producing to confirm the code still works using the JTAG tool -> only then proceed to apply onto the non-working board. In the end, be sure the LED is able to flash in every test. Again, no clock = no love and the design will fail to operate.