Snapdragon 410 clock tuning

96boards

#1

I’m having issues with EMI from the Snapdragon 410 on an eragon SOM from e-infochips which is a clone of the Dragonboard. That SOM is radiating clock noise and jamming our 915MHz radio. I’m currently observing very strong harmonics at n*150MHz as well as a few others at higher frequencies which could be related to the clock PLL. One guess is that the DDR clock termination is not well routed and the PCB is radiating reflections. It could also be internal clock signals to the chip though I don’t see this problem on the Dragonboard. I was wondering if the linux clock driver, device tree, or sysfs interface has a way for me to change clock speeds or try disabling some of them completely? Even with additional shielding added around the SOM we don’t get the radio sensitivity we need and I now need to look for ways to move the interference signal away from the radio’s receive band.


#2

Hi @dustin:

Based on your description, it does sound like the DRAM clock, but as the chip enters power saving modes the DRAM clock will slow down. The Linux Debian builds do not yet implement all of the possible power savings modes on the 410, however the Android builds do. Have you tried building Android and putting it on the board?

The layouts for the 410c board and the eragon are very different. Since thisis an issue related to the eragon design, you should talk to eInfoChips about your issue and see if they can help. When we designed the 410c board we had emissions jamming the GPS, but we were able to resolve this with a shield over the 410, PMIC, and memory.

Full Disclosure: I am an employee of Qualcomm Canada, any opinions I may have expressed in this or any other post may not reflect the opinions of my employer.


#3

Thanks for the quick response. We have had the exact same issue with jamming our external GPS as wellI and have gone to great efforts to locate one culprit being the SD clock line. The GPS still reports high CW jamming so I expect there is still some other EMI degrading performance. We’ve simply disabled the SD interface for now. I have not tried the Android build on this board, only on the Dragonboard. From your understanding do the power savings modes in Android exist in the kernel or does it involve some daemon controls as well? I don’t know if we can move completely to android at this point but I’d certainly be willing to try running that build as an experiment to see if the clock EMI somehow changes.


#4

Hi @dustin

I wasn’t for a moment suggesting you change over to Android for your product, just as an experiment to see if the jammer is due to the DRAM clock speed. In fact I would strongly recommend you do not use Android as your product for several good reasons:

  1. Android is built on top of a quite old kernel (3.18 I think) and will not receive security updates, except for some of the most serious exploits.
  2. the Android on top of the old kernel is old, and spins to new revs almost every year with huge changes. This makes it difficult to keep up and support
  3. the Linux kernel is very up to date and receives all security updates. This is what you want for your product.

You should talk with eInfoChips, they may be able to reroute the jammer signals and make a SOM that doesn’t emit as much. This would be good, not only for you, but for all of eInfoChips customers.

Full Disclosure: I am an employee of Qualcomm Canada, any opinions I may have expressed in this or any other post may not reflect the opinions of my employer.