HopeRF feedback

Werner Almesberger werner at openmoko.org
Mon Jun 21 09:58:21 EDT 2010


Hi all,

Wolfgang told me that you're looking into using an ISM RF module
from HopeRF. I sent him a number of comments and he asked me to
post it to the list. Here we go:

One of my first questions was how you'd use this to connect to the
rest of the world. I didn't find the idea of requiring another
Nanonote plus USB cable plus USB-Ethernet or such particularly
appealing. However, if this kind of protocol has any popularity,
one would expect that someone is at least making a USB dongle for
it.

I poked around a bit and indeed found an RF-to-USB dongle at
Silabs:
http://www.silabs.com/products/mcu/Pages/RFtoUSBRD.aspx

Unfortunately, it comes only in a develoment kit priced at USD 49:
http://www.supplier-direct.com/silabs/Cart.aspx?supplierUVID=63410000&quantity=1&issample=0&partnumber=RF-to-USB-RD&pd=RF to USB Reference Design

But it gets better - here are the schematics (page 12):
http://www.silabs.com/Support Documents/TechnicalDocs/RF_to_USB-RD.pdf

That's just how I had imagined one should to this :-) Perhaps one
could even use a cheaper microcontroller, e.g., the quite similar
C8051F326/7 at less than half the price. There, SPI would have to
be implemented by bit-banging.

There's more. Silabs also have a complete MAC:
http://www.silabs.com/Support Documents/TechnicalDocs/EZMac-EZHop_UG.pdf

Depending on TX power, with or without FCC-mandated frequency
hopping. This MAC is a bit more featureful than rfm12lib. They
even put in link-layer forwarding for meshes :-) Naturally, it's
not compatible to rfm12lib. The sources, unfortunately with a
restrictive license, are here:

http://www.silabs.com/pages/DownloadDoc.aspx?FILEURL=Support Documents/Software/EZMacPRO_setup.exe&src=DesignToolsWebPart

The frequency hopping may be worth stealing. The addressing
scheme doesn't scale. (A BT-like approach with short local
"addresses" may be a lot more useful.) I also have some doubts
about the efficiency of their forwarding mechanism. At least
the documentation doesn't make it sound particularly nice.

By the way, if there's a sufficiently stable 30 MHz clock in the
system, you can even save that pesky crystal.

- Werner




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