[John Ryan] posted his dual core Arduino rig on the Arduino forums. These two ATMega168 chips share the same 16Mhz resonator and I2C bus - allowing them to run in semi-parallel. The uC’s don’t actually communicate with each other, but they run rather nicely as concurrent circuits. It’s a pretty interesting method of adding I/O pins to a project for a minimal cost.
There are plenty of i2c I/O expanders out there that are much cheaper than a full ATMega168. Not nearly as cool though :p.
Posted at 2:03 am on Apr 12th, 2008 by pc486
I am surprised this is considered a hack…
This is like connecting two lamps in parallel and being surprised that both glow.
Posted at 5:39 pm on Apr 12th, 2008 by Lupin
why?
Posted at 6:15 pm on Apr 12th, 2008 by chupa
Useful or not, “dual-core arduino” sounds very macho
Posted at 8:13 pm on Apr 12th, 2008 by DarkLightConnection
He needs to get a truckload of arduinos (1024 maybe?) and build the computer with the most processers that can still fit in a small bathroom.
Posted at 12:30 am on Apr 13th, 2008 by Takato
What about an 8-to-1 multiplexer chip? $1?
Posted at 9:07 am on Apr 14th, 2008 by Ric Johnson
Isn’t a 74HC595 shift register, like, a quarter?
Concurrent firmware (on something other than the Propeller) would be interesting, though. 
Posted at 10:10 am on Apr 14th, 2008 by dro kulix
hack a day serves up a fresh hack each day, every day from around the web and a special how-to hack each week.
have a hack you'd like to see here? tell us about it
powered by wordpress
He wired the chips in parallel - this doesn’t seem to be a good idea. Resonators are usually driven by a not gate - one pin is an input, the other is an output. It’s wired so that the two outputs are driving each other — bad practice! This will waste energy and stress the parts. It’ll also create a lot of electrical noise as the power supply is repeatedly asked to supply the short circuit current every oscillation.
The better solution is a master/slave setup. One device will control the oscillator (connect XTAL1 and XTAL2 as usual) - this is the master. The other device is set up in “external oscillator” mode, with its input driven from the first. I looked at the spec sheet quickly, and it looks like XTAL2 is the output and XTAL1 is the input - this means you should connect XTAL2 of the master to XTAL1 of the slave. (you can verify this with an oscilliscope - the output waveform should be the squarer one)
—
On another note, I’ve thought it would be fun to take a 10 x 10 array of these tiny 100 MHz processors and make a 10GHz-equivalent computer on a 4×4″ pc board:
http://www.silabs.com/public/documents/tpub_doc/dshort/microcontrollers/small_form_factor/en/c8051f365_short.pdf
but, with limited communication, it could only run certain apps well.
Posted at 1:56 am on Apr 12th, 2008 by morcheeba