S.N.A.K.E.: A Dynamically Reconfigurable Artificial Sensate Skin ...

arbitrarily assigned. In the specific case of the Skin Patch assembled to test the project, 12
nodes were connected and addresses 0x100–0x10A were assigned, to consequent nodes.
Now, all nodes are first loaded with the Bootloader program described before, however
what was not mentioned before is that when the Bootloader is programmed using the
JTAG debugger, the I 2 C address assigned to the node is assembled into location 0x1080 in
flash memory. This is done so that I 2 C addresses are only assigned once in the programming
lifetime of the nodes, and no conflicts are created in the bus. If two nodes with the same
address are placed on the same bus, the system will eventually reach a deadlock state which
must be manually reset, which is why it is so critical to avoid this situation.
The solution to the last question, ”How is the bus arbitrated?” is actually really simple:
the MSP430 does it. The I 2 C hardware module of the MSP430 microcontroller, can detect
collisions in the bus and has a method to resolve contentions. Whenever a collision is
detected, the device that first takes the data line to ground loses arbitration and an interrupt
flag is generated. Processing this interrupt flag allows communications to remain reliable.
These cases are rare though, as only one master exists in every Skin Patch: the Brain.
The last consideration to make about the I 2 C backbone bus is the speed at which it is run.
This is the second parameter that limits the amount of nodes that can be connected to
the same patch. Although bandwidth requirements are supposed to be brought down with
the addition of processing power into each node, there is still the need to transfer either
higher-order features into a PC, or simple raw data if the Skin Patch is to be used as a data
extraction device. The amount of bandwidth needed will depend on a variety of factors,
like the number of nodes, the sampling rates, and the resolution at which each sensor is
sampled. Making the bus run at fast-speed (3.4Mbps) would have placed huge constraints
on the length of the bus lines, and running it at the standard speed of 100Kbps would not
have been enough for more than a couple of nodes. Therefore high-speed mode was chosen,
and the bus is then run at 400Kbps 13 .
13 Although these are the standard speeds of the I 2 C standard, the I 2 C module of the MSP430 can run
the bus at any arbitrary speed, up to a limit of close to 500Kbps
102