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

4.3 Communications Software
The last part of the software section corresponds to the specifics of the communications
channels used in the entire project. This will briefly outline some of the necessary design
choices that were taken along the whole process.
4.3.1 I2C Backbone
We have talked about the hardware considerations of the I 2 C backbone, but nothing has
been mentioned about the software that runs the bus. The I 2 C bus requires two lines to
be routed among all devices which are to be connected to it plus a common ground. This
means that whenever a device needs to talk to another one in the bus, it needs a way to
address it. In fact the I 2 C bus has two different addressing schemes: 7-bit and 10-bit.
The addressing scheme is actually the theoretical limit to the number of devices that can
coexist in the same bus. The first one allows a maximum of 2 7 − 1 = 128 devices while the
second one allows 2 10 − 1 = 1023 devices. Naturally, the second one was chosen to allow
a larger number of devices to be connected to the bus. These numbers are not the actual
allowed limit, because the input capacitance of all pins connected to the bus add to the
total, which as we already mentioned cannot exceed 400 pico-farads. We already mentioned
some ways to get around this limitation, however for the case of the current project, the
real limit to this will be presented in the Results Chapter.
There are however several questions that must be answered though: How are these addresses
assigned? Is it the Bootloader which assigns the address or is it the application code or
both? What happens if there are more than one device in a Skin Patch with the same
address? How is the bus arbitrated?
First of all, the scope of an I 2 C address is limited to the Patch of Skin to which the node is
connected. So although there can’t be two nodes with the same address in the same Patch
of Skin, addresses can be reused if a different Brain is used. In other words, as long as
there are not two nodes with the same address on the same Skin Patch, addresses can be
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