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

3.4.3 Communications unit
The Communications unit is made up by the I 2 C bus that connects it to a Skin Patch, and
the USB link that connects it to a PC. Furthermore, the I 2 C module is composed of the
active pull-up devices described before, a header to connect the brain to a Skin Patch, and
the I 2 C module of the MSP430 microcontroller. The USB module is formed by the USB
controller circuitry, the USB headers, and an external crystal.
Active pull-up devices were created with two XP02401 dual PNP transistors connected
as a current mirror, and a biasing resistor. The biasing resistor was chosen so that the
current injected by the current mirror to each one of the two I 2 C lines was not over the
allowed maximum input current of the MSP430 microcontroller (2mA). This can be easily
calculated as follows:
Imax > Vcc − 0.6V
Rmin
3.3V − 0.6V
Rmin >
2ma
(3.2)
(3.3)
Rmin > 1350Ω (3.4)
A 1.5KΩ resistor was then used for biasing the current mirror.
USB communication was achieved thanks to FTDI’s FT245BM USB chip. This device is a
USB to FIFO interface chip, that allows any microcontroller to easily have USB connectivity.
The device requires its own clocking source, so a 6.000Mhz crystal is also included. Also,
if the device is to be named and given a serial number 21 then an EEPROM must be used,
and so a 93LC46B EEPROM chip from Microchip was also included. FTDI provides a very
detailed design guide[12] for their chips, the Brain’s USB circuitry is based on their “self
powered 3.3V example”.
Finally, it would be prudent to mention that if multiple Brains will be connected to a PC,
21 This is usually done so that multiple devices of the same type can be later identified
76