there are a couple of Microchip
MCP2551 CAN transceivers
mounted on the Cerebot. These two
transceivers allow you to set up two
separate CAN networks or a single
two-node CAN network.
Physically, a CAN network is
very simple. A regulation CAN
network is terminated with 120Ω
resistors at both ends. A Cerebot
32MX7 operating in CAN mode
may be in the middle of a CAN
device string or at an extreme end
of the network. Thus, the Cerebot’s
CAN termination resistors are
jumper selectable. The presence of
jumpers JP5 and JP7 in Photo 6
determine if the associated 120Ω
CAN termination resistor is or is not
electrically connected to the CAN
network.
PHOTO 5. Each of the MCP2551 CAN transceivers pictured here is positioned over its
respective interface header. R81 and R91 are 120W CAN termination resistors. You can
see the jumper points that enable termination to the right of each termination resistor.
Cerebot 32MX7
I2C
There are a bunch of really cool
I2C devices out there and it would
be a shame if we couldn’t use them
with the Cerebot. So, the designers
exposed two of the
PIC32MX795F512L’s five available
I2C interfaces to header points. The
first I2C interface (I2C #1) has
permanent pull-up resistors and
drives the 32MX7’s resident
24LC256 EEPROM. I2C #2’s pull-ups
are actually sourcing current mirrors
instead of resistors and can be
inserted and removed with the
placement of a pair of jumpers.
Photo 7 captures both I2C interfaces
with I2C #2’s pull-up current mirrors
enabled.
PHOTO 6. Jumpers JP5 and JP7 enable and disable the 120W CAN termination resistors.
JP6 and JP8 route + 5.0 VDC to the CAN headers.
Cerebot 32MX7
Power
The Cerebot 32MX7 can be
powered externally from the debug
USB connector or from the USB
device connector. A barrel
connector and screw terminal
connector act as inlets for external power. The external
power source should be a regulated 5.0 VDC supply rated
at 500 mA or better. The 500 mA design point is based on
the maximum current a USB device can draw from a host. If
the current consumption of your Cerebot design will exceed
the 500 mA point, Digilent offers an inexpensive 5.0 VDC
switching power supply that is rated at 4,000 mA.
PHOTO 7. The I2C #2 pull-ups are enabled in this shot. IC2 is an 8 MHz resonator that
produces the primary clock for the PIC32MX795F512L.
The Cerebot’s USB, Ethernet, CAN, and I2C hardware is
backed up by free Microchip source code. Drivers and
example source code for all of the aforementioned
hardware entities can be had for a download from the
Microchip website. With that, there’s one more Microchip
firmware-supported Cerebot communications device we
need to examine.
SERVO 07.2011 41
