User manual

chipKIT Network Shield Reference Manual
www.digilentinc.com page 8 of 11
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The CAN network standard requires that the
nodes at each end of a network provide 120
ohm termination. The Network Shield provides
the termination resistors and jumpers to
enable/disable them depending on the location
of the board in the network. Jumper JP6 is
used to enable/disable the termination resistor
for the CAN1 network, and JP8 is used to
enable/disable the termination resistor for
CAN2. Install a shorting block on the jumper
pins to enable the termination resistor, or
remove the shorting block to disable the
termination resistor.
I
2
C Busses and Connectors
The Inter-Integrated Circuit (I
2
C
TM
) Interface
provides a medium speed (100K or 400K bps)
synchronous serial communications bus. The
I
2
C interface provides master and slave
operation using either 7 bit or 10 bit device
addressing. Each device is given a unique
address, and the protocol provides the ability
to address packets to a specific device or to
broadcast packets to all devices on the bus.
Refer to the Microchip PIC32MX7XX Data
Sheet and the PIC32 Family Reference
Manual for detailed information on configuring
and using the I
2
C interface.
The PIC32MX795 microcontroller on the
Max32 provides for up to five independent I
2
C
interfaces. The Network Shield is designed to
provide access to two of these interfaces I
2
C
#1 (SCL1, SDA1) and I
2
C #2 (SCL2, SDA2).
I2C #1 is the bus accessed through the
standard chipKIT Wire library. There are two
sets of connectors on the board for access to
the two I
2
C ports. Connector J7 provides
access to I
2
C port #1 while connector J6
provides access to I
2
C port #2.
The user should note that external interrupt 3
and SCL1 share the same pin on the
PIC32MX795. External interrupt 4 and SDA1
also share the same pin. Therefore, external
interrupts 3 and 4 should not be used
simultaneously with I2C bus #1.
One I
2
C device is provided on the Network
Shield. This is a 256Kbit EEPROM connected
to the I
2
C #1 bus.
I
2
C Connectors: Connectors J6 and J7 can be
used to extend the I
2
C busses off of the board
to connect to external I
2
C devices. These are
standard 2x4 pin header connectors with
0.100” spaced pins. They provide access to
the I
2
C signals, SCL and SDA, plus VCC3V3
and ground. The VCC3V3 can be used to
power external I
2
C devices.
The I
2
C bus uses open collector drivers to
allow multiple devices to drive the bus signals.
This means that pull-up resistors must be
provided to supply the logic high state for the
signals. The Network Shield provides 2.2Kohm
pull-up resistors on I
2
C #1. As I
2
C #1 is the bus
with the EEPROM, these pull-up resistors are
permanently connected.
Jumpers JP9 & JP12 are provided to allow I
2
C
#1 to be disconnected from the Network
Shield, if it not being used and is interfering
with the use of the associated pins. There are
cut-able traces on the underside of the board
between the pins of these jumpers. Cut these
traces to disconnect SCL1 and SDA1 from the
Network Shield. To restore the connection,
load two pin headers for JP9 and JP12 and
install shorting blocks. If this is done, it is still
possible to access the on-board EEPROM by
connecting SCL and SDA from I
2
C #2 by
installing jumper wires between connector J6
and J7. The EEPROM will then appear on I
2
C
bus #2.
The logic high pull-up for I
2
C #2 is provided by
sourcing current mirrors instead of resistors.
These current mirrors source approximately
1.7mA. The use of current mirrors provides
faster rise times on the I
2
C signals and
provides the ability to drive longer cable runs
reliably than would be the case with simple
pull-up resistors.
Generally, only one set of pull-ups are used on
the bus. Jumpers JP10 and JP11 can be used
to disable the on-board pull-ups on I
2
C #2 if a