1 eeprom – Digilent 410-211P-KIT User Manual

Page 8

chipKIT™ Network Shield™ Board Reference Manual

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Page 8 of 10

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 I

C#1 bus.

One I

C device is provided on the Network Shield, a 256Kbit EEPROM connected to the I

C #1 bus.

Connectors J6 and J7 can be used to extend the I

C busses off of the board to connect to external I

C devices.

These are standard 2x4 pin header connectors with 0.100” spaced pins. They provide access to the I

C signals, SCL

and SDA, plus VCC3V3 and ground. The VCC3V3 can be used to power external I

C devices.

The I

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

C #1. Because I

C #1 is the bus with the EEPROM, these pull-up resistors are permanently

connected.

Jumpers JP9 & JP12 let I

C #1 be disconnected from the Network Shield if it is not in use but is interfering with the

associated pins. There are cuttable 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

C #2 by installing jumper wires between connector J6 and J7. The

EEPROM will then appear on I

C #2 bus.

The logic high pull-up for I

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

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 is used on the bus. Jumpers JP10 and JP11 can be used to disable the on-board
pull-ups on I

C #2 if a different value is needed or some other device on the bus is providing the pull-ups or if I

C #2

isn’t being used and the pull-ups are interfering with the use of the pins. The on-board pull-ups are enabled by
install shorting blocks on JP10 and JP11. Removing the shorting blocks disables the pull-ups.

Digilent has several small I/O peripheral modules (Pmods™) available that can be connected using the I

connector. These include a 3-axis accelerometer, a 4-channel 12-bit A/D converter, a serial character LCD panel, a
3-axis gyroscope, a real-time clock/calendar, and an I/O expander.

2.5.1 EEPROM

A 256Kbit (32Kbyte) I

C EEPROM is provided using a Microchip 24LC256. This EEPROM, IC5, is located on the

bottom of the board.

The EEPROM is on the I

C #1 bus, and its 7-bit I

C device address is '1010000'.

Digilent has a library for using the EEPROM. It is contained in document # DSD-0000311 (chipKIT IOShield
Library.zip) which can be downloaded from the Basic I/O Shield product page at

www.digilentinc.com

. The

EEPROM library is IOShieldEEPROM.

For details on the 24LC256, see the data sheet at