2 low frequency oscillator, 8 i2c interface, Low frequency oscillator – Digilent 6021-210-000P-KIT User Manual

Page 14: C interface

Universal Development Board™ Reference Manual

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Page 14 of 27

7.2 Low Frequency Oscillator

An unloaded footprint is provided for a 32.768KHzlow frequency oscillator. If desired, an oscillator such as an
Abracon ASVK-32.768ZEC can be loaded at IC1. This is a 3.3V CMOS crystal oscillator in a 7.0x5.0mm package. It
will provide a low frequency time source that can be used either as a low frequency processor clock, or as a time
source for a real time clock calendar (RTCC) peripheral in one of the microcontrollers. The IC1 footprint is on the
bottom of the board in the lower left corner underneath the area where the user LEDs are located.

The output of the low frequency oscillator, labeled CLK32K, appears on pin 14 of header connector J3 along with
the user I/O devices on the board. A jumper wire can be used to route this any suitable location on the board.

To use the low frequency oscillator as a clock input source: Remove the oscillator from IC6; use a jumper wire to
connect J3, pin 14 (CLK32K) to J7, pin 3 (CLKIN); select the appropriate socket using JP7. This applies the 32KHz
oscillator output to the external clock input pin on the PIC microcontroller in the selected DIP socket.

C Interface

The Inter-Integrated Circuit (I

) Interface provides a medium speed (100K or 400K bps) synchronous serial

communications bus. The I

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 for the particular
microcontroller used for detailed information on configuring and using the I

C interface.

The UDB provides one I

C daisy-chain connector, J4. This connector is a standard 2x4 pin header connector with

0.100” spaced pins. It provides access to the I

C signals, SCL, and SDA, plus VCC3V3 and ground. The VCC3V3 can

be used to power external I

C devices. This connector provides two positions for connecting to the I

C bus. By

using two-wire or four-wire MTE cables (available separately from Digilent) a daisy chain of multiple I

C-capable

devices can be created.

The I

C bus uses open collector drivers to allow multiple devices to drive the bus signals. Devices on the bus

actively drive the signals low. The high state on the I

C signals is achieved by pull-up resistors when no device is

driving the lines low. One device on the I

C bus must provide the pull-up resistors.

The UDB provides 2.2K-ohm pull-up resistors. Generally, only one set of pull-up resistors is used on the bus.
Jumpers JP3 and JP4 can be used to disable the on-board pull-up resistors if a different value is needed or some
other device on the bus is providing the pull-ups. The on-board pull-up resistors are enabled by install shorting
blocks on JP3 and JP3. Removing the shorting blocks disables the pull-up resistors.