Digilent Cerebot II Board Manuel d'utilisateur télécharger pdf (Page 6)

Cerebot II Reference Manual Digilent, Inc.

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up resistors when no device is driving the lines

low. One device on the TWI bus must provide

the pull-up resistors. The Cerebot II board

provides pull-up resistors that can be enabled

or disabled via jumper blocks on the ‘pull-up’

positions on J3. The pull-ups are enabled by

installing jumper blocks on J3 and are disabled

by removing the jumper blocks. The shorting

blocks are placed so that they line up with the

SCL and SDA labels on the board. Only one

device on the bus should have the pull-ups

enabled.

SCL

SDA

TWI-1

TWI-2

SCL

SDA

TWI-1

TWI-2

Pull-ups

Enabled

Pull-ups

Disabled

Jumper Settings for TWI Pull-Up Resistors

Connectors J4 and J5 are provided for daisy-

chaining power along with the TWI signals.

These can be use to pass power from the

Cerebot II to other devices on the TWI bus.

Either of these connectors could also be used

to provide power to the Cerebot II from some

other board on the TWI bus.

Crystal Oscillator

The ATmega64 microcontroller supports

numerous clock source options for the main

processor operating clock. The Cerebot II has

an 8MHz oscillator crystal for use with the

crystal oscillator option. The Cerebot II comes

from the factory with the external crystal

oscillator source selected.

The ATmega64 microcontroller also provides

an internal RC oscillator that can operate at

nominal frequencies of either 1MHz, 2MHz,

4MHz, or 8MHz. This internal oscillator has a

frequency variability of approximately 2-3%,

which is suitable for many applications.

The RC oscillator’s nominal frequency

assumes operation at 5V. The Cerebot II

normally operates at 3.3V. See the oscillator

frequency vs. supply voltage chart in the

ATmega64 data sheet to determine the

nominal frequency at 3.3V.

Although the Cerebot II will normally be

operated using the 8MHz crystal oscillator, the

internal oscillator can be selected to operate

the board at a lower frequency if desired. The

clock source to be used by the board is chosen

using the fuse settings in the in-system

programmer.

When changing the clock source fuse settings,

it is extremely important to ensure that the

clock source chosen actually exists on the

board (i.e., only choose the crystal oscillator or

internal oscillator). The ATmega64 internal in-

system-programming state machine operates

from the selected clock source and if an

unavailable clock source is selected, the board

may no longer be programmable.

In some cases, it is possible to recover the

board if an improper clock source is chosen,

but not always. There is an applications note

“Cerebot Clock Source Fix” (available from

www.digilentinc.com) that explains the

procedure for recovering a board that has had

an improper clock source programmed.

User I/O Devices

The Cerebot II board has a two-position jumper

for user input and four LEDs for output. The

user input jumper, JP5, is connected to I/O port

G, bit 4. To read this jumper, bit 4 of port G

must be set as an input by clearing bit 4 in the

port G data direction register (DDRG) and

reading the pin register for port G (PING).

When the shorting block is in the ‘0’ position,

bit 4 in the pin register will be 0. When the

shorting block is in the ‘1’ position, bit 4 will be

The four LEDs are connected to bits 4-7 of I/O

port E. LED 1 is connected to bit 4, LED 2 is

connected to bit 5, and so on. These four bits

are also shared with the four I/O signals on

1 2 3 4 5 6 7 8 9 10

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Digilent Cerebot II Board Manuel d'utilisateur Page 6

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