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ページを印刷するCerebot II Reference Manual
TWI-1
TWI-2
Pull-ups
Enabled
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.
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TWI-1
TWI-2
Pull-ups
Disabled
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
1.
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
Pmod connector JF. To use the LEDs, set the
desired bits as outputs by setting the
corresponding bits in the port E data direction
register (DDRE) and set the bits to the desired
level in the port E output register (PORTE).
Setting a bit to 1 will illuminate the LED and
setting the bit to 0 will turn it off.
Digilent, Inc.
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