Digilent Nanocon - Manual (Page 4)

Digilent, Inc. Nanocon Reference Manual
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position for in-system-programming, and in
the SS position for user access to the SPI
port.
AVR Clock Fuse Settings
AVR microcontrollers use control bits called
fuses to set basic operating parameters for the
device. The SPI controller uses the clock
source set by the fuses for its clock. If the clock
source fuses are set to select a clock source
that doesn’t exist on the board, the SPI
controller won’t work and it will no longer be
possible to program the microcontroller via the
in-system programming protocol. The Nanocon
can only use the internal RC oscillator as the
clock source.
If the external oscillator, or one of the
crystal/resonator clock sources is selected, it
may be possible to recover the board by
applying a suitable clock signal to pin 1 of
connector JA. There is an applications note on
the Digilent web site illustrating this technique
for the Cerebot board.
In addition, the maximum SPI clock frequency
is the selected clock frequency divided by four.
If the 128KHz internal oscillator is selected as
the clock source, the SPI clock would need to
be set to a frequency of 32KHz or less. The
Digilent programming cables do not support
frequencies that low, so if the 128KHz internal
oscillator is selected for the clock source, the
board will no longer be programmable using
the Digilent programming cable.
Two-Wire Serial Interface
The Atmel TWI interface is a medium
speed (400 Kbps), synchronous, serial,
communications bus. The TWI interface
supports master or slave operation with up
to 128 devices on the bus. Each device is
given a unique address, and the protocol
has the ability to address packets to a
specific device or to broadcast packets to
all devices on the bus. For detailed
information on configuring and using the
two-wire interface see the ATmega168
data sheet at
www.atmel.com .
The Nanocon has two ways to connect to a
TWI bus. The TWI signals, SCL and SDA,
are available on 6-pin connector JA or on
the 2-pin connector J2.
Pins one and two of connector JA and
connector J2 provide two positions for
connecting to the TWI signals. By using
two-wire cables (available separately from
Digilent), a daisy chain of Nanocons or
other TWI-capable boards can be created.
The TWI bus is an open-collector bus.
Devices on the bus actively drive the
signals low. When no device is driving the
lines low, pull-up resistors achieve the high
state on the TWI lines. A single device on
the TWI bus must provide the pull-up
resistors.
The Nanocon provides pull-up resistors
that are controlled by software. I/O port B,
bits 6 and 7 (PB6 and PB7), is connected
to the pull-up resistors. To enable the pull-
ups, configure these pins as outputs and
set the I/O port output bits to “1”. To
disable the pull-ups, configure these pins
as inputs with the internal pull-ups
disabled. Both TWI pull-ups should be
enabled or disabled together. Only one
device on the TWI bus should have pull-
ups enabled.
A port bit is configured as an input or an
output by setting the corresponding bit in
the DDR register. The pin becomes an
output by writing a “1” and an input by
writing a “0”. When a pin is configured as
an input, an internal pull-up resistor is
enabled by writing the corresponding
output port bit to “1” and disabled by writing