Here's a very informative posting from Craig L. discussing the port assignments of the ATMEL AVR...


I've been doing some analysis of the AVR Butterfly to see what
usefulness it might have as a robotics controller and thought I would
share the results with everyone. Everything I found I got from the
schematics in the butterfly's userguide I downloaded from Atmel.
It was a pretty brief look so if anyone finds any booboos let me know
and I will post a correction.

The big thing I was looking for was useful I/O pins, especially
I/O pins with analog to digital converters or pulse width modulation
channels.

Two 8-bit ports (with associated I/O pins) have connectors that we
can get at: port B and port D. Unfortunately, those ports are
shared with lots of other (but useful) stuff.

All four PWM channels are either shared with the navigation switch or
the piezo speaker. We are stuck with the speaker unless we disconnect
it. If we want to use the remaining PWM channels we just have to make
sure that no stupid humans fiddle with the button at the same time.
I would add an external switch that is monitored by the AVR through
one of the other usable ports and determines whether the motors are
enabled or not. You could end up with a short if the PWM channels are
in use and the user starts navigating with the switch.

The rest of port B is shared with the ISP port (which isn't a big
deal as long as we use those pins for output only), the navigation
switch or the dataflash.

Port D also has a connecter but those pins are shared with the LCD.
According to the schematic they are connected but I am not sure if the
LCD actually uses them. If they are used by the LCD then that entire
port will be unusable unless the LCD is disabled.

However, all is not lost. There are 4 pins on the JTAG port that are
useful (PF4-7). They are also ADC channels. That's cool.

The USI interface is also open for us (pe4-6). The UART pins,
however, have voltage converters attached going to j406. This
converts the serial voltages to the usual +12/-12 so we can hook it up
to a PC. If we want to run that to a low voltage serial device we
would need to keep the high voltages in mind.

The dataflash is tres cool. It's accessible through the SPI interface
(you must disable the AVR's SPI interface before you can do that but
getting back from that state could be challenging) and, of course,
you can get at it through the AVR. It has 4 megabits of data or 512
kliobytes. You could, say, have the butterfly record the temperature
every 60 seconds for an entire year. You will overwrite the
tunes that comes with the butterfly if you do that.

Overall, we don't have a huge number of ports to work with but with
clever design it can still be quite useful.

Here is a little table that shows the pin -> device/connector mappings.

Port refers to the AVR I/O port as described in the atmega169 docs.

Purpose is the pin's function. SegXX refers to the attached LCD segment.

Connector refers to the holes on the butterfly that we can solder headers
into.

Device refers to the device on the butterfly.

Note that some I/O pins have multiple uses. i.e. they are hooked up to
a port and a device.

There are 7 groups of locations where we can solder in headers. Below is
a little table that describes the header group (jXXXX) and what it does.