Intro
======

This project contains schematics and firmware for a little
board with an Atmel AVR ATtiny85 microcontroller that
allows you to connect Dallas/Maxim DS18b20 temperature
probes to an USB port on your computer.

Some linux software to read the data from the USB device is also included.

Licence for my own source code is GPL,
Licence for the included libavrusb is their Licence - see the files in the
usbdrv directory.

Features
=========

The attiny85 is very small, it only has 8 pins. All of these pins are used
by this project.
An external crystal is used to generate a 15 MHz clock.
In theory, you can connect an unlimited amount of temperature probes to the
onewire-bus, and read them. However, in reality there are a few limitations:
- There is a maximum number of probes supported set at compiletime. You can
  increase the define in ds1820.c, but be aware that each additional
  probe will require memory, and you will run out of memory at some point.
  You could use an attiny85 (that has the same pinout but more memory)
  to increase the number of probes possible.
- The length of the bus, and the power used by the probes. If the bus gets
  too long, signals and power won't be delivered anymore. The same is true
  for too many probes on the bus.
I currently have NO idea what these limits are - I only tested 2 probes on
a short cable so far. Feel free to send me success-reports.

This project originally used an ATtiny45, which is a slightly smaller
version of the ATtiny85 with 4 instead of 8 KB of Flash. However, it is
no longer possible to fit the code into the smaller ATtiny45, at least not
with more recent gcc and avr-libc versions. The last version that used to
work was the avr-libc 1.4.7 and avr-gcc 4.2.2 (and -Os as a compiler
switch to optimize for size) included with Ubuntu 8.04. Instead of wasting
time getting outdated compiler/library versions to run, just use the
attiny85 instead.

How to use
===========

To make any use of this, you will need:

To compile the AVR firmware: avr-gcc, avr-libc
   use 'make all' to compile.
To compile the linux host software: libusb, libusb-dev
   use 'make hostsoftware' to compile.
For flashing the AVR with avrdude, there is a 'make upload' target.
That target is configured for a STK500 on /dev/ttyS0 - you will have to
adapt the makefile if you use a different programmer and/or port.

For hostsoftware usage information, call it with parameter --help.