Add README.md

Author: matthijskooijman

README.md

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+Standalone AVR programmer
+=========================
+This sketch can be used to let one Arduino program a sketch or
+bootloader into another one, using the ISP programming pins.
+
+This sketch is based on [OptiLoader][], but modified for use with
+Adafruit's adaloader and to be used without a serial connection. It is
+intended to be used with [Adafruit's standalone programmer kit][kit] (a
+protoshield with ZIF socket, buttons, leds and a buzzer), but it should
+be usable with any Arduino wired up to any AVR chip (respecting the
+pinout, see below).
+
+[OptiLoader]: https://github.com/WestfW/OptiLoader/
+[kit]: https://www.adafruit.com/product/462
+
+The sketch to be flashed is stored in the `images.cpp` file inside this
+sketch. By default, it contains the adaLoader bootloader, but you can
+replace it with your own bootloader or sketch as well. To do so,
+compile that sketch into a .hex file (using the Arduino IDE, you can
+use "Sketch" -> "Export compiled Binary" to get the compiled .hex file)
+and paste its contents (it's just a text file) into `images.cpp` (see
+that file for details). There you can also configure the target chip
+signature and fuse settings.
+
+For more info on setting this up, see [Adafruit's tutorial][tutorial].
+
+[tutorial]: https://learn.adafruit.com/standalone-avr-chip-programmer/overview
+
+Pinout
+------
+To use this sketch, you can use the following pinout. Here, "programmer"
+means the Arduino running this particular sketch, "target" means the
+chip being programmed. For reference, this also lists pin numbers for an
+Arduino Uno programmer and bare atmega328p target chip, adapt these if
+you use a different setup.
+
+| Programmer pin | Uno pin number | Connects to
+| -------------- | -------------- | --------------
+| Digital pin 10 | 10             | Target RESET (atmega328p pin 1)
+| MOSI           | 11             | Target MOSI (atmega328p pin 17)
+| MISO           | 12             | Target MISO (atmega328p pin 18)
+| SCK            | 13             | Target SCK (atmega328p pin 19)
+| Digital pin 9  | 9              | Target XTAL1 (optional, atmega328p pin 9)
+| Digital pin 8  | 8              | Error LED (active high)
+| Analog pin 0   | A0             | Activity LED (active high)
+| Analog pin 1   | A1             | Button (active low, internal pullup enabled)
+| Analog pin 3   | A1             | Piezo (outputs 4kHz square wave)
+
+Leds & buzzer
+-------------
+On startup, both leds blink twice and then turn off.
+
+When the target is being programmed the activity led turns on. Once it
+is done, the led turns off and the piezo makes a short beep.
+
+When an error occurs during programming or verification, the error led
+turns on and stays on and the the piezo makes a continuous beep.
+
+Clock output
+------------
+This sketch generates an 8Mhz clock as an extra utility. This can be
+used if the target chip has fuse settings that expect an external clock
+(or crystal, that also seems to work). If you are only configuring the
+fuses to use the internal oscillator, this is not needed.
+
+Button
+------
+A single button can be connected, which can be used to start programming
+the next chip (including the first). Alternatively, you can send a
+command (uppercase `'G'`) through serial to start programming.
+
+Autostart
+---------
+This sketch also supports autostarting the programming when a chip is
+connected. This works by detecting the (internal or external) pullup on
+the reset line. To make sure that the pin reads LOW when nothing is
+connected, this needs a big pulldown on the ISP reset line (so between
+digital pin 10 on the programmer and GND). It should be big enough to
+not interfere with the target's reset pullup, so something like 1M
+should be ok.
+
+To enable autostart, add the pulldown resistor and set `AUTOSTART` to
+`1` in the .ino file.
+
+Now, when you insert a chip and the reset line is stable for a short
+while, the programming will automatically start. If the programmer is
+powered up or reset while a chip is inserted, it must be removed and
+re-inserted to start programming (of course, you can still use the
+button or serial as well).