The ESP-03 has two useful modes of operation that can be initiated by controlling its GPIO pins – normal mode and flash mode.
Normal mode wiring is show in the figure below. ESP-03 shown here is powered using SparkFun’s FTDI Basic Breakout – 3.3V – USB to serial module.
In this mode, ESP-03 executes firmware programmed to the SPI Flash. SPI Flash is an external NOR Flash chip where program instructions are stored, and retrieved during execution.
The ESP-03 has a 4 Mbit 25Q40BT part which allows for 512 KB of program space. Of that, about 423 KB is available for your own programs.
In flash mode, new program instructions can be flashed to SPI Flash, using tools such as the ESP Flash Download Tool. The wiring is similar to that for normal mode, with the addition of GPIO0 connected to GND.
Adafruit provides instructions for configuring the Arduino IDE for ESP8266 development. Here’re the settings I use with the Arduino IDE – under Tools menu.
Try the example project under File -> Examples -> ESP8266WiFi. With it, you’ll be controlling a GPIO pin on the ESP-03, over your Wi-Fi network, in no time at all.