This is a simple In-Circuit (ISP) programmer for Atmel AVR microcontrollers.

This circuit connects the PC via its LPT port to four pins of the microcontroller (MOSI, MISO, SCK and RESET). Power is drawn from the target circuit via the 10 pin ISP connector.

Pinout of the ISP connector:
1. MOSI
2. VCC
3. LED
5. RESET
7. SCK
9. MISO
6, 8, 10. GND

When the AVR is not in programming mode the 74HC244 will act as a sort of isolator. This prevents the programmer influencing the target circuit.

The circuit can be used with:
Ponyprog
avrdude
avrstudio
I prefer to use avrdude as it can be integrated in winavr.

Tips for designing circuits with an ISP connector:
1. Insert a resister between reset pin and reset circuit to avoid interference from the reset circuit.
2. Make sure that ISP action doesn't affect any other functions.
3. Don't drive from external circuit during ISP operation, or a resister should be inserted.
4. Don't connect heavy load such as affects ISP function.

If you are looking for a programmer with more options you can always buy a complete kit: CK1705 - Atmel AVR Programmer

1 x DB25 25 pin connector (male)
1 x 74HC244 IC
1 x 1N4148 diode
1 x 100nF condensator
1 x 100 kOhm resitor