The ZPL—a bootloader for the PIC18Fxxx series—avoids the use of I/O pins by exploiting the *MCLR pin as the only interface between the host PC and the PIC microcontroller. The interface between the PC serial port and the target uses only six components. One of these is the standard *MCLR pull-up resistor. Two others are optional. You could leave out two more resistors, leaving just a single transistor. The interface circuit can be easily put in a DB9 shell, with a DIP clip for the connection to the target. The software runs on Windows and Linux (and other POSIX-compatible systems), and can be easily ported to other operating systems.