Simple Universal PIC Programmer
This simple programmer will accept any device that's supported by software (eg, IC-Prog 1.05 by Bonny Gijzen at www.ic-prog.com). The circuit is based in part on the ISP header described in the SILICON CHIP "PIC Testbed" project but also features an external programming voltage supply for laptops and for other situations where the voltage present on the RS232 port is insufficient. This is done using 3-terminal regulators REG1 & REG2. The PIC to be programmed can be mounted on a protoboard. This makes complex socket wiring to support multiple devices unnecessary. 16F84A, 12C509, 16C765 and other devices have all been used successfully with this device....
It’s entirely logical that low-cost miniature microcontrollers have fewer ‘legs’ than their bigger brothers and sisters – some-times too few. The author has given some consideration to how to economise on pins, making them do the work of several. It occurred that one could exploit the high-impedance feature of a tri-state output. In this way the signal produced by the high-impedance state could be used for example as a CS signal of two ICs or else as a RD/WR signal. All we need are two op-amps or comparators sharing a single operating voltage of 5 V and outputs capable of reaching full Low and High levels in 5-V operation (preferably types with rail-to-rail outputs)....