Microcontrollers family AT89C has a parallel programming interface of flash memory. To write information, we need to supply programming voltage +12V, and for controlling, almost all pins of ports are used. This is why parallel programming is done in special devices – programmers.
Microcontrollers AT89S, besides parallel programming ability, have ISP programming ability. Using the Serial programming interface doesn’t need +12V of programming voltage because the inside interface is voltage converter included.
When programming AT89S microcontroller using ISP method, there are only four lines needed – RESET, SCK, MOSI, MISO. Using the ISP programming method, it is not necessary to remove the microcontroller from the circuit ISP is so-called in-system programmability. How does it look in-circuit shows the following illustration:
The programmer connects to the microcontroller using the ISP header. Resistors R1-R4 are used to avoid conflicts of signals between the programmer and peripheral circuitry.
ISP programmer can be connected to the LPT port, which controls all programming processes and synchronizes the overall process. The programmer is the same as the AVR ISP programmer, and AT89S microcontrollers can be programmed using popular software like PonyProg or AVRdude.