AVR has two programming modes: Mode Programming Parallel (Mode Parallel) and Serial Downloading Mode (ISP mode).
At Mode Parallel, device which will be program placed at programming socket and required tension 12V at pin RESET. Communications between programmers and device is done with instruction of programming in parallel. Programming speed of twice the quickly is compared to ISP Mode. Way of this programming usually applied in pre-program and/or ISP Mode cannot be applied by factor design. Most of the programmer, except STK500 seen to be doesn't support this mode.
At ISP MODE, device communicates through SPI interface to programming and verification. This mode only requires three lines signal programming without tension of 12 Volts, causing earns target system program direct - ISP ( In-System Programming). But, ISP MODE cannot change fuse bit at some device, and some device doesn't have features ISP. Device like this must be program in parallel.
When using AVR with ISP Mode, target of board must be designed by considering function of ISP, what applies pin ISP: RESET, SCK, MISO and MOSI. Pin ISP can share between function of I/O and ISP satisfactorily. When using pin I/O having function of ISP, hopes paid attention of this things:
a. Inserts a resistor between pins RESET and reset network to avoid interference from reset network.
b. Ensures that action ISP doesn't generate effect at other function.
c. Men-drive doesn't from external network when operation ISP takes place, or if desired that way, need to be inserted a resistor.
d. Doesn't attribute to heavy burden because will influence function of ISP.
