ICSP

ICSP, short for In-Circuit Serial Programming, is a programming interface used to program microcontrollers while they remain installed in the target circuit. It is most closely associated with Microchip's PIC family, but similar in-circuit programming techniques are used by other vendors.

The ICSP interface exposes a small set of signals: Vdd (power), Vss (ground), PGD (data), PGC (clock), and Vpp/MCLR (programming voltage). Some devices also provide PGM for low-voltage programming. In practice, a 5- or 6-pin header is wired between the target board and a programming tool such as a PICkit, MPLAB REAL ICE, or third-party programmers. The protocol is built around a serial data stream delivered over PGD with timing driven by PGC, and the tool supplies the necessary MCLR Vpp to enter programming mode.

Using ICSP, the tool can erase or program the device’s flash and configuration words, read device IDs, verify written data, and in some devices modify fuses or oscillator settings. Programming is typically performed with the MCU powered and in-circuit, though some devices require powering through the programmer or using high-voltage programming modes.

Tools range from inexpensive USB programmers to full-featured debuggers. In-circuit debugging or emulation capabilities (ICD/ICE) are related concepts but provide interactive debugging beyond basic programming.

ICSP has been integral to Microchip's programming ecosystem since the 1990s and remains a standard means of firmware updates and recovery for PIC microcontrollers and compatible devices.