Boot Mechanism
On power up, brownout detection, software reset, or external hardware reset, the Propeller will load a machine-code boot routine from the internal ROM into the RAM of its first (primary) cog and execute it. This code emulates an I2C interface in software, temporarily using two I/O pins for the needed serial clock and data signals to load user code from an external I2C EEPROM.
Simultaneously, it emulates a serial port, using two other I/O pins that can be used to upload software directly to RAM (and optionally to the external EEPROM). If the Propeller does not see any commands from the serial port, it will load the user program (the entry code of which must be written in SPIN, as described above) from the serial EEPROM into the main 32K RAM. After that it will load the SPIN interpreter from its built-in ROM into the dedicated RAM of its first cog, thereby overwriting most of the bootloader.
Regardless of how the user program is loaded, execution starts by interpreting initial user bytecode with the SPIN interpreter running in the primary cog. After this initial SPIN code runs, the application can turn on any unused cog to start a new thread, and/or start assembler code routines.
Read more about this topic: Parallax Propeller
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