# OSTC4 Code Modules #+
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## Main CPU - aka CPU1-Discovery ##+
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For historical reasons, the firmware of the main CPU was divided in three parts.+
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It helps development, allow smalled upgrades, and improve security as a bad manipulation won't kill the bootloader, responsible fot the next firmware upgrade.+
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### Bootloader ##+
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The code responsible to upload new firmwares for both CPU.+
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### Font Pack ###+
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This module does not contains any code. Just font data and images.+
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### Firmware code ###+
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The main OSTC4 firmware. The more important modules are:+
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- Buehlmann decompression algorithm.+
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- VPM decompression algorithm.+
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- The simulator and dive planner.+
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- Logbook display (surface and dive mode).+
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- All user interface code, during dive or surface mode (`t*.c` files).+
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- Graphics (gfx) engine.+
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- Plus interface to the other CPU, the management of the _LCD_ screen, and the optional _Bonex_ scooter interface (No longer supported)+
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## Small CPU - aka CPU2-RTE ##+
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The _Real Time_ CPU handle everything that requires precise timming to function perperly.+
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The most important modules are:+
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- Real time computation of gas exchanges during dive (aka the _LifeData_ structure).+
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- Analog to digital conversion for the ... ambient light sensor.+
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- I2C bus interface to the pressure sensor, the magnetic compass and the battery gauge.+
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- SPI bus interface to the buttons and to the other CPU.+
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- Serial interface to the _RS232-over-Wireless_ connection.+
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