Adds Electronics tips

electronics.md

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+# Electronics
+
+Some basic tipe for new embedded Eletronics programmers
+
+## Use of DDx, PORT and PIN registers:
+
+- **DDx**: Choice of pins state: Inputs or Outputs
+- **PORT**: Set the outputs state, rules the outputs (ex : set Pin to 0)
+- **PIN**: Reads the inputs states, or toggle values. (ex: Toogle Pin)
+
+
+## Easily set a bit to 0 in a register:
+
+PPR=PPR & 0b01111111
+
+## Easily set a bit to 1 in a register:
+
+PPR=PPR & 0b01111111
+
+## Debug hardware errors after optimization:
+
+In case your C code does not work any more after Optimization (-O2 -O3)
+
+- Check if global variables are used in interruptions
+- Try to set them as volatile
+
+## Hazardous hardware resets:
+
+**Check if your interruptions are initialized in your code !**
+
+Otherwise, the C code will automatically jump to a non-existing address, causing the reset.
+
+## I/O problems:
+
+In case of I/O problems in your code, **check your Hardware and especially your resistors**
+
+Try to unsolder them and run the code.
+
+## Embedded electronics main architecture code:
+
+Generally, embedded architecture software is divided into 3 main layers :
+
+- **Drivers:** Whose job is to interact with Hardware at a low level (Ex : Read spi data)
+- **Services/Components:** Intermediate layer, interface between drivers and Applications
+- **Application:** Higher layer, which aims at achieving the main purpose of the project (Ex: Calculate movement using an accelerometer)
+Here is a simple drawing to get the idea:
+
+![Embedded Architecture Drawing](https://dl.dropbox.com/u/4286043/01_Git/Embedded_architecture.png)
+
+__**NOTE:**__ The main idea behind that when you code is the reuse! The application may not be reusable, but drivers and components must !
+
+If you did good, you should be able to use **the same application** in two architectures, **changing only components and drivers**.
+