Update tuto_nmigen.md, added part Transition to Nmigen

tuto_nmigen.md

@@ -39,3 +39,41 @@ Note that you need to replace these terms :
 *- ICEBreakerPlatform -> ICEStickPlatform*
 
 *- led = platform.request("led_r") -> led = platform.request("led")*
+
+## Transition to Nmigen
+
+### From variables to signals
+
+Nmigen uses **Constants** and **Signals**.
+
+**Constants** have a fixed value that cannot change.
+
+Regarding the **Signals**, they can replace Python's **Variables**.
+
+When using **Constants** and **Signals**, the number of bits used can be specified, aswell as whether it is signed or not. By default, it uses the least possible amount of bits to represent the value.
+
+#### Constructors :
+
+`a = Const(10, unsigned(16))`
+
+`b = Signal(signed(4))`
+
+#### Warning :
+
+**Constants** and **Signals** are encoded on a limited number of bits, leading to issues when representing floats.
+
+To face this issues, we chose to left shift all our values by 16 bits, effectively multplying them by 2<sup>16</sup>.
+
+A float *x* is represented by the closest integer to _x*2<sup>16</sup>_, allowing us to keep track of the decimals.
+
+This changes the way we do calculations :
+
+>0.8 + 0.5 = 1.3 => 52 429 + 32 768 = 85 197
+>
+>0,8 * 0,5 = 0,4  =>  52 429 * 32 768 = 1 717 993 472
+
+The sum of 2 numbers left shifted by 16 bits gives us the correct result left shifted by 16 bits.
+
+The product of 2 numbers left shifted by 16 bits gives us the correct result left shifted by 32 bits.
+
+Since we dicided to store our values left shifted by 16 bits, we will need to right shift the result of our products before storig it.