added cheatsheet avr_basics.md

cheatsheets/embedded/avr_basics.md

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+---
+title: AVR Basics
+categories: [cheatsheets, embedded]
+tags: [avr, reversing, basics]
+---
+
+# AVR Basics
+
+## Port Operation Registers
+
+* DDRx  = Data Direction Register
+* PORTx = Pin Output Register
+* PINx = Pin Input Register
+
+
+### DDRx - Data Direction Register
+
+This register is used to tell if a output PIN is used as input or output.
+
+* 1 = Output
+* 0 = Input
+
+**The register looks like this**
+
+```
+| Bit No. | 7    | 6    | 5    | 4    | 3    | 2    | 1    | 0    |
+| Name    | DDx7 | DDx6 | DDx5 | DDx4 | DDx3 | DDx2 | DDx1 | DDx0 |
+| Init.   | 0    | 0    | 0    | 0    | 0    | 0    | 0    | 0    |
+```
+
+The register can be set like this:
+
+```C
+// E.g. for Port C
+DDRC = (1<<DDC0)|(1<<DDC4)|(1<<DDC5)|(1<<DDC7);
+
+// the same as
+DDRC = 0x1B;
+
+// in Binary: 0b10110001
+```
+
+**That means:**
+
+* PC7 = Output(1)
+* PC6 = Input(0)
+* PC5 = Output(1)
+* PC4 = Output(1)
+* PC3 = Input(0)
+* PC2 = Input(0)
+* PC1 = Input(0)
+* PC0 = Output(1)
+
+
+PINs marked as Input Pins have the capability to _read_ the voltage-level at that PIN.
+In contrast, PINs marked as _Output_ Pins can be set to either _HIGH_ or _LOW_.
+
+
+### PORTx -  Pin Output Register
+
+This Register determines whether the output of the corresponding output pins should be _HIGH_ or _LOW_.
+
+**The register looks like this**
+
+```
+| Bit No. | 7    | 6    | 5    | 4    | 3    | 2    | 1    | 0    |
+| Name    | Px7  | Px6  | Px5  | Px4  | Px3  | Px2  | Px1  | Px0  |
+| Init.   | 0    | 0    | 0    | 0    | 0    | 0    | 0    | 0    |
+```
+
+* Set the PINS:
+```C
+PORTC = (1<<PD0)|(1<<PD3)|(1<<PD6);
+```
+
+**Info:** `PORTx` can only modify the PINS that are marked as output pins with `DDRx`.
+
+
+### PINx - Pin Input Register
+
+In contrast to the `PORTx` register, this register is used to read the state of the corresponding pin `x`.
+
+```
+| Bit No. | 7    | 6    | 5    | 4    | 3    | 2    | 1    | 0    |
+| Name    | Px7  | Px6  | Px5  | Px4  | Px3  | Px2  | Px1  | Px0  |
+| Init.   | 0    | 0    | 0    | 0    | 0    | 0    | 0    | 0    |
+```
+
+To read the PINS state:
+
+```C
+if (PINC == 0b01000000)
+    PORTC = 0x0B;
+else
+    PORTC = 0x00
+```
+
+**Info:** As already noted aboved, `PINx` can only read the PINS that are marked as input pins with `DDRx`.
+
+
+