Add interrupt notes

interrupts/pin-interrupt-notes.md

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+# External Interrupt Configuration (ATmega328P)
+
+## SREG (Status Register)
+
+SREG is AVR status register with 8-bit status:
+
+- Bit 0 → Carry Flag (C) – for ALU operations  
+- Bit 1 → Zero Flag (Z) – result is zero  
+- Bit 2 → Negative Flag (N) – result is negative  
+- Bit 3 → Two’s Complement Overflow Flag (V) – arithmetic overflow  
+- Bit 4 → Sign Bit (S) – N ⊕ V  
+- Bit 5 → Half Carry Flag (H) – carry from bit 3 to 4  
+- Bit 6 → Bit Copy Storage (T) – used in bit operations  
+- Bit 7 → Global Interrupt Enable (I) – Important for interrupt  
+
+### Enable Global Interrupt
+    SREG |= (1 << 7);
+    // or
+    sei();
+
+### Disable Global Interrupt
+    SREG &= ~(1 << 7);
+    // or
+    cli();
+
+---
+
+## External Interrupt Pins
+
+- PD2 → INT0  
+- PD3 → INT1  
+
+---
+
+## EICRA (External Interrupt Control Register A)
+
+Controls how external interrupts trigger.
+
+### ISC (Interrupt Sense Control)
+
+| ISCx1 | ISCx0 | Mode               |
+|------|------|--------------------|
+| 0    | 0    | Low Level          |
+| 0    | 1    | Any Logical Change |
+| 1    | 0    | Falling Edge       |
+| 1    | 1    | Rising Edge        |
+
+### Bit Positions
+
+- INT0 → ISC01 (bit1), ISC00 (bit0)  
+- INT1 → ISC11 (bit3), ISC10 (bit2)  
+
+---
+
+# Interrupt Trigger Modes (INT0)
+
+## 1. Low Level (00)
+
+    EICRA &= ~((1 << ISC01) | (1 << ISC00));
+
+Bit calculation:
+- (1 << ISC01) = 00000010  
+- (1 << ISC00) = 00000001  
+- OR            = 00000011  
+- NOT           = 11111100  
+- Result        → ISC01=0, ISC00=0  
+
+---
+
+## 2. Any Logical Change (01)
+
+    EICRA &= ~(1 << ISC01);
+    EICRA |=  (1 << ISC00);
+
+Bit calculation:
+- Clear ISC01 → 0  
+- Set ISC00   → 1  
+- Result      → 01  
+
+---
+
+## 3. Falling Edge (10)
+
+    EICRA |=  (1 << ISC01);
+    EICRA &= ~(1 << ISC00);
+
+Bit calculation:
+- Set ISC01   → 1  
+- Clear ISC00 → 0  
+- Result      → 10  
+
+---
+
+## 4. Rising Edge (11)
+
+    EICRA |= (1 << ISC01) | (1 << ISC00);
+
+Bit calculation:
+- (1 << ISC01) = 00000010  
+- (1 << ISC00) = 00000001  
+- OR            = 00000011  
+- Result        → ISC01=1, ISC00=1  
+
+---
+
+## INT1 (Same Logic)
+
+Replace:
+- ISC01 → ISC11  
+- ISC00 → ISC10  
+
+Example (Rising Edge INT1):
+
+    EICRA |= (1 << ISC11) | (1 << ISC10);
+
+---
+
+# Practical Note (Real Hardware Behavior)
+
+- Button press is not 100% stable  
+- Causes:
+  - Mechanical bounce  
+  - Loose wiring / plug-in noise  
+  - Floating inputs  
+
+### Recommendation
+
+- Avoid: Any Logical Change (too sensitive)  
+- Prefer:
+  - Falling Edge (more stable detection)  
+  - Or implement HOLD logic in software  
+
+---
+
+## EIMSK (External Interrupt Mask Register)
+
+Controls enabling/disabling of interrupts.
+
+### Bit Positions
+
+- Bit 0 → INT0 enable  
+- Bit 1 → INT1 enable  
+
+---
+
+## Enable / Disable Interrupt Lines
+
+### Enable INT0
+    EIMSK |= (1 << INT0);
+
+### Disable INT1
+    EIMSK &= ~(1 << INT1);
+
+---
+
+## Why Disable Unused Interrupts
+
+- Prevent false triggering from floating pins  
+- Reduce noise impact  
+- Ensure controlled initialization  
+- Avoid unintended behaviour  
+
+---
+
+## Interrupt Execution Flow
+
+1. Configure trigger in EICRA  
+2. Enable interrupt in EIMSK  
+3. Enable global interrupt (sei)  
+4. Event occurs → CPU jumps to ISR  
+
+---
+
+## ISR (Interrupt Service Routine)
+
+ISR is executed automatically when interrupt occurs.
+
+---
+
+## Flag-Based Handling
+
+### Global Flag
+    volatile uint8_t flag = 0;
+
+- `volatile` required (value changes outside normal flow)
+
+---
+
+## ISR Definitions
+
+### INT0 Interrupt
+    ISR(INT0_vect)
+    {
+        flag = 1;
+    }
+
+### INT1 Interrupt
+    ISR(INT1_vect)
+    {
+        flag = 1;
+    }
+
+---
+
+## Important Notes
+
+- Keep ISR short  
+- No delay or heavy processing inside ISR  
+- Use flag and handle logic in main loop  
+- Always use `volatile` for shared variables