Branch Instructions in AVR Microcontrollers: Implementing Loops and Conditional Logic
Learn how branch instructions are used to implement loops and conditional logic in AVR microcontroller programming. This guide explains different types of branch instructions (conditional, unconditional), their syntax, and how they efficiently control program flow.
Branch Instructions in AVR Microcontrollers
Looping in AVR Microcontrollers
Loops are fundamental to programming. In AVR (Alf and Vegard's RISC Processor) assembly language, a simple loop involves repeatedly writing the same instructions. This is inefficient and uses a lot of memory. Branch instructions offer a much better way to implement loops and conditional logic.
Branch Instructions: Controlling Program Flow
Branch instructions alter the sequence of instructions executed by the microcontroller. This allows for loops, conditional statements, and other program control structures. Branch instructions check a condition (often a flag in the status register) and jump to a different location in the program if the condition is met.
Types of Branch Instructions
AVR branch instructions are mainly categorized as conditional and unconditional:
1. Conditional Branch Instructions
Conditional branch instructions check a condition (using flags in the Status Register) before branching.
| Instruction | Mnemonic | Condition |
|---|---|---|
| Branch if Equal | BREQ | Zero Flag (Z) is set (result of previous operation was zero) |
| Branch if Not Equal | BRNE | Zero Flag (Z) is clear (result was non-zero) |
| Branch if Same or Higher (Unsigned) | BRSH | Carry Flag (C) is clear |
| Branch if Lower (Unsigned) | BRLO | Carry Flag (C) is set |
| Branch if Less Than (Signed) | BRLT | Sign Flag (S) is set |
| Branch if Greater Than or Equal (Signed) | BRGE | Sign Flag (S) is clear |
| Branch if Overflow Set | BRVS | Overflow Flag (V) is set |
| Branch if Overflow Cleared | BRVC | Overflow Flag (V) is clear |
(The descriptions of how these instructions work and the conditions they test are shown in the original text and should be included here. A clear description of what conditions trigger a branch for each instruction should be added. The explanation that these are all relative branches, with the target address specified as an offset from the program counter, should also be provided.)
Example: Loop using BRNE
(The example from the original text demonstrating a loop using the BRNE instruction is given here.)
2. Unconditional Branch Instructions
Unconditional branch instructions always jump to the specified address, regardless of any conditions. The AVR has three types of unconditional branch instructions:
- JMP (Jump): A long jump; the target address can be anywhere in the 4MB address space.
- RJMP (Relative Jump): A shorter instruction; the target address is relative to the current program counter. The range is limited to -2048 to +2047 bytes from the current address.
- IJMP (Indirect Jump): The jump target is given by the contents of the Z register.
Conclusion
Branch instructions are essential for controlling program flow in AVR microcontrollers, enabling the creation of loops, conditional statements, and other control structures. Understanding the different types of branch instructions and how they use flags in the status register is crucial for effective programming.