AVR Delay Loop Generator: Configurable Loops for Stack-Safe Delays

1. Compute total cycles required = round(T * f).
2. Choose an outer loop structure. Typical cost per inner iteration (cycles_per_iter) determined by instructions used. For example, a common 3-cycle-per-iteration loop uses dec + brne (taken).
3. Determine number of iterations needed and whether nesting is required (e.g., 16-bit loop = outer * inner).
4. Compute remainder cycles to fill with NOPs or one-off instructions.
5. Generate assembly with correct initial load values (ldi) adjusting for off-by-one effects from branch behavior.

Example: 1 ms delay at 8 MHz

Target: 1 ms -> 8000 cycles.

Using a two-level loop where inner loop is 256 iterations costing 3 cycles each (dec + taken brne = 1+2), inner total = 256 * 3 = 768 cycles. Using an outer loop of N iterations:

Let outer_count * 768 ≈ 8000 → outer_count = 10 gives 7680 cycles. Remainder = 320 cycles. Fill remainder with smaller loop or NOPs.

A calculator would pick inner and outer counts to reduce remainder, possibly using a 16-bit combined loop to hit 8000 cycles exactly or within 1 cycle.

Example output from the calculator

For a target of 1 ms at 8 MHz, the tool might produce:

• Registers used: r18 (inner), r19 (outer)
• Assembly:

  
  ldi r19, 10      ; outer outer_loop: ldi r18, 256     ; inner inner_loop: dec r18 brne inner_loop dec r19 brne outer_loop ; remaining NOPs... 

  (Actual code would adjust counts and insert NOPs to match 8000 cycles precisely.)

Handling different clock speeds and accuracy

• For low clock speeds (1 MHz), larger loop counts are required for long delays; nesting deeper or using ⁄₂₄-bit counters helps.
• For high speeds (16–20 MHz), you may need small loops and NOPs to achieve very short delays (microseconds). The calculator should allow a minimum achievable delay based on loop granularity.
• Accuracy depends on rounding to whole cycles; a good calculator reports achieved delay and error in microseconds and percentage.

Stack safety and register clobbering

• Generated assembly should document which registers it uses and whether it preserves them. Common safe registers: r18–r27, r30–r31 (if not using indirect addressing). Avoid r0, r1, r2 and call-saved registers unless documented.
• For use in C projects, provide an inline-asm wrapper that saves/restores any used registers if necessary, or mark them as clobbered in the asm directive.

Integration with C and inline assembly

Two ways to use generated delays:

1. As a standalone assembly routine you call from C (requires proper calling convention and stack handling).
2. As inline asm for small delays (use GCC’s asm volatile and clobber list).

Example inline asm snippet:

asm volatile (     "ldi r18, %[inner] 	"     "inner_loop: 	"     "dec r18 	"     "brne inner_loop 	"     :     : [inner] "M" (value)     : "r18" ); 

Edge cases and practical tips

• Watch interrupts: If interrupts are enabled, they add variable delay (ISR execution) and can ruin cycle-accurate timing. Disable interrupts around timing-critical loops if necessary, or use hardware timers.
• Compiler optimizations: Always use asm volatile for inline assembly to prevent reordering or removal.
• Power modes: If entering sleep modes, CPU halts and timing semantics change; delays based on CPU cycles won’t progress during sleep.
• Instruction set variants: Some AVRs (AVR32, XMEGA) have different timings or instructions; verify timing tables for your specific core.

Building a user-friendly calculator (UI ideas)

• Input fields: clock frequency, desired delay, acceptable error, preferred registers.
• Output options: assembly (ATT syntax), C inline asm, preconfigured functions for common delays (e.g., 1 ms, 10 ms).
• Visualization: show cycles breakdown (total cycles, cycles per loop, remainder).
• Export: copy-to-clipboard and downloadable .S files.

Conclusion

A fast AVR delay loop calculator is a practical tool for embedded developers who need precise, cycle-accurate delays across microsecond to second ranges. By automating cycle counting, nesting loop selection, and remainder handling, such a calculator saves time and reduces subtle timing bugs. Remember to consider interrupts, register usage, and compiler interactions when integrating generated code into real projects.