Delay calculation and applications

Lesson progress 0%

Sign in with CALETAS to take the exam (10 questions) and save your result.

BRA vs GOTO (exam question)

Both use 2 cycles. BRA = relative jump for local loops. GOTO = absolute jump. Course expects BRA in delay loops.

NOP tuning

Each NOP = 1 cycle. Two NOPs add +2 to Σ CI to hit 3 ms exactly after rounding A.

3 ms @ 20 MHz — worked example

• CM = 0.2 µs
• Target 3000 µs → Σ CI = 15,000
• Two loops, N = 29, solve M = A:

Σ CI = 3NM + 4M + 5 + 2(NOP) = 91A + 7
91A + 7 = 15,000  →  A ≈ 165

Check: 91×165 + 7 = 15,022 → 15,022 × 0.2 µs ≈ 3.00 ms

RETARDO_3MS
    MOVLW   .29
    MOVWF   CONTADOR_1
    MOVLW   .165            ; A
    MOVWF   CONTADOR_2
BUCLE_2
    MOVLW   .29
    MOVWF   CONTADOR_1
BUCLE_1
    DECFSZ  CONTADOR_1,F
    BRA     BUCLE_1
    DECFSZ  CONTADOR_2,F
    BRA     BUCLE_2
    NOP
    NOP
    RETURN

10 Hz square wave

T = 100 ms → 50 ms per half period. Use 3 nested loops + 3 NOP (PDF: M=2, inner values 252 and 164).

Partial checklist

• CM @ 20 MHz
• Table 4.1
• DECFSZ = N+1 cycles
• Solve A for 3 ms
• Explain NOP and BRA

Interactive exam

Delays — Calculation and partial exam

Based on: Clase Tema 4 y 5.pdf

0 of 0 answered

With a 4 MHz crystal, the machine cycle (CM) duration is...

1 µs 0.2 µs 4 µs 250 ns

The software delay formula is...

Delay = CM × Σ CI Delay = Fosc × N Delay = TRIS × PORT Delay = WREG / STATUS

How many cycles does BRA normally consume?

1 2 3 4

DECFSZ uses more cycles when...

It skips the next instruction (counter reached zero) It never skips It is only used in interrupts WREG is zero

With a 20 MHz crystal, CM equals...

0.2 µs 1 µs 5 µs 20 µs

To fine-tune a delay by a few microseconds you use...

NOP (1 cycle each) Multiple GOTOs MOVLW only RETFIE

In a local delay loop BRA is preferred over GOTO because...

It is a relative jump to a nearby loop (course convention) BRA uses 0 cycles GOTO cannot be used in .asm GOTO does not affect the PC

For a ~10 s delay @ 20 MHz you typically need...

Three nested loops (N,M,P up to 255) Only one MOVLW Only configure TRIS Enable the ADC

For a 3 ms delay @ 20 MHz, Σ CI should be approximately...

15,000 605 770 3,000

The course formula for Σ CI with two nested loops (no CALL) is...

3NM + 4M + 5 N + M 2NM 255 × 255

Check answers