Computer Systems Experiments 9 | Scalable Larson Scanner

Series: Computer Systems Experiments

Computer Systems Experiments 9 | Scalable Larson Scanner

This experiment is just a simple application of our last experiment. The word “scalable” actually means that we can change the length of the scanner by changing just one line of the code. For example, we have 8 LEDs and we want to make the scanner shorter, to 4 LEDs (Only GPIO 20 to GPIO 23 will work). The code for this feature is,

.equ DELAY, 0x3F0000
.equ AMOUNT, 4   // set the scale of 2-8 LEDs

// set output to GPIO 20 to 27
ldr r0, FSEL2
ldr r1, =0x00249249
str r1, [r0]

// set the starting status
mov r1, #(1<<20) // current pin
mov r3, #1
mov r4, #(1<<20)
mov r5, #(1<<18)
lsl r4, #AMOUNT
lsl r5, #AMOUNT

loop:
  ldr r0, SET0
  str r1, [r0]

  // delay a certain time
  mov r2, #DELAY
  wait1:
      subs r2, r2, #1
      bne wait1

  // clear GPIO status
  ldr r0, CLR0
  str r1, [r0]

  tst r3, #1
  bne forward
  beq backward
 
forward:
  lsl r1, #1
  b compar

backward:
  lsr r1, #1
  b compar

compar:
  cmp r1, r4
  moveq r1, r5
  moveq r3, #2

  cmp r1, #(1<<19)
  moveq r1, #(1<<21)
  moveq r3, #1

  b loop

FSEL2: .word 0x20200008
SET0: .word 0x2020001c
CLR0: .word 0x20200028

Note that we can change the second line of this code to change the length of our scanner. You can also find this code form my repo. In the next experiment, we are going to change the brightness of LEDs to make a smoother scanner.