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Mission 01 / 07

Dark Light

Sixteen dead control lines. Bring them back to life, one bit at a time.

GPIOOSCCONTRISAPORTATRISBPORTB
On this page
  1. The situation
  2. Why this matters
  3. The idea in one picture
  4. Your mission
  5. How you know it worked
  6. Debrief

01The situation

The Dark Light station has gone silent. Every control line is dead and every fault light glows red. Make the pins speak again.

02Why this matters

GPIO is how a chip touches the world. Every button you press and every LED you see on a device is a pin set as an input or an output.

  • ButtonsA button is a pin set as an input. The chip reads it.
  • LEDsAn LED is a pin set as an output. The chip drives it.
  • Relays & motorsOne output pin can switch real power on and off.

03The idea in one picture

TRIS = 0 → pins become outputsPORT A·RA7·RA6·RA5·RA4·RA3·RA2·RA1·RA0PORT B·RB7·RB6·RB5·RB4·RB3·RB2·RB1·RB0
TRIS makes each pin an output, then PORT drives it to 5 V.

Think of it as: TRIS is the tap, PORT is the water. Open the tap (TRIS = 0) and switch the water on (PORT = 1); only then does it flow.

Go deeper
TRIS bitPORT bitThat pin…
01is an output at 5 V (what this mission needs)
00is an output held at 0 V
10 or 1is an input: it listens and drives nothing

04Your mission

  • OSCCONStart the clock. Any speed works.
  • TRISAMake every Port A pin an output.
  • PORTADrive every Port A pin high.
  • TRISBMake every Port B pin an output.
  • PORTBDrive every Port B pin high.
Hint 1
Direction first, level second. TRIS sets each pin's direction, PORT sets its level, and each port has its own pair.
Hint 2
In TRIS a 0 means output. In PORT a 1 means 5 V. All eight bits, on both ports.
Show the solution
  1. 1Step 1 — OSCCON (0x20)Set it to 0x05 (0000 0101). Switch the internal oscillator on at 4 MHz (any speed wins).
  2. 2Step 2 — TRISA (0x21)Set it to 0x00 (0000 0000). Make every Port A pin an output.
  3. 3Step 3 — PORTA (0x22)Set it to 0xFF (1111 1111). Drive every Port A pin to 5 V.
  4. 4Step 4 — TRISB (0x23)Set it to 0x00 (0000 0000). Make every Port B pin an output.
  5. 5Step 5 — PORTB (0x24)Set it to 0xFF (1111 1111). Drive every Port B pin to 5 V.
  6. 6Program & ExecutePress P to compile your configuration into machine code in ROM, then press E to run it. When every register matches you get Target Reached.

The same thing as a program (MOVLW loads a value into W, MOVWF copies W into a register):

MOVLW 0x05
MOVWF 0x20   ; OSCCONMOVLW 0x00
MOVWF 0x21   ; TRISAMOVLW 0xFF
MOVWF 0x22   ; PORTAMOVLW 0x00
MOVWF 0x23   ; TRISBMOVLW 0xFF
MOVWF 0x24   ; PORTB
Bit-by-bit breakdown

OSCCON0x20= 0x05

Switch the internal oscillator on at 4 MHz (any speed wins). 0x05 = 0000 0101 = SCS0 = 1, IRCF = 10

Bit76543210
Value00000101
Field—IRCFSCS0
  • IRCF (bits 2:1) = 10 → 4 MHz
  • SCS0 (bit 0) = 1 → Internal oscillator on
  • Every remaining bit is 0: those bits are not needed for this level, but they must stay clear for the value to match.

TRISA0x21= 0x00

Make every Port A pin an output. 0x00 = 0000 0000 = RA7…RA0 all 0

Bit76543210
Value00000000
FieldRA7RA6RA5RA4RA3RA2RA1RA0
  • RA7…RA0 (bits 7:0) are all 0 → RAn is an output

PORTA0x22= 0xFF

Drive every Port A pin to 5 V. 0xFF = 1111 1111 = RA7…RA0 all 1

Bit76543210
Value11111111
FieldRA7RA6RA5RA4RA3RA2RA1RA0
  • RA7…RA0 (bits 7:0) are all 1 → RAn drives 5 V (needs TRISA bit n = 0)

TRISB0x23= 0x00

Make every Port B pin an output. 0x00 = 0000 0000 = RB7…RB0 all 0

Bit76543210
Value00000000
FieldRB7RB6RB5RB4RB3RB2RB1RB0
  • RB7…RB0 (bits 7:0) are all 0 → RBn is an output

PORTB0x24= 0xFF

Drive every Port B pin to 5 V. 0xFF = 1111 1111 = RB7…RB0 all 1

Bit76543210
Value11111111
FieldRB7RB6RB5RB4RB3RB2RB1RB0
  • RB7…RB0 (bits 7:0) are all 1 → RBn drives 5 V (needs TRISB bit n = 0)

Try it: build the values

Flip the bits until each register matches the mission. It's practice only; nothing is saved.

2 of 5 registers match.

OSCCON 0x20

✗ Not yet – 2 bits differ
7–
6–
5–
4–
3–
2IRCF1
1IRCF0
0SCS0

Hex 0x00 Binary 0000 0000 Target 0x05 (0000 0101)

  • IRCF (bits 2:1) = 00 → 1 MHz
  • SCS0 (bit 0) = 0 → Internal oscillator not enabled

TRISA 0x21

✓ MATCH
7RA7
6RA6
5RA5
4RA4
3RA3
2RA2
1RA1
0RA0

Hex 0x00 Binary 0000 0000 Target 0x00 (0000 0000)

  • RA7 (bit 7) = 0 → RA7 is an output
  • RA6 (bit 6) = 0 → RA6 is an output
  • RA5 (bit 5) = 0 → RA5 is an output
  • RA4 (bit 4) = 0 → RA4 is an output
  • RA3 (bit 3) = 0 → RA3 is an output
  • RA2 (bit 2) = 0 → RA2 is an output
  • RA1 (bit 1) = 0 → RA1 is an output
  • RA0 (bit 0) = 0 → RA0 is an output

PORTA 0x22

✗ Not yet – 8 bits differ
7RA7
6RA6
5RA5
4RA4
3RA3
2RA2
1RA1
0RA0

Hex 0x00 Binary 0000 0000 Target 0xFF (1111 1111)

  • RA7 (bit 7) = 0 → RA7 low / not driving 5 V
  • RA6 (bit 6) = 0 → RA6 low / not driving 5 V
  • RA5 (bit 5) = 0 → RA5 low / not driving 5 V
  • RA4 (bit 4) = 0 → RA4 low / not driving 5 V
  • RA3 (bit 3) = 0 → RA3 low / not driving 5 V
  • RA2 (bit 2) = 0 → RA2 low / not driving 5 V
  • RA1 (bit 1) = 0 → RA1 low / not driving 5 V
  • RA0 (bit 0) = 0 → RA0 low / not driving 5 V

TRISB 0x23

✓ MATCH
7RB7
6RB6
5RB5
4RB4
3RB3
2RB2
1RB1
0RB0

Hex 0x00 Binary 0000 0000 Target 0x00 (0000 0000)

  • RB7 (bit 7) = 0 → RB7 is an output
  • RB6 (bit 6) = 0 → RB6 is an output
  • RB5 (bit 5) = 0 → RB5 is an output
  • RB4 (bit 4) = 0 → RB4 is an output
  • RB3 (bit 3) = 0 → RB3 is an output
  • RB2 (bit 2) = 0 → RB2 is an output
  • RB1 (bit 1) = 0 → RB1 is an output
  • RB0 (bit 0) = 0 → RB0 is an output

PORTB 0x24

✗ Not yet – 8 bits differ
7RB7
6RB6
5RB5
4RB4
3RB3
2RB2
1RB1
0RB0

Hex 0x00 Binary 0000 0000 Target 0xFF (1111 1111)

  • RB7 (bit 7) = 0 → RB7 low / not driving 5 V
  • RB6 (bit 6) = 0 → RB6 low / not driving 5 V
  • RB5 (bit 5) = 0 → RB5 low / not driving 5 V
  • RB4 (bit 4) = 0 → RB4 low / not driving 5 V
  • RB3 (bit 3) = 0 → RB3 low / not driving 5 V
  • RB2 (bit 2) = 0 → RB2 low / not driving 5 V
  • RB1 (bit 1) = 0 → RB1 low / not driving 5 V
  • RB0 (bit 0) = 0 → RB0 low / not driving 5 V

05How you know it worked

  1. 1The clock startsOSCCON switches the oscillator on and the CPU runs your program.
    Screenshot coming soon
  2. 2Pipes turn greenEvery pin you made an output gets a green output pipe.
    Screenshot coming soon
  3. 3The lights come onWith PORT high, all 16 fault-panel LEDs go from red to green.TARGET REACHED
    Screenshot coming soon

Stuck?

  • PORT changes nothing, pipes stay darkThe pins are still inputs. TRIS needs 0x00, not 0xFF.
  • Pipes are green but the LEDs stay redOutputs are held low. Set PORT to 0xFF.
  • Only some LEDs turn greenSet all 8 bits (0x0F lights only half), on both ports.
  • Nothing happens at allThe clock is off. OSCCON needs bit 0 set.

06Debrief

You just wrote the first lines of every firmware project.

Setting a pin's direction and level is embedded "hello world". Every blinking LED starts exactly here.

// the same idea in CTRISA = 0x00;  // Port A: all outputsPORTA = 0xFF;  // Port A: all highTRISB = 0x00;  // Port B: all outputsPORTB = 0xFF;  // Port B: all high

On a real chip the same switches can sit at different bit positions; its datasheet has the map. The idea is identical.

Next mission · 02Data Hunter (ADC)