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Mission 04 / 07

FlashBack Fighter

The ship forgot its mission. Make it remember.

EEPROMOSCCONEECON0EECON1
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

Mid-flight, the memory failed: flight paths, calibration, the whole mission record. Write it back somewhere a power cut can't erase it.

02Why this matters

RAM forgets everything when the power goes. EEPROM doesn't, so it's where every device keeps what must survive a restart.

  • Wi-Fi settingsYour router still knows its password after a power cut.
  • CalibrationSensors store their correction values once, at the factory.
  • Saved stateLast volume, last channel, high scores.

03The idea in one picture

WRITE addr 0x00 'E'RAMEEPROM000102030405060708090A0B0C
Each character is written to EEPROM and read back. Cut the power: RAM is wiped, EEPROM still has it.

Think of it as: RAM is a whiteboard that's wiped when the lights go out; EEPROM is a notebook. EEPGD picks the notebook (EEPROM) or the ship's printed manual (flash), and RD / WR say whether you read a page or write one.

04Your mission

  • OSCCONStart the clock. Any speed works.
  • EECON0Point at EEPROM and start a write.
  • EECON1Point at EEPROM and start a read.
Hint 1
Each register makes two choices: which memory (EEPGD) and which action (RD or WR). EECON0 writes, EECON1 reads back.
Hint 2
EEPGD is bit 0, and 0 means EEPROM. RD is bit 1 and WR is bit 2. Set only the one action each register needs.
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 — EECON0 (0x29)Set it to 0x04 (0000 0100). Select EEPROM (not program memory) and initiate a write.
  3. 3Step 3 — EECON1 (0x2A)Set it to 0x02 (0000 0010). Select EEPROM (not program memory) and initiate a read.
  4. 4Program & 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 0x04
MOVWF 0x29   ; EECON0MOVLW 0x02
MOVWF 0x2A   ; EECON1
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.

EECON00x29= 0x04

Select EEPROM (not program memory) and initiate a write. 0x04 = 0000 0100 = EEPGD = 0, RD = 0, WR = 1

Bit76543210
Value00000100
Field—WRRDEEPGD
  • WR (bit 2) = 1 → Initiate a write
  • RD (bit 1) = 0 → No read initiated
  • EEPGD (bit 0) = 0 → Access EEPROM data memory
  • Every remaining bit is 0: those bits are not needed for this level, but they must stay clear for the value to match.

EECON10x2A= 0x02

Select EEPROM (not program memory) and initiate a read. 0x02 = 0000 0010 = EEPGD = 0, RD = 1, WR = 0

Bit76543210
Value00000010
Field—WRRDEEPGD
  • WR (bit 2) = 0 → No write initiated
  • RD (bit 1) = 1 → Initiate a read
  • EEPGD (bit 0) = 0 → Access EEPROM data memory
  • Every remaining bit is 0: those bits are not needed for this level, but they must stay clear for the value to match.

Try it: build the values

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

0 of 3 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

EECON0 0x29

✗ Not yet – 1 bit differ
7–
6–
5–
4–
3–
2WR
1RD
0EEPGD

Hex 0x00 Binary 0000 0000 Target 0x04 (0000 0100)

  • WR (bit 2) = 0 → No write initiated
  • RD (bit 1) = 0 → No read initiated
  • EEPGD (bit 0) = 0 → Access EEPROM data memory

EECON1 0x2A

✗ Not yet – 1 bit differ
7–
6–
5–
4–
3–
2WR
1RD
0EEPGD

Hex 0x00 Binary 0000 0000 Target 0x02 (0000 0010)

  • WR (bit 2) = 0 → No write initiated
  • RD (bit 1) = 0 → No read initiated
  • EEPGD (bit 0) = 0 → Access EEPROM data memory

05How you know it worked

  1. 1Write: address, then dataFor each character the address appears, then the byte travels into its EEPROM cell.
    Screenshot coming soon
  2. 2Read it straight backRight after each write, the same cell is read back to the read block.
    Screenshot coming soon
  3. 3The message is restored"EmbeddedPlayX" sits in memory from address 0x00 up, and WR and RD clear themselves.TARGET REACHED
    Screenshot coming soon

Stuck?

  • Nothing is ever writtenWR belongs in EECON0 (0x04). Check the two values aren't swapped.
  • It never starts, even with WR setThe write waits for the read-back: EECON1 needs RD (0x02).
  • Data goes to the wrong memoryEEPGD = 1 selects flash. Keep bit 0 at 0.
  • It runs, but no Target ReachedOne action per register: 0x06 sets both RD and WR.

06Debrief

You just made the ship remember.

Every product with settings uses non-volatile memory. Real firmware also waits for each write to finish before starting the next, because EEPROM writes are slow.

// the same idea in CEECON0 = 0x04;  // EEPROM, start a writeEECON1 = 0x02;  // EEPROM, start a read

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

Next mission · 05Byte Buster (UART)