# 3. EEPROM

EEPROM is a small non-volatile memory inside the microcontroller.

Unlike SRAM:

* SRAM loses data when power is removed
* EEPROM keeps data even after reset or power loss

EEPROM is commonly used to store:

* Settings
* Calibration values
* System state

EEPROM has limited write endurance, so it should only be used sparingly

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## EEPROM Registers

EEPROM operations are controlled using dedicated I/O registers.

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### EEARH:EEARL — EEPROM Address Register

Stores the memory address to be accessed.

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### EEDR — EEPROM Data Register

Stores the data to be written or read.

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### EECR — EEPROM Control Register

Controls read and write operations.

Important control bits:

| Name  | Bit | Function               |
| ----- | --|---------------------- |
| EERE  | 0 | EEPROM Read Enable      |
| EEWE  | 1| EEPROM Write Enable     |
| EEMWE | 2| EEPROM Host Write Enable |

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## EEPROM Write Operation

Writing EEPROM requires a specific sequence:

1. Wait until EEWE becomes ‘0’.
2. Write new EEPROM address to EEAR (optional).
3. Write new EEPROM data to EEDR (optional).
4. Write a logical ‘1’ to the EEMWE bit while writing a ‘0’ to EEWE in EECR.
5. Within four clock cycles after setting EEMWE, write a logical ‘1’ to EEWE.

```asm
; Writes R21 to EEPROM address in 0x0052
EEPROM_write:
  SBIC  EECR, 1
  RJMP  EEPROM_write

  LDI R22, hi8(0x0052)
  OUT   EEARH, R22
  LDI R22, lo8(0x0052)
  OUT   EEARL, R22

  OUT   EEDR, R21

  SBI   EECR, 2
  SBI   EECR, 1
  RET
```

---

## EEPROM Read Operation

Reading EEPROM is simpler:

1. Load address into EEAR
2. Start read operation by setting EERE
3. Read data from EEDR

```asm
; Read R21 from EEPROM address in 0x0052
EEPROM_read:
  SBIC  EECR, 1
  RJMP  EEPROM_read

  LDI R22, hi8(0x0052)
  OUT   EEARH, R22
  LDI R22, lo8(0x0052)
  OUT   EEARL, R22

  SBI   EECR, 0
  IN    R21, EEDR
  RET
```