Programming or writing data to EEPROM (electrically erasable programmable read-only memory), which is a user-modifiable ROM, is accomplished by applying an electrical voltage that is higher than normal. This higher voltage is necessary to alter the state of the individual memory cells.
Understanding the Programming Process
EEPROM cells store data by trapping electrical charge on a floating gate within a transistor structure. To program a cell (typically setting it to represent a logical '0' or '1'), a voltage significantly higher than the standard operating voltage is applied across the transistor's terminals.
Here's a simplified look at the write (programming) operation:
- Targeting: Specific rows and columns are addressed to select the target memory cell(s) for writing.
- Applying Voltage: A higher-than-normal electrical voltage is applied across the cell's control gate and source/drain terminals.
- Charge Tunneling: This high voltage provides the energy needed for electrons to tunnel through a thin insulating layer onto the floating gate.
- Storing Data: The presence or absence of trapped charge on the floating gate determines whether the cell represents a '0' or a '1'. Trapping charge (adding electrons) typically changes the threshold voltage of the transistor, altering how it conducts when read.
Erasing and Reprogramming
As the name suggests, EEPROM is electrically erasable. Erasing also involves applying a high voltage, often with the polarity reversed compared to writing, to remove the trapped charge from the floating gate.
The reference notes that EEPROM can be erased and reprogrammed repeatedly. This cycle of applying higher-than-normal voltages for both erasing and writing allows the memory to be updated multiple times, unlike older ROM types that were write-once or erased by UV light.
Key Aspects of EEPROM Programming
- Electrical Method: The core distinction is that programming and erasing are done purely using electrical signals.
- High Voltage Requirement: A voltage significantly higher than the normal operating voltage (e.g., 12V, 20V, or even higher depending on the specific EEPROM type, compared to typical 3.3V or 5V operation) is crucial for enabling charge tunneling to/from the floating gate.
- Byte or Page Programming: Many EEPROMs allow programming a single byte at a time, although some modern ones support page writes for faster bulk programming.
- Limited Write Cycles: While repeatedly programmable, EEPROM cells have a finite number of erase/write cycles they can endure before degrading (typically in the range of 100,000 to 1,000,000 cycles).
EEPROM vs. Other Memory Types
| Feature | EEPROM | EPROM | Flash Memory |
|---|---|---|---|
| Programming | Electrical (using high voltage) | Electrical (using high voltage) | Electrical (using high voltage) |
| Erasing | Electrical (byte or page erase) | UV Light (requires special eraser) | Electrical (block erase) |
| Modifiability | User-modifiable (byte by byte usually) | User-modifiable (requires erase) | User-modifiable (block by block) |
| Write Cycles | Moderate (100k - 1M) | Limited (due to UV erase complexity) | High (100k - 100M+) |
| Complexity | Relatively complex cell structure | Simpler cell structure | Optimized for density/speed |
In summary, programming EEPROM relies on applying a precisely controlled, higher-than-normal electrical voltage to force electrons onto the floating gate of a memory cell, thereby storing data electrically.
