Different Ways ROM Can Be Programmed

ROM (Read Only Memory) programming refers to how data is written into ROM. Based on programmability and erasability, types are:

1. Mask Programmed ROM (MROM)

• When programmed: At time of manufacturing by manufacturer using photographic mask.
• Mechanism: Transistor connections at crosspoints defined by mask. Presence/absence of transistor = 1/0.
• Features: Non-field programmable, cheapest per unit in mass production but high setup cost, 100% non-erasable.
• Use: Firmware, character generators, microcode where volume high and code never changes.

2. PROM (Programmable ROM) - One Time Programmable (OTP)

• Mechanism: Manufactured as all 0s/1s with fusible NiCr links at each bit. User programs with PROM programmer which applies high current pulse (20-50mA, ~10-12V) to blow fuses for desired bits.
• Process: Address selected -> data applied -> programming pulse for each word.
• Features: Field programmable once, cannot be erased; if mistake chip discarded.

3. EPROM (Erasable PROM)

• Uses FAMOS transistors with floating gate storing charge.
• Programming: Electrical - with EPROM programmer, apply Vpp ~21-25V pulse to inject hot electrons into floating gate via avalanche. 0 = charged, 1 = uncharged.
• Erasing: UV light (~253.7nm) through quartz window for 15-20 min erases entire chip by photoelectric discharge. Then reprogrammable (typically ~1000 cycles).
• Types: UV-EPROM

4. EEPROM (Electrically Erasable PROM) / E²PROM

• Uses FLOTOX/ MNOS structure with thin oxide allowing Fowler-Nordheim tunneling.
• Programming & Erasing: Both electrically - no UV, no removal from circuit. Byte/word erasable using ~20V internal charge pump.
• Features: ~10^4-10^5 cycles, slower write (ms vs ns read), more expensive per bit due to 2 transistor per cell.

5. Flash Memory (Advanced EEPROM)

• Bulk/ sector erase instead of byte erase using same tunneling.
• Programming: Electrical with on-chip charge pump. Erase entire block at once (~1s), program byte/page wise.
• Features: High density, in-system programmable, ~10^5-10^6 cycles, used in SSD, pen drives, BIOS.

Summary Table:

Type	Programmability	Erasability	Volatility
MROM	Factory mask only	No	Non-volatile
PROM	Once by user (fuse)	No	Non-volatile
EPROM	User, electrical	UV whole chip	Non-volatile
EEPROM	User, electrical byte-wise	Electrical byte-wise	Non-volatile
Flash	User, electrical sector-wise	Electrical block-wise	Non-volatile

For exam, diagram of ROM array with row/column decoders and fuse/ floating gate cell should be drawn.