Read Only Memories

Read Only Memories

1. Non-Volatile memories

   • retain their state (0/1) even with no power supply
   • required to store data for initialization system operation

2. Only read memory - no write

3. Types

   • FLASH
     • Possible to write an entire block of memory cells
     • Previous contents of the block are erased prior to writing
     • Higher density, higher capacity with lower cost per bit
     • Required a single power supply voltage, less power during operation
   • EEPROM
     • Electrical erasable programmable ROM
       • can be programmed, erased and reprogrammed electrically
     • Possible to erase the cell contents selectively
     • Does not have to be removed physically for erasure
     • Disadvantage
       • Different voltages are required for erasing, writing, and reading the stored data
   • PROM
     • Programmable ROM
       • allows user to load some data
     • Programmability
       • using fuse, blowing it creates logic 1, keeping it creates logic 0
   • ROM
     • Information can be written only once at the time of manufacture
     • Logic 0 when connected to ground, otherwise logic 1
     • The state of connection to ground in each cell is determined during manufacturing using a mask
     • Block diagram
   • EPROM
     • Erasable programmable ROM
       • allows to store data to be erased and new data to be written into it

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