Page 114 - Digital Electronics by harish

 Electrically Erasable (EEPROM)
o R/W
o Takes much longer to write than read
o Individual bytes programmable
 Flash memory
o Faster erase (block erase)
o Higher density than EEPROM

4.8 ROM organization

ROM is a read only memory. We can‟t write data in this memory. It is a non-volatile
memory i.e. it can retain the data even when the power is switched off. Generally ROM is
used to store permanent data like computer programs and look-up tables. Figure shows the
organization of simple ROM using diode matrix. Diodes are physically fabricated in the
required positions during manufacturing of the memory chip. ROMs are cheap when
produced in large volumes.

Figure : Four-byte Diode Matrix ROM

The address lines A0 and A1 are decoded by 2:4 decoder and used to select one of the
four rows. The active low output of the decoder produces a „0‟ on the selected row. When the
row line is „0‟ and if the diode is present in the column, we get „0‟ at the particular column.
Data is available on the output lines only when the output enable () signal is low. Table
shows the contents of ROM at four locations.

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