Computer Science Principles V10

Clive W. Humphris


Programming a ROM (Read Only memory) permanently writes a data state (logic 1 or logic 0) to each memory location or cell.

Here we are using random binary 0 and 1 for each cell, generated by a Re-Program operation. In practice each cell is manufactured as logic 1 (diode present). A cell is then programmed to logic 0 as required by fusing the diode connection, shown here as a greyed out device.

Reading is a measurement of the voltage developed across the Data resistor due to the current flowing through a good-diode. An open circuit diode will produce logic 0 as no current flows to generate a voltage or logic level across the resistor for that particular data line.

Note that a HIGH data output can only be derived from the logic 1 output of the 2 - 4 line decoder when that line is selected, this is because the decoder provides the 5V to bias on the good-diode.

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