Draw the logic diagram of 16-bit ROM Array and explain its principle of operation.

Ans:

16-bit ROM Array: A ROM that is read-only memory is an array of selectively closed and open unidirectional contacts. A 16-bit ROM array is demonstrated in Fig. (b). To choose any one of the 16 bits, a 4-bit address (A₃, A₂, A₁, A₀) is needed. The lower order two bits (A₁, A₀) are decoded through the decoder DL that selects one of the four rows, while the higher order two bits (A₃, A₂) are decoded through the decoder DH that activates one of the 4 column sense amplifiers.

Fig.(a) Logic Diagram of 16-bit ROM array

The diode matrix is made by connecting one diode with a switch among each row and column. For illustration the diode D₂₁ is connected in between column 1 and row 2. The output is enabled through applying logic 1 at the CS input that is chip select. Programming a ROM implies to selectively open and close the switches in series along with the diodes. For illustration, if the switch of diode D₂₁ is into closed position and if the address input is 0110, the row 2 is activated linking this to the column 1. As well the sense amplifier of column 1 is enabled that provides logic 1 output if the chip is selected means CS = 1. It shows as a logic1 is stored at the address 0110. Conversely, if the switch of diode D₂₁ is open, logic 0 is stored on the address 0110.