Memory Interfacing

The memory is made up of semiconductor material used to store the programs and data. Three types of memory is,

  • Process memory
  • Primary or main memory
  • Secondary memory

Typical EPROM and Static RAM

A typical semiconductor memory IC will have n address pins, m data pins (or output pins).

  • Having two power supply pins (one for connecting required supply voltage (V and the other for connecting ground)
  • The control signals needed for static RAM are chip select (chip enable), read control (output enable) and write control (write enable).
  • The control signals needed for read operation in EPROM are chip select (chip enable) and read control (output enable).


It is used to select the memory chip of processor during the execution of a program. No of IC's used for decoder is,

  • 2-4 decoder (74LS139)
  • 3-8 decoder (74LS138)


Block Diagram of 2-4 Decoder

Truth Table for 2-4 Decoder

Example for Memory Interfacing

Consider a system in which the full memory space 64kb is utilized for EPROM memory. Interface the EPROM with 8085 processor.

  • The memory capacity is 64 Kbytes. i.e 2^n = 64 x 1000 bytes where n = address lines.
  • So, n = 16.·        In this system the entire 16 address lines of the processor are connected to address input pins of memory IC in order to address the internal locations of memory.
  • The chip select (CS) pin of EPROM is permanently tied to logic low (i.e., tied to ground).
  • Since the processor is connected to EPROM, the active low RD pin is connected to active low output enable pin of EPROM.
  • The range of address for EPROM is 0000H to FFFFH.