Interfacing:Interfacing means a device or program enabling a user to communicate with a computer, or for connecting two items of hardware or software.The interfacing process involves designing a circuit that will match the memory requirements with the microprocessor.
Memory interfacing Memory is an integral part of a microcomputer system. Memory has certain signal requirements to write into & read from its registers. Similarly the micro processor initiates a set of signals when it wants to read from & write into memory.
Memory structure & its requirements Read/Write memory is a group of registers to store binary information. It has 2048 registers & each register can store 8 bits indicated by 8 input & 8 output data lines. The chip has 11 address lines A10-A0, one chip select (cs), & two control lines:Read (rd) to enable the output buffer and Write (wr) to enable the input buffer.
Eprom: It is a group of 4096 registers. It has 12 address lines A11-A0, one chip select (cs), & one read control signal. This chip must be programmed before it can be used as a read-only memory.
Basic concepts in memoryinterfacing: The primary function of memory interfacing is that the microprocessor should be able to read from and write into a given register of a memory chip. The microprocessor should perform these operations: Be able to select the chip. Identify the register. Enable the appropriate buffer.
Address decoding The process of address decoding should result in identifying a register for a given address. It should be able to generate a unique pulse for a given address. The address lines (A11-A0) are connected to the memory chip, & the remaining four address lines (A15- A12) of the 8085 microprocessor must be decoded.
It shows two methods of decoding these lines. One by using a NAND gate Other by using a 3-8 decoder. The output of the NAND goes active & selects the chip only when all address lines A15-A12 are at logic 1. The same result can be obtained by using 3-8 decoder , which is capable of decoding 8 different input addresses.
In the decoder circuit, three input lines can have 8 different logic combinations from 000 to 111; each input combination can be identified by the corresponding output line if Enable lines are active. In this circuit enable lines are enabled by grounding. This address decoding scheme can be used to interface a 4K EPROM and a 2K R/W memory.
Address decoding & memoryaddresses The address range of the memory chip can be obtained by analyzing the possible logic levels on the 16 address lines. The logic levels on the address lines A15-A12 must be 0000 to assert the chip enable, & the address lines A11- A0 can assume any combinations from all 0s to all 1s. Therefore the memory address of the chip ranges from 0000H to 0FFFH.