Memory Devices and Interfacing
Any application of a microprocessor based system requires the transfer of data between external circuitry to the microprocessor and microprocessor to the external circuitry. Most of the peripheral devices are designed and interfaced with a CPU either to enable it to communicate with the user or an external process and to ease the circuit operations so that the microprocessor works more efficiently.
The use of peripheral integrated devices simplifies both the hardware circuits and software considerable. The following are the devices used in interfacing of Memory and General I/O devices
• 74LS138 (Decoder / Demultiplexer).
• 74LS373 / 74LS374 3-STATE Octal D-Type Transparent Latches.
• 74LS245 Octal Bus Transceiver: 3-State.
74LS138 (Decoder / Demultiplexer)
The LS138 is a high speed 1-of-8 Decoder/ Demultiplexer fabricated with the low power Schottky barrier diode process. The decoder accepts three binary weighted inputs (A0, A1, A2) and when enabled provides eight mutually exclusive active LOW Outputs (O0–O7).
The LS138 can be used as an 8-output demultiplexer by using one of the active LOW Enable inputs as the data input and the other Enable inputs as strobes. The Enable inputs which are not used must be permanently tied to their appropriate active HIGH or active LOW state.
74LS373 / 74LS374 3-STATE Octal D-Type Transparent Latches and Edge-Triggered Flip-Flops
These 8-bit registers feature totem-pole 3-STATE outputs designed specifically for implementing buffer registers, I/O ports, bidirectional bus drivers, and working registers. The eight latches of the 74LS373 are transparent D type latches meaning that while the enable (G) is HIGH the Q outputs will follow the data (D) inputs.
When the enable is taken LOW the output will be latched at the level of the data that was set up. The eight flip-flops of the 74LS374 are edge-triggered D-type flip flops. On the positive transition of the clock, the Q outputs will be set to the logic states that were set up at the D inputs.
• Choice of 8 latches or 8 D-type flip-flops in a single package
• 3-STATE bus-driving outputs
• Full parallel-access for loading
• Buffered control inputs
• P-N-P inputs reduce D-C loading on data lines
74LS245 Octal Bus Transceiver: 3-State
The 74LS245 is a high-speed Si-gate CMOS device. The 74LS245 is an octal transceiver featuring non- inverting 3-state bus compatible outputs in both send and receive directions. The 74LS245 features an Output Enable (OE) input for easy cascading and a send/receive (DIR) input for direction control. OE controls the outputs so that the buses are effectively isolated. All inputs have a Schmitt-trigger action.
These octal bus transceivers are designed for asynchronous two-way communication between data buses. The 74LS245 is a high-speed Si-gate CMOS device. The 74LS245 is an octal transceiver featuring non-inverting 3-state bus compatible outputs in both send and receive directions.
The 74LS245 features an Output Enable (OE) input for easy cascading and a send/receive (DIR) input for direction control. OE controls the outputs so that the buses are effectively isolated. All inputs have a Schmitt-trigger action. These octal bus transceivers are designed for asynchronous two-way communication between data buses.
Memory Devices And Interfacing
The memory interfacing circuit is used to access memory quit frequently to read instruction codes and data stored in the memory. The read / write operations are monitored by control
signals. Semiconductor memories are of two types. Viz. RAM (Random Access Memory) and ROM (Read Only Memory) The Semiconductor RAM’s are broadly two types-
static Ram and dynamic RAM
Memory structure and its requirements
The read / write memories consist of an array of registers in which each register has unique address. The size of memory is N * M as shown in figure.
Where N is number of register and M is the word length, in number of bits. As shown in figure(a) memory chip has 12 address lines Ao–A11, one chip select (CS), and two control lines, Read (RD) to enable output buffer and Write (WR) to enable the input buffer.
The internal decoder is used to decoder the address lines. Figure(b) shows the logic diagram of a typical EPROM (Erasable Programmable Read-Only Memory) with 4096 (4K) register. It has 12 address lines Ao – A11, one chip select (CS), one read control signal. Since EPROM does not require the (WR) signal.
EPROM (or EPROMs) is used as a program memory and RAM (or RAMs) as a data memory. When both, EPROM and RAM are used, the total address space 1 Mbytes is shared by them.
Address Decoding Techniques
• Absolute decoding
• Linear decoding
• Block decoding
In the absolute decoding technique the memory chip is selected only for the specified logic level on the address lines: no other logic levels can select the chip. Below figure the memory interface with absolute decoding. Two 8K EPROMs (2764) are used to provide even and odd memory banks. Control signals BHE and Ao are use to enable output of odd and even memory banks respectively. As each memory chip has 8K memory locations, thirteen address lines are required to address each locations, independently. All remaining address lines are used to generate an unique chip select signal. This address technique is normally used in large memory systems.
In small system hardware for the decoding logic can be eliminated by using only required number of addressing lines (not all). Other lines are simple ignored. This technique is referred as linear decoding or partial decoding. Control signals BHE and Ao are used to enable odd and even memory banks, respectively. Figure shows the addressing of 16K RAM (6264) with linear decoding. The address line A19 is used to select the RAM chips. When A19 is low, chip is selected, otherwise it is disabled. The status of A14 to A18 does not affect the chip selection logic. This gives you multiple addresses (shadow addresses). This technique reduces the cost of decoding circuit, but it gas drawback of multiple addresses
In a microcomputer system the memory array is often consists of several blocks of memory chips. Each block of memory requires decoding circuit. To avoid separate decoding for each memory block special decoder IC is used to generate chip select signal for each block.
Static Memory Interfacing
The general procedure of static memory interfacing with 8086 as follows:
1. Arrange the available memory chips so as to obtain 16-bit data bus width. The upper 8-bit bank is called ‘odd address memory bank’ and the lower 8-bit bank is called ‘even address memory bank’.
2. Connect available memory address lines of memory chips with those of the microprocessor and also connect the memory RD and WR inputs to the corresponding processor control signals. Connect the 16-bit data bus of the memory bank with that of the microprocessor 8086.
3. The remaining address lines of the microprocessor, BHE and Ao are used for decoding the required chip select signals for the odd and even memory banks. The CS of memory is derived from the output of the decoding circuit.
4. As a good and efficient interfacing practice, the address map of the system should be continuous as far as possible
Dynamic RAM Interfacing
The basic Dynamic RAM cell uses a capacitor to store the charge as a representation of data. This capacitor is manufactured as a diode that is reverse-biased so that the storage capacitance comes into the picture. This storage capacitance is utilized for storing the charge representation of data but the reverse-biased diode has a leakage current that tends to discharge the capacitor giving rise to the possibility of data loss.
To avoid this possible data loss, the data stored in a dynamic RAM cell must be refreshed after a fixed time interval regularly. The process of refreshing the data in the RAM is known as refresh cycle. This activity is similar to reading the data from each cell of the memory, independent of the requirement of microprocessor, regularly. During this refresh period all other operations (accesses) related to the memory subsystem are suspended.
The advantages of dynamic RAM. Like low power consumption, higher packaging density and low cost, most of the advanced computer systems are designed using dynamic RAMs. Also the refresh mechanism and the additional hardware required makes the interfacing hardware, in case of dynamic RAM, more complicated, as compared to static RAM interfacing circuit.
Interfacing I/O Ports
I/O ports or input/output ports are the devices through which the microprocessor communicates with other devices or external data sources/destinations. Input activity, as one may expect, is the activity that enables the microprocessor to read data from external devices, for example keyboard, joysticks, mouser etc. the devices are known as input devices as they feed data into a microprocessor system.
Output activity transfers data from the microprocessor top the external devices, for example CRT display, 7-segment displays, printer, etc, the devices that accept the data from a microprocessor system are called output devices.
Steps in Interfacing an I/O Device
The following steps are performed to interface a general I/O device with a CPU:
1. Connect the data bus of the microprocessor system with the data bus of the I/O port.
2. Derive a device address pulse by decoding the required address of the device and use it as the chip select of the device.
3. Use a suitable control signal, i.e. IORD and /or IOWR to carry out device operations, i.e. connect IORD to RD input of the device if it is an input devise, otherwise connect IOWR to WR input of the device. In some cases the RD or WR control signals are combined with the device address pulse to generate the device select pulse.
The input device is connected to the microprocessor through buffer. The simplest form of a input port is a buffer as shown in the figure. This buffer is a tri-state buffer and its output is available only when enable signal is active. When microprocessor wants to read data from the input device (keyboard), the control signals from the microprocessor activates the buffer by asserting enable input of the buffer. Once the buffer is enabled, data from the device is available on the data bus. Microprocessor reads this data by initiating read command.
It is used to send the data to the output device such as display from the microprocessor. The simplest form of the output port is a latch.
The output device is connected to the microprocessor through latch as shown in the figure. When microprocessor wants to send data to the output device it puts the data on the data bus and activates the clock signal of the latch, latching the data from the data bus at the output of latch. It is then available at the output of latch for the output device.
I/O Interfacing Techniques
Input/output devices can be interfaced with microprocessor systems in two ways:
1. I/O mapped I/O
2. Memory mapped I/O
1. I/O mapped I/O:
8086 has special instructions IN and OUT to transfer data through the input/output ports in I/O mapped I/O system. The IN instruction copies data from a port to the Accumulator. If an 8-bit port is read data will go to AL and if 16-bit port is read the data will go to AX. The OUT instruction copies a byte from AL or a word from AX to the specified port. The M/IO signal is always low when 8086 is executing these instructions. In this address of I/O device is 8-bit or 16-bit. It is 8-bit for Direct addressing and 16-bit for Indirect addressing.
2. Memory mapped I/O
In this type of I/O interfacing, the 8086 uses 20 address lines to identify an I/O device. The I/O device is connected as if it is a memory device. The 8086 uses same control signals and instructions to access I/O as those of memory, here RD and WR signals are activated indicating memory bus cycle.