1. Logic Device and Memory

2. Tri-state Devices Tri-state logic devices have three states: logic 1, logic 0, and high impedance. A tri-state device has three lines- 1) Input 2) Output 3) Enable When Enable line is activated the tri-state device works as ordinary logic devices. While if this line is disabled, the device goes into high impedance state (as if it is disconnected from system) In microcomputer systems, peripherals are connected in parallel between address bus & data bus through tri-state devices. Because of tri-state device peripherals do not load the system.

3. Active High Buffer when Enable pin is High- output is same as input When Enable pin is Low- buffer stays in high impedance state Active Low Buffer when Enable pin is Low- output is same as input When Enable pin is High- buffer stays in high impedance state Active High Inverter when Enable pin is High- output is complement of input When Enable pin is Low- inverter stays in high impedance state Active Low Inverter when Enable pin is Low- output is complement of input When Enable pin is High- inverter stays in high impedance state

4. Buffers Buffer is a logic circuit that enhances the current or power. This is similar to non-inverting amplifier with unit gain. The buffer is used to increase the driving capability of a logic circuit. It has one input line & one output line. The logic level of output is the same as that of input Simple Buffer Tri-state Buffer It has one input line, one output line & one Enable line Depending on Enable line it is further classified into two types 1) Active High Buffer 2) Active Low Buffer

5. Active Low Buffer When Enable is Low- Buffer is active. Therefore logic level of output is the same as that of input When Enable is High- Buffer goes into high impedance state (Disabled) When Enable is High- Buffer is active. Therefore logic level of output is the same as that of input When Enable is Low- Buffer goes into high impedance state (Disabled) Active High Buffer

6. Inverters Inverter is a logic circuit that inverts the input signal. This is exactly opposite to buffer. The output is complement of input. It has one input line & one output line. The output is complement of input. It is similar to NOT gate. Simple Inverter Tri-state Inverter It has one input line, one output line & one Enable line Depending on Enable line it is further classified into two types 1) Active High inverter 2) Active Low inverter

7. Active Low Inverter When Enable is Low--- Inverter is active. Therefore logic level of output is complement of the input When Enable is High--- Inverter goes into high impedance state (Disabled) When Enable is High--- Inverter is active. Therefore logic level of output is complement of the input When Enable is Low--- Inverter goes into high impedance state (Disabled) Active High Inverter

8. Encoder Encoder is a logic circuit that provides the appropriate code (Binary, BCD, etc.) as output for each input signal. 8 to 3 Encoder D7D6D5D4D3D2D1D0D7D6D5D4D3D2D1D0 Y2Y1Y0Y2Y1Y0

9. Construction & Working 8 to 3 priority encoder has 8 inputs (D 0 to D 7 ) & 3 outputs (Y 0, Y 1, Y 2 ) When any one of the inputs goes HIGH then code for that input is generated at the output. Example: When input line D 3 is made high keeping all other lines low then code for it is generated at output (i.e., 011) making Y 2 =0, Y 1 =1, Y 0 =1. Term Priority means if more than one inputs are made HIGH simultaneously then the code of the selected input of highest priority is generated at output. Example: When input line D 2 & D 6 are made high simultaneously then code for D 6 is generated at output (i.e., 110) making Y 2 =1, Y 1 =1, Y 0 =0 ignoring line D 2 completely.

10. InputsOutputs D7D6D5D4D3D2D1D0Y2Y1Y0 00000001000 0000001X001 000001XX010 00001XXX011 0001XXXX100 001XXXXX101 01XXXXXX110 1XXXXXXX111

11. Decoder Decoder is a logic circuit that identifies code present in the input & produces the output accordingly. 3 to 8 Decoder Y7Y6Y5Y4Y3Y2Y1Y0Y7Y6Y5Y4Y3Y2Y1Y0 D2D1D0D2D1D0

12. Construction & Working 3 to 8 decoder has 3 inputs (D 0, D 1, D 2 ) & 8 outputs (Y 0 to Y 7 ) When a 3 bit code is fed to input lines then equivalent output line for that code goes high keeping all other output lines low. Example: When code 101 is given at input (i.e., D 2 =1, D 1 =0, D 0 =1) then output pin Y 5 goes high keeping all other output pins low.

13. InputsOutputs D2D1D0Y7Y6Y5Y4Y3Y2Y1Y0 00000000001 00100000010 01000000100 01100001000 10000010000 10100100000 11001000000 11110000000

14. Introduction to Memory Device Memory is the storage device which can be used to store monitor program, user program or user data. The memory consists of the thousands of memory cells arranged to store data. Each memory cell is capable of storing 1 bit of data. Memory must be properly interfaced with microprocessor properly, so that it can be accessed while reading or writing the data or program from/to it. Similarly input/output devices are also required to read or write data. So these devices must be interfaced properly with microprocessor. Therefore a microprocessor can read data from input or memory. Also it can write data to output or memory.

15. Memory plays a significant role in the following important aspects of a computer system 1.Performance: The amount & type of system memory is an important contributing factor to overall performance. Insufficient memory can cause a processor to work at 50% or even more below its potential. 2.Software Support: Newer programs require more memory than old ones. More memory will give you access to programs that you cannot use with a lesser amount. 3.Reliability and Stability: Bad memory is a leading cause of mysterious system problems. Ensuring you have high quality memory, will result in a smooth running PC will lesser problems. 4.Upgradability: There are many different types of memory available, and some are more universal than others. Making a wise choice can allow you to migrate your memory to a future system or continue to use it after you upgrade your motherboard.

16. Types of memory Random Access Memory (RAM) It is the kind of memory used for holding programs and data being executed. RAM can be read as well as written, therefore also known as Read Write memory (RWM). It is a volatile memory. The volatility of RAM also means that you risk losing what you are working on unless you save it frequently. RAM is much faster than ROM. That is why RAM is often used to shadow the BIOS ROM to improve performance when executing BIOS code. There are different types of RAM….. Static RAM & many flavors of Dynamic RAM

17. SRAM & DRAM SRAM (Static RAM)DRAM (Dynamic RAM) Each SRAM cell is a flip flopEach DRAM cell consists of one MOSFET & a capacitor More number of components per cell. Less number of components per cell. Memory cell area is less than DRAM Memory cell area is more than SRAM Refreshing is not requiredRefreshing is required Access time is less, so these are faster memories Access time is more, so these are slower memories Less power consumptionMore power consumption

18. EPROM Erasable Programmable Read Only Memory Data can be erased by exposing EPROM to ultra violet light, hence also called UV PROM. Selective erasing is not possible. All the locations get erased. Time required for erasing is 10 to 15 min. It is necessary to remove the EPROM chip from the circuit in order to erase the data.

19. EEPROM Electrically Erasable Programmable Read Only Memory Data can be erased by applying voltage of 20 to 25 V. Selective erasing is possible. A particular location only can be erased. Time required for erasing is less than 10 ms. It is not necessary to remove the EEPROM chip from the circuit in order to erase the data.

20. Different signals required for memory interfacing Each memory chip like RAM, ROM, EPROM, EEPROM etc have pins and these pins are used to accept different kind of signals. Normally every memory chip has pins for address, data, control signals and chip select signals.

21. Different signals required for memory interfacing Address Pins: Address pins are used to accept address from the system address bus transmitted by the microprocessor. The number of address pin depends upon size of the memory

22. Different signals required for memory interfacing Data Pins: The size of the data bus depends upon the data bits, which can be stored in memory location. Standard memory data bits stored in a memory location available are 1, 4, 8 bits.

23. Chip Select Logic Chip select logic can be developed using either combination of different gates such as AND, NAND, NOT etc. or Decoders.

24. Chip Select Logic Using Logic Gates: Take an example of interfacing of 2K of RAM with microprocessor 8085 which is an 8-bit microprocessor. Hence all 8 data bus lines can be directly connected after de-multiplexing to D 0 -D 7 of RAM memory. Total 11 address lines required to access any memory location within 2K memory, so out of 16 address lines (A0-A15) of microprocessor, A0-A10 address lines can be connected directly to RAM memory. The remaining address lines can be used to generate chip select signals using NAND and NOT gates depending on the addresses required as shown in the fig.