Bus organization of 8085 microprocessor

Introduction :

The bus organization of the 8085 microprocessor is the way in which the microprocessor communicates with other devices in a computer system. The 8085 microprocessor has a 16-bit address bus, an 8-bit data bus, and various control signals that are used to manage data transfer and other operations.

The address bus is used to specify the memory location or device with which the microprocessor wants to communicate. It is 16 bits wide, which allows the microprocessor to address up to 64K bytes of memory. The address bus is unidirectional, which means that data can only flow in one direction from the microprocessor to the addressed device.

The data bus is used to transfer data between the microprocessor and other devices. It is 8 bits wide, which means that data can be transferred in byte-sized chunks. The data bus is bidirectional, which means that data can flow in either direction between the microprocessor and other devices.

In addition to the address and data buses, the 8085 microprocessor has various control signals that are used to manage data transfer and other operations. These control signals include the read (RD), write (WR), and hold (HLDA) signals, among others. The RD and WR signals are used to control data transfer to and from memory or other devices, while the HLDA signal is used to indicate that the microprocessor is in a hold state and cannot execute instructions.

Bus is a group of conducting wires which carries information, all the peripherals are connected to microprocessor through Bus. Diagram to represent bus organization system of 8085 Microprocessor. Why use Bus organization in 8085 microprocessor  ?

 There are several reasons why bus organization is used in the 8085 microprocessor:

1. Memory access: The bus organization is used for accessing memory by transferring the address of the memory location through the address bus and the data to be stored or retrieved through the data bus. This enables the microprocessor to read and write data to and from memory, which is essential for executing instructions and storing data.
2. I/O operations: The bus organization is used for performing input/output (I/O) operations by transferring the input/output device address through the address bus and the data to be input or output through the data bus. This enables the microprocessor to communicate with peripheral devices such as keyboards, displays, and sensors.
3. Interrupt handling: The bus organization is used for interrupt handling, where the microprocessor uses the address bus to fetch the interrupt vector and the data bus to fetch the interrupt service routine. This enables the microprocessor to respond to external events and perform time-critical operations.
4. DMA operations: The bus organization is used for performing Direct Memory Access (DMA) operations, where the data transfer between the memory and I/O devices takes place without the intervention of the microprocessor. This enables high-speed data transfer between devices and reduces the load on the microprocessor.
5. Control signal transfer: The bus organization is used for transferring control signals between the microprocessor and other components of the system. This enables the microprocessor to control the operation of devices and coordinate the execution of instructions.

There are three types of buses.

• Address bus - 

The address bus is a unidirectional bus that is used to carry the memory or I/O device address to which the data is to be transferred. The address bus in the 8085 microprocessor is 16-bit wide.

It is a group of conducting wires which carries address only.Address bus is unidirectional because data flow in one direction, from microprocessor to memory or from microprocessor to Input/output devices (That is, Out of Microprocessor). Length of Address Bus of 8085 microprocessor is 16 Bit (That is, Four Hexadecimal Digits), ranging from 0000 H to FFFF H, (H denotes Hexadecimal). The microprocessor 8085 can transfer maximum 16 bit address which means it can address 65, 536 different memory location. The Length of the address bus determines the amount of memory a system can address.Such as a system with a 32-bit address bus can address 2^32 memory locations.If each memory location holds one byte, the addressable memory space is 4 GB.However, the actual amount of memory that can be accessed is usually much less than this theoretical limit due to chipset and motherboard limitations.

• Data bus -

The data bus is an 8-bit bidirectional bus that is used to transfer data between the microprocessor and other components such as memory and I/O devices. It is used to carry data to or from the memory or input/output devices.

 It is a group of conducting wires which carries Data only.Data bus is bidirectional because data flow in both directions, from microprocessor to memory or Input/Output devices and from memory or Input/Output devices to microprocessor. Length of Data Bus of 8085 microprocessor is 8 Bit (That is, two Hexadecimal Digits), ranging from 00 H to FF H. (H denotes Hexadecimal). When it is write operation, the processor will put the data (to be written) on the data bus, when it is read operation, the memory controller will get the data from specific memory block and put it into the data bus. The width of the data bus is directly related to the largest number that the bus can carry, such as an 8 bit bus can represent 2 to the power of 8 unique values, this equates to the number 0 to 255.A 16 bit bus can carry 0 to 65535.

• Control bus - 

The control bus is a bidirectional bus that is used to carry control signals between the microprocessor and other components such as memory and I/O devices. It is used to transmit commands to the memory or I/O devices for performing specific operations.

It is a group of conducting wires, which is used to generate timing and control signals to control all the associated peripherals, microprocessor uses control bus to process data, that is what to do with selected memory location. Some control signals are:

Memory read

Memory write

1. I/O read
2. I/O Write
3. Opcode fetch

Uses of Bus organization in 8085 microprocessor :

Some of the important uses of bus organization in the 8085 microprocessor are:

1. Memory access: The bus organization is used for accessing the memory by transferring the address of the memory location through the address bus and the data to be stored or retrieved through the data bus.
2. I/O operations: The bus organization is used for performing input/output operations by transferring the input/output device address through the address bus and the data to be input or output through the data bus.
3. Interrupt handling: The bus organization is used for interrupt handling, where the microprocessor uses the address bus to fetch the interrupt vector and the data bus to fetch the interrupt service routine.
4. DMA operations: The bus organization is used for performing Direct Memory Access (DMA) operations, where the data transfer between the memory and I/O devices takes place without the intervention of the microprocessor.

Advantages:

Flexibility: The bus organization used in the 8085 microprocessor allows it to communicate with a wide range of devices. This flexibility makes it well-suited for use in a variety of computer systems, including embedded systems, personal computers, and other devices.

Modularity: The bus organization makes it easy to add or remove devices from a computer system. This modularity allows system designers to customize the system to meet the needs of specific applications.

Scalability: The bus organization used in the 8085 microprocessor is scalable, which means that it can be used in systems of varying sizes and complexity. This scalability makes it well-suited for use in systems that require a wide range of performance levels.

Low Cost: The bus organization used in the 8085 microprocessor is relatively simple and inexpensive to implement. This makes it an attractive option for low-cost, embedded applications.

Disadvantages:

Limited Bandwidth: The bus organization used in the 8085 microprocessor has a limited bandwidth, which can limit the performance of the processor in high-performance applications.

Latency: The bus organization can introduce latency, which is the delay between the time a command is issued and the time the response is received. This latency can be a problem in real-time applications that require immediate responses.

Data Integrity: The bus organization used in the 8085 microprocessor is vulnerable to data corruption due to electromagnetic interference and other sources of noise. This can lead to errors in data transmission and processing.

Complexity: The bus organization used in the 8085 microprocessor can be complex to implement and troubleshoot, which can increase the cost and time required to develop and maintain computer systems.

Issues of Bus organization in 8085 microprocessor :

Some of the main issues with bus organization in the 8085 microprocessor are:

1. Limited data transfer rate: The 8085 microprocessor has an 8-bit data bus, which means that it can transfer only 8 bits of data at a time. This limited data transfer rate can be a bottleneck in systems that require faster data transfer.
2. Limited address range: The 8085 microprocessor has a 16-bit address bus, which limits the addressable memory to 64 KB. This can be a limitation in systems that require larger memory addressing.
3. Bus contention: Bus contention occurs when two or more devices try to use the bus at the same time. This can cause data corruption and other errors in the system.
4. Timing issues: The bus organization requires precise timing for the signals to be transmitted correctly. Any timing errors can cause data corruption or other errors in the system.
5. Limited number of devices: The bus organization of the 8085 microprocessor can support a limited number of devices due to its limited bus width and address range. This can be a limitation in systems that require more devices to be connected.
6. Noise interference: The signals on the bus can be affected by noise interference, which can cause errors in the system.
7. Power consumption: The bus organization can consume significant power, especially when many devices are connected to the bus. This can be a limitation in portable or low-power systems.