The 8255 microprocessor is an input/output (I/O) device that can be used to interface with various peripheral devices. It has three operating modes: mode 0, mode 1, and mode 2.
Mode 0 is the basic input/output mode. In this mode, the 8255 can be used as an 8-bit input or output device, with each of the three ports (Port A, Port B, and Port C) being configured individually as input or output.
Mode 1 is the strobed input/output mode. This mode is similar to mode 0, but includes additional control signals to enable latching of input data and/or output data.
Mode 2 is the bidirectional mode. In this mode, Port A and Port B are configured as bidirectional I/O ports, while Port C is configured as a control port for setting various operational modes.
Each of these operating modes can be further customized through programming of various control registers, allowing for flexible and efficient interfacing with a wide range of peripheral devices.
There are 2 modes in 8255 microprocessors: 1. Bit set reset (BSR) mode – This mode is used to set or reset the bits of port C only, and selected when the most significant bit (D7) in the control register is 0. Control Register is as follows: This mode affects only one bit of port C at a time because, as user set the bit, it remains set until and unless user changes it. User needs to load the bit pattern in control register to change the bit. 2. Input/output mode (I/O) – This mode is selected when the most significant bit (D7) in the control register is 1.
- Mode 0 – Simple or basic I/O mode: Port A, B and C can work either as input function or as output function. The outputs are latched but the inputs are not latched. It does not have interrupt handling capability.
- Mode 1 – Handshake or strobed I/O: In this either port A or B can work and port C bits are used to provide handshaking. The outputs as well as inputs are latched. It has interrupt handling capability. Before actual data transfer there is transmission of signal to match speed of CPU and printer. Example: When CPU wants to send data to a slow peripheral device like printer, it will send handshaking signal to printer to tell whether it is ready or not to transfer the data. When the printer is ready it will send one acknowledgement to CPU then there will be transfer of data through data bus.
- Mode 2 – Bidirectional I/O: In this mode only port A will work, port B can either is in mode 0 or 1 and port C bits are used as handshake signal. The outputs as well as inputs are latched. It has interrupt handling capability. Control Register is as follows: The most significant bit (D7) is 1 for the I/O mode and 0 for the BSR mode. D6 & D5It is used to set the port A mode. D4 is used to tell whether port A is taking input or displaying the result. If it is 1 then it is taking input otherwise displaying output. D3 is used to tell whether port C higher bits is taking input or displaying the result. If it is 1 then it is taking input otherwise displaying output. D2 tells the mode of port B. If it is 0 then port B is in m0 mode otherwise in m1 mode. D1 is used to tell whether port B is taking input or displaying the result. If it is 1 then it is taking input otherwise displaying output. D0 is used to tell whether port C lower bits is taking input or displaying the result. If it is 1 then it is taking input otherwise displaying output.
When 8255 microprocessor is reset, it will clear the control word register contents, setting all the ports to input mode.
Why use 8255 microprocessor operating modes?
The 8255 microprocessor is a versatile I/O device that can be used to interface with various peripheral devices in a computer system. The use of different operating modes allows for flexibility and customization of the interface, which can be optimized for the specific requirements of the connected devices.
For example, mode 0 is the simplest and most basic operating mode, and is suitable for simple input/output operations where no additional control signals or latching is required. Mode 1 adds strobed input/output capability, which can be useful in applications where input data needs to be latched for synchronization purposes, or where output data needs to be held until a certain condition is met.
Mode 2 is the most advanced operating mode, offering bidirectional I/O capability and a dedicated control port for setting various operational modes. This mode is useful in more complex applications where bidirectional data transfer is required, such as in communication protocols or control systems.
By using the appropriate operating mode and configuring the various control registers, the 8255 can be tailored to the specific requirements of the connected peripheral devices, providing efficient and effective interfacing.
Advantages of 8255 microprocessor operating modes:
The advantages of using the 8255 microprocessor operating modes include:
- Flexibility: The different operating modes of the 8255 provide flexibility in interfacing with various peripheral devices, allowing for customization of the interface to meet the specific requirements of the connected devices.
- Efficiency: The use of the appropriate operating mode and configuration of the control registers can improve the efficiency of the interface, allowing for faster and more reliable data transfer.
- Ease of use: The 8255 is a popular I/O device and its operating modes are well-documented, making it relatively easy to implement and use in a variety of applications.
- Low cost: The 8255 is a low-cost device compared to other I/O devices, making it an attractive option for applications where cost is a consideration.
- Compatibility: The 8255 is compatible with a wide range of microprocessors and can be easily interfaced with various computer systems.
- Versatility: The 8255 can be used in a variety of applications, including communication protocols, control systems, and general-purpose I/O operations.
Dis-advantages of 8255 microprocessor operating modes:
- Limited functionality: While the 8255 offers flexibility and versatility, it is still a relatively simple I/O device and may not offer the same level of functionality as more advanced devices.
- Limited data transfer rate: The 8255 has a limited data transfer rate compared to other I/O devices, which may be a disadvantage in applications where high-speed data transfer is required.
- Limited number of ports: The 8255 has only three I/O ports (Port A, Port B, and Port C), which may be a disadvantage in applications where multiple ports are required.
- Programming complexity: While the 8255 is relatively easy to use, programming the control registers to configure the various operating modes can be complex and time-consuming.
- Compatibility issues: While the 8255 is compatible with a wide range of microprocessors, there may be compatibility issues when interfacing with newer microprocessors or computer systems.
- Obsolescence: The 8255 is an older technology and may be obsolete in newer applications where more advanced I/O devices are required.
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