The Intel 8155 is a multifunction peripheral chip developed by Intel for use with microprocessor systems such as the Intel 8085 Microprocessor and Intel 8086 Microprocessor. It integrates memory, programmable input/output ports, and a timer in a single chip. The main purpose of the 8155 is to reduce the number of external components required in a microprocessor-based system. By combining RAM, I/O ports, and timing functions, it simplifies hardware design and improves system efficiency.
Table of Contents
Main Features
• 256 × 8 bits of internal RAM
• Three programmable I/O ports
• Programmable timer
• Compatible with 8085 microprocessor
• Internal address decoding
• Multiple functional units in a single chip
These features allow the device to perform several system functions without requiring separate chips.
Internal Architecture
The internal architecture of the 8155 consists of several functional blocks that allow it to interact with the microprocessor and external devices.
The major blocks include:
• Data bus buffer
• Address decoder
• Internal RAM
• I/O ports
• Timer section
• Control logic
The data bus buffer connects the 8155 to the system data bus, allowing data transfer between the microprocessor and the internal units of the device.
The address decoder identifies whether the microprocessor is accessing RAM, I/O ports, or control registers within the chip.
Internal RAM
The 8155 contains 256 bytes of static RAM. This memory can be used for temporary storage of data and instructions during program execution.
The microprocessor can read from or write to this RAM using standard memory read and write operations. This internal RAM reduces the requirement for external memory chips in small systems.
Input and Output Ports
The 8155 provides three programmable ports that allow communication between the microprocessor and external devices.
Port A is an 8-bit programmable input/output port that can be configured as either input or output depending on the control word.
Port B is also an 8-bit programmable input/output port used for interfacing with external peripherals such as keyboards, switches, or displays.
Port C is a 6-bit port that can be used for control signals or additional input/output operations.
Although the chip provides multiple lines, some of them are used for control functions, so the number of freely programmable I/O lines is reduced.
Programmable Timer
The 8155 includes an internal timer that can be programmed by the microprocessor to generate time delays or count events.
Important characteristics of the timer include:
• 14-bit programmable counter
• Can operate as an interval timer
• Can generate square wave outputs
• Can be used for event counting
The timer is useful in applications where precise timing or periodic signals are required.
Control Word Register
The behavior of the ports and timer is controlled by a control word written into the control register by the microprocessor.
The control word determines:
• Whether a port operates as input or output
• Timer operation mode
• Control functions for the ports
By changing the control word, the microprocessor can dynamically modify how the 8155 operates.
Interfacing with 8085
The Intel 8085 Microprocessor can communicate directly with the 8155 through the system bus.
The main signals used during interfacing include:
• Data bus lines (D0 – D7)
• Address lines
• Read signal (RD)
• Write signal (WR)
• Chip select signal
• IO/M control signal
When the microprocessor performs a read or write operation at the address assigned to the 8155, the corresponding internal register, RAM location, or port is accessed.
Applications
The 8155 is widely used in microprocessor-based control systems. Typical applications include:
• Keyboard and switch interfacing
• Display control systems
• Industrial control equipment
• Data acquisition systems
• Embedded systems
• Timing and delay generation circuits
Because it integrates multiple hardware functions in one device, it significantly simplifies system design.
Comparison with Other Peripheral ICs
Several peripheral integrated circuits are used in microprocessor systems.
For example, the Intel 8255 provides programmable input/output ports but does not include internal RAM. The Intel 8253 is designed specifically as a programmable timer, while the Intel 8259 is used for interrupt control.
The 8155 differs from these devices because it combines memory, I/O ports, and a timer in a single chip.