The Intel 82C55A is a high-performance, CHMOS version of the industry standard 8255A general purpose programmable I/O device which is designed for use with all Intel and most other microprocessors. It provides 24 I/O pins which may be individually programmed in 2 groups of 12 and used in 3 major modes of operation. The 82C55A is pin compatible with the NMOS 8255A and 8255A-5. In MODE 0, each group of 12 I/O pins may be programmed in sets of 4 and 8 to be inputs or outputs. In MODE 1, each group may be programmed to have 8 lines of input or output. 3 of the remaining 4 pins are used for handshaking and interrupt control signals. MODE 2 is a strobed bi-directional bus configuration. The 82C55A is fabricated on Intel’s advanced CHMOS III technology which provides low power consumption with performance equal to or greater than the equivalent NMOS product. The 82C55A is available in 40-pin DIP and 44-pin plastic leaded chip carrier (PLCC) packages.
Intel 82C55A Chip Architecture
82C55A FUNCTIONAL DESCRIPTION
General
The 82C55A is a programmable peripheral interface device designed for use in Intel microcomputer systems. Its function is that of a general purpose I/O component to interface peripheral equipment to the microcomputer system bus. The functional configuration of the 82C55A is programmed by the system software so that normally no external logic is necessary to interface peripheral devices or structures.
Data Bus Buffer
This 3-state bidirectional 8-bit buffer is used to interface the 82C55A to the system data bus. Data is transmitted or received by the buffer upon execution of input or output instructions by the CPU. Control words and status information are also transferred through the data bus buffer.
Read/Write and Control Logic
The function of this block is to manage all of the internal and external transfers of both Data and Control or Status words. It accepts inputs from the CPU Address and Control busses and in turn, issues commands to both of the Control Groups.
Group A and Group B Controls
The functional configuration of each port is programmed by the systems software. In essence, the CPU “outputs” a control word to the 82C55A. The control word contains information such as “mode”, “bit set”, “bit reset”, etc., that initializes the functional configuration of the 82C55A. Each of the Control blocks (Group A and Group B) accepts “commands” from the Read/Write Control Logic, receives “control words” from the internal data bus and issues the proper commands to its associated ports. Control Group A – Port A and Port C upper (C7±C4) Control Group B – Port B and Port C lower (C3±C0) The control word register can be both written and read as shown in the address decode table in the pin descriptions. Figure 6 shows the control word format for both Read and Write operations. When the control word is read, bit D7 will always be a logic “1”, as this implies control word mode information. Ports A, B, and C
The 82C55A contains three 8-bit ports (A, B, and C). All can be configured in a wide variety of functional characteristics by the system software but each has its own special features or “personality” to further enhance the power and flexibility of the 82C55A.
Port A. One 8-bit data output latch/buffer and one 8-bit input latch buffer. Both “pull-up” and “pulldown” bus hold devices are present on Port A.
Port B. One 8-bit data input/output latch/buffer. Only “pull-up” bus hold devices are present on Port B.
Port C. One 8-bit data output latch/buffer and one 8-bit data input buffer (no latch for input). This port can be divided into two 4-bit ports under the mode control. Each 4-bit port contains a 4-bit latch and it can be used for the control signal outputs and status signal inputs in conjunction with ports A and B. Only “pull-up” bus hold devices are present on Port C.
82C55A Block Diagram Showing Data Bus Buffer and Read-Write Control Logic Functions
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