Data Bus Buffer

This 3-stable bi-directional 8-bit buffer is used to interface the 8255A to the systems 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 business and in turn, issues commands to both of the Control Groups.

(CS)

Chip Select. A ¡§low¡¦ on this input pin enables the communication between the 8255A, and the CPU.

(RD)

Read. A ¡§low¡¨ on this Input pin enables the 8255A to send the data or status information to the CPU on the data bus. In essence, it allows the CPU to ¡§read from the 8255A.

(WR)

Write. A. ¡§ low¡¨ on the input pin enables the CPU to write data or control words into the 8255A.

(A0 and A1)

Port Select 0 and Port Select 1. The Input signals, in conjunction with the RD and WR Inputs, controls the selection of one of the three ports or the control word registers. They are normally connected to the least significant bits of the address bus (A0 and A1).

8255A BASIC OPERATION

(RESET)

Reset. A ¡§high¡¨ on this Input clears the control register and all ports (A, B, C) are set to the Input mode.

Group A and Group B Controls

The functional configuration of each port is programmed by the systems software. In essence, the CPU ¡§output¡¨ a control word to the 8255A. The control word contains information such as ¡§mode¡¨, bit set¡¨, bit reset¡¨, etc. that Initializes the functional configuration of the 8255A.

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 ¡V Port A and Port C upper (C7 C4)

Control Group B ¡V Port B and Port C lower (C3 C0)

The Control Word Register can only be written into. No.

Read operation of the Control Word Register is allowed.

Ports A, B, and C

The 8255A 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 personally to further enhance the power and flexibility of the 8255A.

Port A. One 8 bit data output latch/buffer and one 8-bit data input latch.

Port B. One 8-bit data output latch/buffer and one 8-bit data input buffer.

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 controls signal outputs and status signal inputs in conjunction with ports A and B.

Mode Selection

There are three basic modes of operation that can be selected by the systems software:

Mode O ¡V Basic Input/Output

Mode 1 ¡V Strobed Input/Output

Mode 2 ¡V Bi-Directional Bus

When the reset Input goes ¡§high¡¨ all ports will be set to the Input mode (i.e., all 24 lines will be in the high Impedance state). After the reset is removed the 8255A can remain in the input mode with no additional Initialization required. During the execution of the systems program any of the other modes may be selected using a single output Instruction. This allows a single 8255A to service a variety of peripheral devices with a simple software maintenance routine.

The modes for Ports A and Port B can be separately defined, while Port C is divided into two portions as required by the Port A and Port B definitions. All of the output registers, including the status flip-flops, will be reset whenever the mode is changed. Modes may be combined so that their functional definition can be ¡§tailored¡¨ to almost any I/O stricture. For instance; Group B can be programmed in Mode 0 to monitor simple switch closing or display computational results, Group A could be programmed in Mode 1 to monitor a keyboard or tape reader on an interrupt-driven basis.

The Mode definitions and possible mode combinations may seem confusing at first but after a cursory review of the complete device operation a simple , logical I/O approach will surface. The design of the 8255A has taken into account things such as efficient PC board layout, control signal definition vs PC layout and complete functional flexibility to support almost any peripheral device with no use of the available pints.

Single Bit Set/Reset Feature

Any of the eight bits of Port C can be Set or Reset using a single OUT put Instruction. This feature reduces software requirements in Control-based applications.

Interrupt Control Functions

When the 8255A is programmed to operate in mode 1 or mode 2, control signals are provided that can used as interrupt request input to the CPU. The interrupt request signal generated from port C, can be inhibited or enabled by setting or resetting the associated INTE flip-flop, using the bit set/reset function of port C.

This function allows the Programmer to disallow or allow a specific I/O device to interrupt the CPU without affecting any other device in the interrupt structure.

INTE flip-flop definition
(BIT-SET) ¡V INTE is SET ¡V Interrupt enable

(BIT-RESET) ¡V INTE is RESET ¡V Interrupt disable

Note: All Mask flip-flops are automatically reset during mode selection and device reset.

Operating Modes

Mode 0 (Basic Input/Output). This functional configuration provides simple input operations for each of the three ports. No ¡§handshaking¡¨ is required data is simply written to or read from a specified port.

Mode O Basic Functional Definitions:

*Two 8-bit ports and two 4-bit port
*Any port can be input or output.
*Outputs are not latched.
*Inputs are not latched.
*16 different Input/output configurations are not possible in this Mode.

MODE 1 (Strobed Input/Output). This functional configuration provides a means for transferring I/O data to or from a specified port in conjunction with strobes or ¡§handshaking¡¨ signals. In mode 1, port A and Port B use the lines on port C to generate or accept these ¡§handshaking¡¨ signals.

Mode 1 Basic Functional Definitions:

*Two groups (Group A and Group B)
*Each group contains one 8-bit data port and one 4-bit control/data port
*The 8-bit data port can be either Inputs or output Both inputs and outputs are latched.
*The 4-bit port is used for control and status of the 8-bit data port.

Input Control Signal Definition

STB (Strobe Input). A ¡§ low ¡§ on the input loads data into the input latch.

IBF (Input Buffer Full F/F)

A ¡§high¡¨ on this output indicates that the data has been loaded into the input latch. In essence, an acknowledgement.

IBF is set by STB input being low and is reset by the rising edge of the RD input.

INTR (Interrupt Request)

A ¡§high¡¨ on this output can be used to interrupt the CPU when an input device is requesting service, INTR is set by the STB is a ¡§one¡¨, IBF is a ¡§one ¡§ and INTE is ¡§one ¡§. It is reset by the falling edge of RD. This procedure allows an input device to request service from the CPU by simply strobing its data into port.

INTE A

Controlled by bit set/reset of PC4

INTE B

Controlled by set/reset PC2

OBF (Output Buffer Full F/F). The OBF output will go ¡§low¡¨ to indicate that the CPU has written data out to the specified port. The OBF F/F will be set by rising edge of the WR input being low.

ACK (Acknowledge Input). A ¡§low¡¨ on this input informs the 8255A that the data from port A or port B has been accepted. In essence, a response from the peripheral device indicating that it has received the data output by CPU.

INTR (Interrupt Request). A ¡§high¡¨ on the output can be used to interrupt the CPU when an output device has accepted data transmitted by the CPU. INTR is set when ACK is a ¡§one¡¨, OBF is a ¡§one¡¨, and INTE is a ¡§one¡¨. It is reset by the falling edge of WR.

INTE A

Controlled by bit set/reset of PC6.

INTE B

Port A and B can be Individually defined as Input or output in Mode 1 to support a wide varlety of strobed I/O application.

1. Interrupt generation and enable/disable functions are also available.

MODE 2 Basic Functional Definitions:

*Used in Group A only.
*One 8-bit, bi-directional bus Port (Port A) and a 5-bit control Port (Port C).
*Both Inputs and Outputs are latched.
*The 5-bit control port (Port C) is used for control and status for the 8-bit,bi-directional bus port (Port A).

Bi-directional Bus I/O Control Signal Definition

INTR (Interrupt Request). A high on this output can be used to interrupt the CPU for both input or output operations.

Output Operations
OBF (Output Buffer Full). The OBF output will go ¡§low¡¨ to indicate that the CPU has written data out to port A.

ACK (Acknowledge). A ¡§low¡¨ on this input enables the iri-state output buffer of port A to send out the data. Otherwise, the output buffer will be in the high impedance state.

INTE 1 (The INTE Flip-Flop Associated with OBF). Controlled by bit set/reset of PC6

Input Operations
STB (Strobe Interrupt)

STB (Strobed Input). A ¡§low¡¨ on this input loads data into the input latch.

IBF (Input Buffer Full F/F). A ¡§high¡¨ on this output indicates that data has been loaded into the input latch.

INTE 2 (The INTE Flip-Flop Associated with IBF). Controlled by bit set/reset of PC4.

Mode Definition Summary

There are several combinations or modes when not all of the bits in Port C are used for control or status. The remaining bits can be used as follows:

If Programmed as Inputs-

All input lines can be accessed during a normal Port C read.

If programmed as Outputs-

Bits in C upper (PC7-PC4) must be individually accessed using the bit set/reset function.

Bits in C lower (PC3_Pco) can be accessed using the bit set/reset function or accessed as a threesome by writing into Port C.

Source Current Capability on Port B and Port C

Any set of eight output buffers, selected randomly from Ports B and Ports C can source 1mA at 1.5volts. This feature allows the 8255A to directly drive Darlington type drivers and high-voltage displays that require such source current.

Reading Port C Status

In Mode O, Port C transfers data to or from the peripheral device. When the 8255 is programmed to function in Modes 1 or 2, Port C generates or accepts ¡§hand shaking¡¨ signals with the peripheral device. Reading the contents of Port C allows the programmer to test or verify the ¡§status¡¨ of each peripheral device and change the program flow accordingly.

There is co special instruction to read the status information from Port C. A normal read operation of Port C is executed to perform this function.