8255 Programmable Peripheral Interface

8255 Programmable Peripheral Interface

The parallel input-output port chip 8255 is also called as programmable peripheral input- output port. It has 24 input/output lines which may be individually programmed in  three groups of eight lines. The two groups of I/O pins are named as Group A and Group B. Each of these two groups contains a subgroup of eight I/O lines called as 8-bit port and another subgroup of four lines or a 4-bit port.

 Thus Group A contains an 8-bit port A along with a 4-bit port C upper. The port A lines are identified by symbols PA0-PA7 while the port C lines are identified as PC4-PC7. Similarly, Group B contains an 8-bit port B, containing lines PB0-PB7 and a 4-bit port C with lower bits PC0- PC3. The port C upper and port C lower can be used in combination as an 8-bit port C. Both the port C are assigned the same address. Thus one may have either three 8-bit I/O ports or two 8-bit and two 4-bit ports from 8255. All of these ports can function independently either as input or as output ports. This can be achieved by programming the bits of an internal register of 8255 called as control word register (CWR). This buffer receives or transmits data upon the execution of input or output instructions by the microprocessor. The control words or status information is also transferred through the buffer.

•  RD: This is the input line driven by the microprocessor and should be low to indicate read operationto 8255.
• WR: This is an input line driven by the microprocessor. A low on this line indicates write operation.
• CS: This is a chip select line. If this line goes low, it enables the 8255 to respond to RD and WRsignals, otherwise RD and WR signal are neglected.
• A1-A0: These are the address input lines and are driven by the microprocessor. These lines A1-A0with RD, WR and CS from the following operations for 8255. These address lines are used for addressing any one of the four registers
• D0-D7: These are the data bus lines those carry data or control word to/from the microprocessor.
• RESET: A logic high on this line clears the control word register of 8255. All ports are set as inputports by default after reset.