OPERATIONAL MODES OF 8255 PPI IC

by Subham · March 28, 2018

Here we come with a new topic operational modes of 8255 PPI IC. In my previous post I discussed on details of “Programmable Peripheral Interface (PPI) IC 8255”. Now here we see how we can classified 8255 PPI IC depending upon its operational modes. The operational modes of 8255 PPI IC can be classified into two broad groups.

I/O Mode
BIT Set Reset (BSR) modes.

Further I/O mode be subdivided into three groups.

Mode 0
Mode 1
Mode 2.

Now we go for some elaborate discussion on all the operational modes of 8255 PPI IC.

I/O Mode

I/O mode refer as input/output mode. So for any input/output operation of microprocessor this mode will activated. As we know the input/output mode can further be subdivided into three groups:

Mode 0 – Simple Input/output mode

In this mode the 8 bit port A (PA0-PA7) of 8255 IC can be configured as input or output port. In the similar fashion port B (PB0-PB7) can also be configured as input or output operation. However there is flexibility for the port C. It can be divided into two 4 bit ports, the port C_Lower (PC0-PC3) and port C_Upper (PC4-PC7). Each of them can be set independently for input or output operation. In this way can say there are four ports (port-A, port-B, port C_Lowerand port C_Upper) and each of them can be set either as an input port or an output port.

Here these ports are simple input or output ports. That means these ports can work without handshaking. In this mode the outputs are latched whereas the inputs are not latched.

Mode 1 – Strobed Input/output or Handshake mode

Mode 1 is a second mode of 8255 I/O mode. In this mode of operation handshaking is used for the input or output data transfer. Another important think we have to remember that there are two groups in 8255 PPI, Group A and Group B. Both these groups have one 8-bit port and one 4-bit port.Group A consist Port-A and Port C_Upper. And group B consist Port-B and Port C_Lower. The 8-bit port of each group can be programmed for input or output operation with latched input and latched output facilities. The bits of Port C are used for handshaking.

Mode 2 – Bidirectional Mode

So the final mode of 8255 is Mode 2. In this mode Port A can be programmed to operate as a bidirectional port. When Port A is programmed in this mode of operation, Port B can be used either in Mode 0 or Mode 1. For mode 2 operation PC3 to PC7 bits are used for handshaking. In this mode too both inputs and outputs are latched.

Bit Set/Reset Mode

In BSR mode, any of the 8-bits of port C can be Set or reset using a single control word written into the control register. Because of this facility it helps the programmer to control the port C pin output individually. It is also used in mode 1 and mode 2 operation. In bellow you will find the control word format for BSR mode.