3.2 The 8255 a PI/O - Chip 82C55A

The 82C55A is a high performance CMOS version of the industry standard 8255A . It is a general purpose programmable I/O device which may be used with many different microprocessors. There are 24 I/O pins which may be individually programmed in 2 groups of 12 and used in 3 major modes of operation. The high performance and industry standard configuration of the 82C55A make it compatible with the 80C86, 80C88 and other microprocessors. Static CMOS circuit design ensures low operating power. TTL compatibility over the full military temperature range and bus hold circuitry eliminate the need for pull-up resistors.

 The device comprises three 8 bit ports whereby port c can be sub-divided into two 4 bit groups. Each of these three ports is addressed by A0 and A1 . With it's read, write, the chip select and data signals it looks and behaves like a tiny ROM or a RAM with only three bytes.

    To switch between 'RAM-' and 'ROM-mode' the 8255 has a mode register where three working modes can be selected. The mode register is selected when A0 and A1 are set to high (+5V).

    The three modes are:

    Mode 0: basic input / output
    Mode 1: strobed input / output
    Mode 2: bi-directional bus

For our purpose only mode 0 was interesting, but the device may be switched into each with the setup supplied in this document.
To work with the three ports they must first be initialized. This is done by writing the proper control word into the control register. Figure 11a shows the definition of the control word. While bit D2, D5, D6 and D7 define mode 0..2 the bits D0, D1, D3 and D4 define the settings of the ports to input- or output-mode. In other words you need to 'program' or switch the 82C55 every time you want to change a port data flow direction.
If 'all input mode' in mode 0 is requested the control word would be 10011011bin = 9Bhex = 155dec. If 'all output mode' in mode 0 shall be selected the control word would be 10000000bin = 80hex = 128dec.