The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Inputs and Outputs
An 8255 Interface
Figure 6-6 shows an 8255 (U15) accessed at FC00h in the 8052-BASIC system. Many of
the pins connect directly to matching signals in the system: D0-D7 connect to the system’s
data bus, A0 and A1 connect to the lowest two address lines, and the RD and WR inputs connect
to U2’s matching outputs. CS connects to pin 7 of U11 (from Figure 6-2), which selects the
chip at addresses from FC00h to FFFFh. U15 actually uses four of these addresses: FC00h
through FC03h.
U15’s RESET input is controlled by the same RESET signal at pin 9 of the 8052-BASIC. The
three new I/O ports are Port A, Port B, and Port C. Address lines A0 and A1 select the port
to be accessed, with Port A at FC00h, Port B at FC01h, and Port C at FC02h.
The port pins should connect only to voltages in the range -0.5V to +6.5V. According to
Intel’s data sheet, the 82C55 has bus-hold circuits that eliminate the need for external
pull-ups on its CMOS inputs.
The Control Word
You configure the 8255 by writing a control word to a control register addressed at FC03h.
The control word has two functions: selecting modes of operation, and setting and clearing
port bits. When bit 7 of a byte written to the control register is 1, the control word selects
modes of operation for each port and determines whether a port is input, output, or
bidirectional. Many combinations of modes and I/O are available. When bit 7 of a byte
written to the control register is 0, the control word sets and clears individual bits of Port C.
The control word is write-only; you can’t read it.
Mode Setting
Figure 6-7, from Intel’s data sheet, describes the mode-set control word. The ports are
divided into two groups. Group A consists of Port A plus bits 4-7 of Port C, and Group B
consists of Port B plus bits 0-3 of Port C. This grouping enables Ports A and B to each use
half of Port C for handshaking, or control, signals.
To set the mode, bit 7 of the control word must be 1. Bit 2 selects mode 0 or 1 for Group B,
and bits 5 and 6 select mode 0, 1, or 2 for Group A. Bits 0, 1, 3, and 4 select whether a port
is input or output, with each half of Port C selected independently.
The simplest mode is Mode 0, Basic Input/Output. The ports behave very much like the
inputs and outputs at U12 and U14. Outputs are latched, so they change only when written
to. The inputs are not latched, so the present, or current, value of the input is always read.
This statement configures all ports as inputs in mode 0:
The Microcontroller Idea Book 103