4-3
Functional Description
General
The 82C54 is a programmable interval timer/counter
designed for use with microcomputer systems. It is a general
purpose, multi-timing element that can be treated as an
array of I/O ports in the system software.
The 82C54 solves one of the most common problems in any
microcomputer system, the generation of accurate time
delays under software control. Instead of setting up timing
loops in software, the programmer configures the 82C54 to
match his requirements and programs one of the counters
for the desired delay. After the desired delay, the 82C54 will
interrupt the CPU. Software overhead is minimal and vari-
able length delays can easily be accommodated.
Some of the other computer/timer functions common to micro-
computers which can be implemented with the 82C54 are:
• Real time clock
• Event counter
• Digital one-shot
• Programmable rate generator
• Square wave generator
• Binary rate multiplier
• Complex waveform generator
• Complex motor controller
Data Bus Buffer
This three-state, bi-directional, 8-bit buffer is used to inter-
face the 82C54 to the system bus (see Figure 1).
Read/Write Logic
The Read/Write Logic accepts inputs from the system bus and
generates control signals for the other functional blocks of the
82C54. A1 and A0 select one of the three counters or the Con-
trol Word Register to be read from/written into. A “low” on the
RD input tells the 82C54 that the CPU is reading one of the
counters. A “low” on the
WR input tells the 82C54 that the CPU
is writing either a Control Word or an initial count. Both
RD and
WR are qualified by CS; RD and WR are ignored unless the
82C54 has been selected by holding
CS low.
CLK 2 18 I CLOCK 2: Clock input of Counter 2.
A0, A1 19 - 20 I ADDRESS: Select inputs for one of the three counters or Control Word Register for read/write
operations. Normally connected to the system address bus.
CS 21 I CHIP SELECT: A low on this input enables the 82C54 to respond to RD and WR signals. RD and
WR are ignored otherwise.
RD 22 I READ: This input is low during CPU read operations.
WR 23 I WRITE: This input is low during CPU write operations.
V
CC
24 V
CC
: The +5V power supply pin. A 0.1µF capacitor between pins VCC and GND is recommended
for decoupling.
Pin Description
(Continued)
SYMBOL
DIP PIN
NUMBER TYPE DEFINITION
A1 A0 SELECTS
0 0 Counter 0
0 1 Counter 1
1 0 Counter 2
1 1 Control Word Register
CONTROL
WORD
REGISTER
COUNTER
2
COUNTER
1
COUNTER
0
INTERNAL BUS
OUT 2
GATE 2
CLK 2
OUT 1
GATE 1
CLK 1
OUT 0
GATE 0
CLK 0
WR
RD
D
7
- D
0
A
0
A
1
CS
FIGURE 1. DATA BUS BUFFER AND READ/WRITE LOGIC
FUNCTIONS
8
DATA/
BUS
BUFFER
READ/
WRITE
LOGIC
82C54
