8254
respectively. Both are normally referred to as one
unit and called just OL. These latches normally ‘‘fol-
low’’ the CE, but if a suitable Counter Latch Com-
mand is sent to the 8254, the latches ‘‘latch’’ the
present count until read by the CPU and then return
to ‘‘following’’ the CE. One latch at a time is enabled
by the counter’s Control Logic to drive the internal
bus. This is how the 16-bit Counter communicates
over the 8-bit internal bus. Note that the CE itself
cannot be read; whenever you read the count, it is
the OL that is being read.
Similarly, there are two 8-bit registers called CR
M
and CR
L
(for ‘‘Count Register’’). Both are normally
referred to as one unit and called just CR. When a
new count is written to the Counter, the count is
stored in the CR and later transferred to the CE. The
Control Logic allows one register at a time to be
loaded from the internal bus. Both bytes are trans-
ferred to the CE simultaneously. CR
M
and CR
L
are
cleared when the Counter is programmed. In this
way, if the Counter has been programmed for one
byte counts (either most significant byte only or least
significant byte only) the other byte will be zero.
Note that the CE cannot be written into; whenever a
count is written, it is written into the CR.
The Control Logic is also shown in the diagram.
CLK n, GATE n, and OUT n are all connected to the
outside world through the Control Logic.
8254 SYSTEM INTERFACE
The 8254 is a component of the Intel Microcomputer
Systems and interfaces in the same manner as all
other peripherals of the family. It is treated by the
system’s software as an array of peripheral I/O
ports; three are counters and the fourth is a control
register for MODE programming.
Basically, the select inputs A
0
,A
1
connect to the A
0
,
A
1
address bus signals of the CPU. The CS can be
derived directly from the address bus using a linear
select method. Or it can be connected to the output
of a decoder, such as an Intel 8205 for larger sys-
tems.
OPERATIONAL DESCRIPTION
General
After power-up, the state of the 8254 is undefined.
The Mode, count value, and output of all Counters
are undefined.
How each Counter operates is determined when it is
programmed. Each Counter must be programmed
before it can be used. Unused counters need not be
programmed.
Programming the 8254
Counters are programmed by writing a Control Word
and then an initial count.
The Control Words are written into the Control Word
Register, which is selected when A
1
,A
0
e
11. The
Control Word itself specifies which Counter is being
programmed.
231164–6
Figure 6. 8254 System Interface
5
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