Ziatech Corporation ZT 8809A, Руководство по эксплуатации

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Counter/Timers (8254)
OPERATION
Reset State
After power-up, the state of the 8254 is undefined. The mode, count
value, and output of all counter/timers are undefined. How each
counter operates is determined when it is programmed; each must be
programmed before it can be used. Unused counters need not be
programmed.
The 8254 does not receive a reset signal; if a board reset occurs due to
power-fail or a pushbutton reset, the timers continue to function as
programmed.
Programming
The three counter/timers are programmed by writing a Control Word
followed by an initial count. The Control Word is written to the
Control Word register at I/O address 0043h. This specifies the counter
being addressed, the mode programmed, whether or not reading or
writing will next take place, and whether or not BCD counting is
desired. Figure 11-3 illustrates the encoding of the Control Word
format.
After the Control Word has been written, the initial count may be
sent. This count must be written following the format specified in the
Control Word. The options are shown in Figure 11-3.
Since each Control Word specifies the counter to which it applies, no
special instruction sequence is required. Any programming sequence
that follows the conventions in Figure 11-3 is acceptable. A new
initial count may be written to a counter at any time without affecting
the counter’s programmed mode. Counting is affected as described
later in this chapter under "Mode Definitions" (see page 11-16). The
new count must follow the programmed count format.
11-6