ST ST7 Series, Руководство пользователя

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10 - Second Application: a Sailing Computer
Thus, displaying a number, say -1.234, consists of finding the corresponding pattern in the
table above and sending all four patterns in a row, starting with the right-most digit. In this ex-
ample, we would send the bytes (in hexadecimal):
The Liquid Crystal Display component used here is said to be static, meaning that there is a
separate pin for each segment or symbol, and no multiplexing is required. Applying a small
voltage between any segment and the backplane will show that segment. The voltage may be
chosen to be either positive or negative.
However, liquid crystal physics are such that a segment cannot be continuously supplied with
a DC voltage, or electrolysis would occur and severely degrade the component. To avoid this,
the voltage polarity must be reversed periodically. This does not alter the clarity of the display,
provided the frequency of the reversing is high enough. In practice, a frequency of 50 Hz is
sufficient, that is, 100 reversals per second.
Since the backplane is connected to one output of the shift register, reversing the polarity
merely involves the one's complem enting of all the bytes sent to the display, every other
frame.
In the example above, the successive frames sent would be:
10.3.2 Push-button circuit
Each of the three push-buttons could have been connected directly to an input of a parallel
port. This is by far the simplest solution. However, there are cases where the number of avail-
Digit or symbol to show 4 3 .2 -1
Value sent C9 8F BE 18
Digit or symbol to show 4 3 .2 -1 Time (ms)
Value sent C9 8F BE 18 0
36 70 41 E7 10
C9 8F BE 18 20
36 70 41 E7 30
C9 8F BE 18 40
etc.