ST AN1781 Application note
AN1781
APPLICATION NOTE
STR71x GPIO
DRIVING FOUR 7-SEGMENT DISPLAY
INTRODUCTION
Seven-segment devices are often preferred in display applications where high luminescence
is required, both for indoor and outdoor applications. Segments are marked with letters: a, b,
c, d, e, f, g and DP, where DP is the decimal point.
The technique described in this note is of a general nature and may be applied to a variety of
applications.
Hardware considerations are reviewed and generation of control software using the STR71x is
described.
Rev. 2
AN1781/0205 1/8
1
STR71x GPIO DRIVING FOUR 7-SEGMENT DISPLAY
1 PRINCIPLE OF OPERATION
A 7-segment display consists of 7 LED’s arranged in a figure-eight pattern, then by selectively
powering-on various combinations of segments, alphanumeric characters may be displayed;
a further LED is present which, when powered-on, causes a dot or decimal point to be dis
played (Figure 1). The LED display can be driven by a common cathode or common anode.
With a common cathode display, the common cathode must be connected to the 0V rail and
the LEDs are turned on with a logic one.
Table 1 illustrates the required segment patterns for each numeric representation, including
the optional decimal point.
Figure 1. Structure of a 7-segment display
a
-
f
b
g
e
d
c
DP
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2
STR71x GPIO DRIVING FOUR 7-SEGMENT DISPLAY
Table 1. LEDs turned on for a given digit
a b c d e f g DP Hex
0 1 1 1 1 1 1 0 x 0x3F
1 0 1 1 0 0 0 0 x 0x06
2 1 1 0 1 1 0 1 x 0x5B
3 1 1 1 1 0 0 1 x 0x4F
4 0 1 1 0 0 1 1 x 0x66
5 1 0 1 1 0 1 1 x 0x6D
6 1 0 1 1 1 1 1 x 0x7D
7 1 1 1 0 0 0 0 x 0x07
8 1 1 1 1 1 1 1 x 0x7F
9 1 1 1 1 0 1 1 x 0x6F
A 1 1 1 1 0 1 1 x 0x77
B 0 1 1 1 1 0 1 x 0x7C
C 1 0 0 1 1 1 0 x 0x39
D 0 1 1 1 1 0 1 x 0x5E
E 1 0 0 1 1 1 1 x 0x79
F 1 0 0 0 1 1 1 x 0x71
DP x x x x x x x 1
x = Don’t care
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