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STR71x GPIO DRIVING FOUR 7-SEGMENT DISPLAY

2 MULTIPLEXING FOUR 7-SEGMENT LED DISPLAYS

2.1 HARDWARE LAYOUT

Biasing is achieved using the STR71x I/O lines. Eight lines (P1.0 - 7) are assigned to LED seg-

ments a - g and the DP. Four lines (P1.8 - 11) are used to drive and select the 7-segment dis-

plays through sink transistors (Figure 2).

Figure 2. Multiplexing four 7-segments displays

2.2 SOFTWARE IMPLEMENTATION

Biasing can be either continuous or multiplexed as long as the refresh frequency is high

enough to ensure image persistence for the human eye (at least 25 cycles per second to avoid

flicker) and it will appear that all the displays are turned on at the same time. As each display

is turned on, the appropriate information must be delivered to it so that it will give the correct

reading.

The multiplexing is achieved by turning on each display for 5 ms duration every 20 ms. This

gives an update rate of 50 Hz. The 5 ms time-base can be generated using a timer overflow in-

terrupt.

The main program uses a global pointer which refers to the variable to display. The lower 4

bits correspond to the least significant digit, the next 4 bits correspond to the second digit and

so on.

P1.0

P1.1

P1.2

P1.3

P1.4

P1.5

P1.6

P1.7

P1.8

P1.9

P1.10

P1.11

4 x 7-segment displays

a

b

c

d

e

f

g

DP

BC 547C

BC 547C

BC 547C

BC 547C

1.2K

1.2K

1.2K

1.2K

0.33K

0.33K

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