Page 386 - ARM Based Microcontroller Projects Using MBED

372 15. Mbed RTOS PROJECTS

FIG. 15.6, CONT’D

15.5 PROJECT 2—REFRESHING A 7-SEGMENT DISPLAY

15.5.1 Description

As we have seen in the project in Chapter 8.2, the digits of a multidigit 7-segment LED need
to be refreshed at about every 5–10ms so that the human eye sees both digits to be ON at all
times. The CPU has to spend all of its time to the refreshing process and as a result it cannot do
any other tasks. In this project we use a multitasking approach where the refreshing process
will be implemented in a separate task. In this project the display will count from 00 to 99
continuously with 1s delay between each count.

15.5.2 Aim

The aim of this project is to show how a multidigit 7-segment display can be refreshed in a
multitasking environment.

15.5.3 Block Diagram
The block diagram of the project is as in Fig. 8.1.

15.5.4 Circuit Diagram

The circuit diagram of the project is as in Fig. 8.3 where the display segments are connected
to PORT C pins of the development board, and an NPN transistor is used to control the dis-
play digits.

15.5.5 Program Listing

The program listing (program: RTOS7Segment) is shown in Fig. 15.7. The program is basi-
cally very similar to the one given in Fig. 8.5, except that here the display refreshing is done as a
separatetask.Atthebeginningoftheprogramheaderfilesmbed.handrtos.hareincludedinthe
program and the RTOS library is loaded to the program. The interface between the 7-segment
display and the development board are then defined as in the program in Fig. 8.5 and global