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Bachelor's Degree in Telecommunications Systems and in Network Engineering |
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Chapter 3 problems |
- B3.7 - |
- 2-digit multiplexed 7-segment display |
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1.- Specifications
We want to use the same 7-segment decoder function for driving multiple common-anode displays. To keep it simple, we implement only a 2-digit multiplexed 7-segment display circuit. In this way, we will multiplex the information to be represented between Digit 1 and Digit 0. As seen in Fig. 1, only one display is active at a given time controlled by input D.
Using the same principle, it is easy to imagine how to expand the circuit for driving a larger number of common-anode 7-segments displays. Thus, this idea on multiplexed display systems or scanning display controllers is generalised in tutorial MDS.
The same project is proposed in D1.7 as a combinational circuit based on logic gates. This Hex_7seg_MUX_2digit.pdsprj simulation in Proteus, give you a better idea how the multiplexer works.
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Fig. 1. Symbol of Hex_7seg_MUX_2digit. |
Complete the symbol in Fig. 1 and the circuit's truth table in Fig. 2 adding a new input E (enable) that blank the two digits when active.
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Fig. 2. Initial idea of the circuit truth table. |
Draw an example of timing diagram.
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Fig. 3. Timing diagram example. |
2. Planning
You will solve the circuit in several steps, adding a few lines of code each time to develop and test immediately to debug any error before adding more code. For instance:
- step #1 read only D and monitor its current value var_D in a watch window.
- step #2 read only E and monitor its current value.
- step #3 read the vector H(3..0) and monitor its current value in a watch window. In this way, your read_inputs() function will be validated.
- step #4 Imagine a given value for the variable that will control the display anodes var_AN, write it in the corresponding pins and observe LED values.
- step #5 Complete and validate your function write_outputs().
- step #6 complete the truth_table() and the project.
A) Planning hardware
Copy and adapt a circuit from any of the LAB9 projects and name it Bin_Johnson_4bit.pdsprj. Assign pins to inputs and outputs accordingly to one of the following options (your instructior will tell you which):
Pin assignment option #1:
E --> RA3; D --> RB7; H(3..2) ---> RC(3..2); H(1..0) ---> RD(7..6); AN(1..0) ---> RA(1..0); a_L ---> RB6; b_L ---> RB5; c_L ---> RB4; d_L ---> RB3; e_L ---> RC7; f_L ---> RC6; g_L ---> RC5
Pin assignment option #2:
E --> RA1; D --> RA0; H(3..2) ---> RD(7..6); H(1..0) ---> RC(3..2); AN(1) ---> RA3; AN(0) ---> RB7; a_L ---> RC7; b_L ---> RC6; c_L ---> RC5; d_L ---> RC4; e_L ---> RB6; f_L ---> RB5; g_L ---> RB4
Pin assignment option #3:
E --> RD7; D --> RD6; H(3..2) ---> RA(1..0); H(1..0) ---> RC(7..6); AN(1) ---> RA2; AN(0) ---> RC5; a_L ---> RB3; b_L ---> RB2; c_L ---> RB1; d_L ---> RD5; e_L ---> RD4; f_L ---> RD3; g_L ---> RD2
Project location:
C:\CSD\P9\Hex_7seg_MUX_2digit\(files)
B) Planning software
Organise the main program in our CSD way. Copy and adapt a circuit from any of the LAB9 projects and name it Hex_7seg_MUX_2digit.pdsprj.
Propose a hardware-software diagram naming all the electrical signals, RAM variables and the software functions.
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Fig. 3. Hardware-software diagram. |
Describe all the RAM memory variables used in this project and its type.
Explain how to configure the µC in init_system().
Organise using a flowchart the interface function read_inputs().
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Fig. 4. Reading inputs. |
Organise using a flowchart the interface function write_outputs().
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Fig. 5. Writing outputs. |
Infer the truth_table() software function using a behavioural plan B interpretation from the corresponding flowchart.
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Fig. 6. Shifter_4bit initial discussion on adapting the truth table using plan B flowcharts. |
3. Developing & 4. Testing (debugging)
- Step #1. Write the Hex_7seg_MUX_2digit.c source code translating the function flowcharts. Start capturing only one input as in (LAB9) and visualising it in the watch window as a RAM variable as indicated in the sequence of steps above in the planning section. And only then go to the next step developing & testing more inputs.
Start a software IDE project for the target microcontroller PIC18F4520 and generate the configuration files ".cof" and ".hex" after compilation. Discuss the project summary: % of ROM used for the code, number of RAM bytes used, etc.
| Note: Step-by-step tactical approach for developing and testing the project: Read one input at a time and run to check that the voltage value is correctly captured as a valid digital value in RAM memory. Write one output at a time and run to check that your code is correct to light the LED connected at the output pin. |
Add a few lines of code every time, compile and run the test intereactively to check results watching variables.
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Fig. 7. Example pictures demonstrating how the circuit works. |
Measure how long does it take to run the main loop code when using a 4 MHz and a 16 MHz quartz crystal oscillator.
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Fig. 8. Loop execution time measurements. |
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