UPC EETAC Bachelor's Degree in Telecommunications Systems and in Network Engineering EEL

Chapter 2 problems

- B3.7 -

Multiplexed displays: A: 7-segment

B: LED bar

Products

B3.6

B3.8

1. Specifications

Because microcontrollers typically have a limited number of pins, implementing a multiplexed display system (MDS) is an effective way to reduce the required pin count when multiple displays are needed. Review the MDS tutorial for more details. For instance, a 10-digit calculator display requires only 17 pins to display any number from 0000000000 to 9999999999, with the Binary-Coded Decimal (BCD) data stored in 10 bytes of RAM.

The same project is proposed in D1.7 as a combinational circuit based on logic gates.

We can design the BCD_7seg_MUX_3digit represented in Fig. 1. To practise with polling inputs, we will imagine that the H, T and U data is external, generated by 12 switches. Select signal S(1..0) can be generated externally for example at 1 kHz by a modulo 3 counter, S = 0, 1, 2, 0, 1, 2, 0, ... and will multiplex the numbers to the corresponding displays.

- When S = 0, CA_L = "110" and the number U is decoded at segment outputs to be represented on digit 0.

- When S = 1, CA_L = "101" and the number T is decoded at segment outputs to be represented on digit 1.

- When S = 2, CA_L = "011" and the number H is decoded at segment outputs to be represented on digit 2.

 

Symbol and application

Fig. 1. Symbol for this MDS_HEX_7seg_3digit and the external driving circuit for the common anodes. Only one digit is activated at a time.

A) Planning hardware

Pin assignment option #1:

H(3..2) ---> RD(7..6) H(1..0) ---> RC(1..0)
T(3..2) ---> RD(5..4) T(1..0) ---> RC(3..2)  
U(3..2) ---> RD(3..2) U(1..0) ---> RC(5..4)  
S(1..0) ---> RD(1..0)    
CA_L(2..0) ---> RB(7..5)  
a_L, b_L, c_L, d_L ---> RB(4..1) e_L, f_L, g_L ---> RA(3..1)  

 

Pin assignment option #2:

H(3..2) ---> RA(5..4) H(1..0) ---> RB(7..6)
T(3..2) ---> RB(5..4) T(1..0) ---> RD(7..6)  
U(3..1) ---> RD(5..3) U(0) ---> RC(7)  
S(1..0) ---> RC(6..5)    
CA_L(2..0) ---> RC(4..2)  
a_L, b_L ---> RC(1..0) c_L, d_L, e_L, f_L ---> RA(3..0) g_L ---> RB(2)

 

Pin assignment option #3:

H(3..1) ---> RC(7..5) H(0) ---> RD(7)
T(3..0) ---> RD(6..3)    
U(3..2) ---> RB(7..6) U(1..0) ---> RA(5..4)  
S(1..0) ---> RA(3..2)    
CA_L(2..0) ---> RB(5..3)  
a_L, b_L, c_L ---> RD(2..0) d_L, e_L, f_L, g_L ---> RC(4..1)  

 

Project location:

 C:\CSD\P9\MDS_BCD_7seg_3digit\(files)

 

Chapter 2 problems

- B3.7 -

Multiplexed displays: A: 7-segment

B: LED bar

Products

B3.6

B3.8

1. Specifications

In this project we propose to use a µC PIC18F48K22 to implement the combinational circuit Bar_28LED_dec represented in Fig. 1. The same project is proposed in D1.7 as a combinational circuit based on logic gates (complete and use the truth table in Fig. 7).

Symbol for the Bar_28LED_dec

Fig. 1. Symbol of the Bar_28LED_dec.

A) Planning hardware

To practise with polling inputs we will imagine a hardware circuit in Proteus where number B and enable E are generated by 6 switches. You can also use the schematic "Hex_7seg_MUX_2digit.pdsprj" to drive le LED bar and check results.

Copy and adapt a circuit from any of the previous projects (LAB9) and name it "Bar_28LED_dec.pdsprj". Assign pins to inputs and outputs accordingly to one of the following options (your instructor will tell you which):

Pin assignment option #1:

B(4..3) ---> RD(7..6) B(2..0) ---> RC(5..3)
E ---> RE(1)  
A_L(3..2) ---> RB(7..6) A_L(1..0) ---> RC(7..6)
K(6..5) ---> RA(3..2) K(4..3) ---> RB(4..3) K(2..0) ---> RD(3..1)

 

Pin assignment option #2:

B(4..3) ---> RC(5..4) B(2..0) ---> RD(7..5)
E ---> RB(1)  
A_L(3..2) ---> RA(3..2) A_L(1..0) ---> RB(6..5)
K(6..5) ---> RC(7..6) K(4..3) ---> RD(4..3) K(2..0) ---> RE(2..0)

 

Pin assignment option #3:

B(4..3) ---> RE(2..1) B(2..0) ---> RA(3..1)
E ---> RD(7)  
A_L(3..2) ---> RB(1..0) A_L(1..0) ---> RC(7..6)
K(6..5) ---> RB(5..4) K(4..3) ---> RC(1..0) K(2..0) ---> RD(5..3)

 

Project location:

 C:\CSD\P9\Bar_LED28_dec\(files)

 

B) Planning software

Organise the main program in our CSD way.

Propose a hardware-software diagram naming all the electrical signals, RAM variables and the software functions.  

Explain how to configure the µC in init_system(). List all RAM variables required and their type.

Organise using a flowchart the interface function read_inputs().

Organise using a flowchart the interface function write_outputs().

Infer the truth_table() software function using a behavioural interpretation and the corresponding flowchart.

 

Developing & testing (debugging)

Write the "Bar_28LED_dec.c" source code translating the function flowcharts. Start capturing only one input as in (LAB9) and visualising it in the watch window. And only then go step by step developing & testing more inputs. 

Start a software IDE project for the target microcontroller PIC18F46K22 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.

Add a few lines of code every time, compile and run the test interactively to check results watching variables.

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.

Measure how long does it take to run the main loop code when using a 4 MHz and a 20 MHz crystal oscillators.

 

NOTE (optional): This project can be continued using the software implementation of FSM proposed in P10 to complete the the full Bar_28LED.