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

P10

P11: peripheral LCD and C libraries for specialised functions.

P12
Resources in lectures and labs: L11, Lab11 Project objectives

Highlighted project: 4-bit serial transmitter with LCD display (phase #2)

1. Specifications Planning Dev. & test Prototype Report

Serial transmitter design phase #2: Serial_transmitter_LCD.

The new symbol serial_transmitter_LCD is represented in Fig. 1.

symbol

Fig 1. Adding an external LCD peripheral to the basic 4-bit serial transmitter from P10. It implies adding new connections to the chip, for instance PORTD pins, and new functions in he source file from an external library, therefore, compiling a hierarchical multiple-file project (equivalent to our plan C2 in previous hardware projects).

 

The usual display LM016L is presented in Fig. 2.

LCd

Fig. 2. The LCD controller chip is the standard Hitachi HD44780 (Dot Matrix Liquid Crystal Display Controller/Driver) or equivalent like the HD44100.

Some displays include LED background illumination (pin 15, A; pin 16, K).

V0 is used to adjust the display contrast by means of a potentiometer. 

HD44100

We can use only a 4-bit bus to send commands or data to the LCD. The low-level communication protocol will be solved using a C language library of functions. It requires up to seven pins for handshaking and data. Optionally, the microcontroller can use another output port pin to switch on and off the LCD power supply to save energy if required.  

LCD display

Fig 3. Parallel interface. If only writing to the device is required, signal R/W (RW_L) can be grounded, thus six port pins are necessary for this parallel interface. Datasheets: (1), (2).

 

Alternatively, we can use a serial 2-wire interface I2C to attach the LCD to the mC.

Adapter I2C

Fig 4. Serial I2C adapter based on the PCF8574 port expander chip. 

  

Other design tutorials and assignments.

 

Specifications 2. Planning Dev. & test Prototype  report

A) Planning hardware

As in previous projects, draw your circuit in a sheet of paper and discuss where to connect: reset (CD_L or MCLR_L), OSC oscillator, digital I/O and push-buttons. We can use free pins from the CSD_PICstick board and reserve some PORTD pins to connect the LCD. 

Circuit hardware with LCD

Fig. 5. Connecting RD(5..0) pins to interface the LCD.

 

B) Planning software

Determine the functions in the main program that have to be modified in order to control the LCD display. Define RAM internal variables to facilitate LCD communication. Use a convenient indicator like var_LCD_flag to prevent writing continuously to the display; this flag is set only when new information has to be shown. For example Fig. 5 shows the hardware-software diagram. Output_logic() generates the variables to interface with LCD high-level functions.

Diagram

Fig. 5. Hardware-software diagram.

And Fig.6 represents the truth table and its flowchart.

output logic

Fig 6. Output_logic function. The new variable var_LCD_flag is required to write the LCD only when new information is available. The flowchart must solve the idea.

Plan a sequence for building and debugging the application: the idea is "plan & develop & test" step by step enhancing the initial state diagram with a new feature at a time. For instance:

Step #1 Start running the copy of P10 project and try to print only the typical ASCII message "Hello World" on the LCD. How long does it take to write a message at the LCD? How can you measure such time? Why a var_LCD_flag is required?

Step #2 Add now the messages you like in FSM states or add new states when necessary.

Step #3 Add numerical information on the display, mixing text and dynamic data, for instance in this example a count of the messages transmitted. 

Fig. 7 below is a possible state diagram modified from P10 to allow printing ASCII messages beginning and ending transmissions.

State diagram

Fig. 7. State diagram to print ASCII messages on the LCD.

 

Specifications Planning 3. Dev. & 4. test Prototype Report

A) Developing hardware

Draw the schematic of the application in Proteus copying the P10 and modifying it accordingly to your new sketch in paper that includes the LCD connections. so that the LCD is made visible on the top schematic. Fig. 8 shows an example circuit Serial_transmitter_LCD.pdsprj.

hardware connections
Hardware wires

Fig. 8. Captured circuit in Proteus. Here we have two options for triggering transmissions: a manual ST button or an periodic train of pulses.

 

B) Developing software

Translate to C language your new flowcharts. Run the microcontroller's IDE MPLABX to develop and compile the C code copying and adapting the previous P10 code. Do it section by section according to your plan, testing whether it works before adding new code. The entity Serial_Transmitter becomes now in this new design phase Serial_Transmitter_LCD.

NOTE: here we have shortened the project folder name, because using the MPLABX tool is easy to reach Windows file names length limit:

C:\CSD\P11\s_trans_LCD\(files)

Project options

Fig 9. XC8 project options to generate the COF file to run Proteus simulations.

 In Fig. 10 is an example list of source files when using the libraries to interface this specific LCD hardware.

Library

Fig. 10. Example of a typical project that includes LCD libraries to drive specific and advanced hardware subsystems (peripherals), thus, becoming a multiple-file project, as a kind of software plan C2 with several components (functions) under the top schematic.

The current XC8 compiler version is v3.0, C standard is C99. And these LCD library files lcd.c, lcd.h has to be included in the project. The file config.h contains all the microcontroller configuration bits.

This is an example C code translating the planned FSM: Serial_transmitter_LCD.c.

Asjusting the OSC frequency

Fig 11. Edit the variable _XTAL_FREQ to be of the same value set in the hardware circuit (PIC18F46K22 parameter Processor Clock Frequency).

 

C) Step-by-step testing

Run the Proteus simulator. Do it in step by step mode while watching variables and placing break points, specially to follow the interrupt flags.

Example of circuit running while watching variables

Fig. 12.  Circuit running and showing ASCII messages in the LCD display.

Learn how to use the One-shot (single) trigger operation mode of the oscilloscope and how the frame of bit is transmitted once the ST button is detected. Can you add a pulse generator to automate ST pulses (20 ms duration, 1.5 Hz) ?

Measure how long does it take to write one row of 16 ASCII characters to the LCD screen.

 

Specifications Planning Dev. & Test 5. Prototype Report

Example 1:  CSD_PICstick board. Target chip: PIC18F46K22

 

Example 2: PICDEM 2 PLUS board. Target chip: PIC18F4520

 

Example 3: Explorer 8 board. Target chip: PIC18F4520

 

Example 4: PICDEM 2 PLUS board. Target chip: PIC16F877A

- This example  is copied  from (1). Thus, you can integrate these libraries into your project and use the LCD functions.

- This obsolete example was planned for HI-TECH C compiler which is no longer in use. 

 

Example 5: Optional, beyond the CSD learning objectives. Training board: Explorer 8 + MCC + PIC18F46K22 + MPLAB X + XC8 + Proteus

 

Specifications Planning Dev. & Test Prototype 6. Report

Follow this rubric for writing reports.

Further discussion and enhancements to this basic circuit:

- How to generate several transmission frequencies?

- How to transmit 8-bit of data?

-- How to add a parity bit to the transmission frame, for instance an odd parity bit?

- How to design a serial receiver? How to build a prototype for both transmitter and receiver modules?

- How the embedded USART peripheral works, so that the same functionality can be implemented in real-world applications liberating the mC of such time-consuming tasks?

You can even imagine different communication channel scenarios:

- How to adapt 0 V and 5 V to the standard RS232?

- How to invent a MODEM (1.2 kHz / 2.4 kHz) to be able to use telephone copper lines?

- How to invent a fiber optic adapter (for instance to implement a kind of HI-FI audio TOSH-LINK) or an infra-red (IR) link for a remote control?

- How to invent a radio link?