Lab 6 : LCD and Interrupt
Seneca Polytechnic SEP600 Embedded Systems
Introduction
Documentation for the Cortex-M4 instruction set, the board user's guide, and the microcontroller reference manual can be found here:
Documentation for the Freedom K64 and K66 boards and their microcontrollers can be found here:
- FRDM-K64F Freedom Module User’s Guide (PDF)
- Kinetis K64 Reference Manual (PDF)
- FRDM-K64F Mbed Reference
- FRDM-K64F Mbed Pin Names
- FRDM-K66F Freedom Module User’s Guide (PDF)
- Kinetis K66 Reference Manual (PDF)
- FRDM-K66F Mbed Reference
- FRDM-K66F Mbed Pin Names
Documentation for the Cortex-M4 instruction set can be found here:
- Arm Cortex-M4 Processor Technical Reference Manual Revision (PDF)
- ARMv7-M Architecture Reference Manual (PDF)
16×2 (or other size) LCD Module
A 16×2 Liquid Crystal Display module has 16 columns and 2 rows of characters. Most 16×2 (as well as 8×1, 20×2, and 20×4) LCD modules use a Hitachi HD44780 (or compatible) LCD controller. Each character space can display a single alphanumeric character, symbol, or custom character. The display operates by selectively controlling the liquid crystal pixels, making them opaque or transparent to create characters or graphics. The LCD controller works in 2 main modes:
- 4-bit Mode: Data is sent to the LCD module in two consecutive nibbles. The higher nibble, consisting of data lines D4 to D7, is sent first, followed by the lower nibble with data lines D0 to D3. This configuration allows us to send 8-bit data using only four data lines, conserving valuable I/O pins on the microcontroller.
- 8-bit Mode: The LCD can directly receive 8-bit data in a single transmission, using all eight data lines (D0 to D7). As a result, this mode offers faster and more efficient data transfer compared to the 4-bit mode. However, it requires more I/O pins on the microcontroller, potentially limiting its application in projects with limited resources.
When using the LCD module in 4-bit mode, only 4 wires are required for parallel data transfer, plus 2 wires for enable. However, with the help of an I2C parallel port expander (I2C backpack), only 2 wires through I2C are required to work with the LCD module.
Figure 6.1 16x2 LCD
Some common commands are:
| Command Name | HEX Value | Description |
|---|---|---|
| Clear Display | 0x01 | This command clears the entire display, resetting the cursor position to the home position (0, 0). |
| Return Home | 0x02 | Sending this command moves the cursor to the home position (0, 0) without clearing the display. |
| Entry Mode Set | 0x04 | This command determines the cursor movement direction and whether the display should shift. |
| Display On/Off Control | 0x08 | This command controls the display, cursor, and cursor blinking options. |
| Cursor or Display Shift | 0x10 | Used to shift the cursor or the entire display left or right without changing the display data. |
| Function Set | 0x20 | This command sets the LCD data length (4-bit or 8-bit), number of display lines, and font size. |
| Set CGRAM Address | 0x40 | This command sets the address of the Character Generator RAM (CGRAM) for custom character creation. |
| Set DDRAM Address | 0x80 | This command sets the address of the Display Data RAM (DDRAM), allowing data to be written to a specific location on the LCD. |
Reference: HD44780 LCD Controller
Materials
- Safety glasses (PPE)
- Freedom K64F or K66F Board
- Breadboard
- Jumper Wires
- LCD Display (Parallel or I2C)
- Various 1kΩ–10kΩ resistors
- Button
If you are using an I2C LCD, connect the LCD to the I2C pins and use the I2C library instead of the parallel LCD library.
Preparation
Lab Preparation Question
- Read over the lab and understand the procedures.
Procedures
Part 1: LCD Module
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Acquire an LCD and a resistor, then connect them to the Freedom K64F/K66F board as per the connection table and the diagram below. If you are using an I2C LCD, connect the LCD to the I2C pins.
Figure 6.2 LCD connection with Freedom board
The typical pinout and connection for a parallel 16x2 LCD are given below. Please keep in mind that depending on the manufacturer, some labels and configurations may vary.
LCD Pin # LCD Label K64F/K66F Pin 1 GND / VSS GND / 0V 2 VDD / VCC 5V 3 VO 1kΩ to GND / 0V 4 RS D9 5 R/W GND / 0V 6 E D8 7 DB0 N/C 8 DB1 N/C 9 DB2 N/C 10 DB3 N/C 11 DB4 D4 12 DB5 D5 13 DB6 D6 14 DB7 D7 15 LED+ 1kΩ to 5V 16 LED- N/C - Some models work with 3.3V instead of 5V.
- VO pin configuration varies depending on the manufacturer. A potentiometer can be used instead of a 1kΩ resistor for adjustable contrast.
You may change the pins used on the K64F/K66F board depending on your application and pin availability.
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Open Keil Studio and install the following library to your project depending on whether you are using the Parallel or I2C version of the LCD.
- Parallel LCD: https://os.mbed.com/users/sstaub/code/mbedLCD/
- I2C LCD: https://os.mbed.com/users/sstaub/code/mbedLCDi2c/
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Use the following code to output a message on the display. Uncomment the necessary lines for the LCD screen you are using:
#include "mbed.h" // #include "LCD.h" // for parallel LCD // #include "LCDi2c.h" // for I2C LCD // LCD lcd(D9, D8, D4, D5, D6, D7, LCD16x2); // for parallel LCD: RS, EN, D4-D7, Type // LCDi2c lcd(I2C_SDA, I2C_SCL, LCD16x2); // for I2C LCD: SDA, SCL int main() { lcd.cls(); // clear display lcd.locate(0, 0); // set cursor location lcd.printf("SEP600\n"); // display text ThisThread::sleep_for(2s); lcd.cls(); // clear display lcd.locate(0, 0); // set cursor location lcd.printf("Hello World!\n"); // display text }When the LCD is operating in 4-bit mode, command and data are sent to the LCD 4-bit at a time throught D4-D7. When the RS pin on the LCD is low (0), the LCD treat the parallel input as command. When the RS pin on the LCD is high (1), the LCD treat the parallel input as data to be displayed.
Here's the sequence for printing "SEP":
Command/Data RS Pin Description 0x80 LOW Display the next charater at Row 0, Col 0 0x53 HIGH ASCII of captial "S" in HEX 0x81 LOW Display the next charater at Row 0, Col 0 0x45 HIGH ASCII of captial "E" in HEX 0x82 LOW Display the next charater at Row 0, Col 0 0x50 HIGH ASCII of captial "P" in HEX -
After uploading your code, the LCD should show "SEP600" for 2 seconds, then "Hello World!".
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If you are using a parallel LCD, you may skip this steps as tools for sniffing the four data line will be required to read the signals. If you are using an I2C LCD module, use the oscilloscope to take a look at the I2C data frame. You should be able to see the command and data that you are sending to the module.
The I2C I/O expander is attached to the LCD as follow:
Bit-7 (MSB) Bit-6 Bit-5 Bit-4 Bit-3 Bit-2 Bit-1 Bit-0 (LSB) DB7 DB6 DB5 DB4 BL E RW RS - BL (Backlight) is LED+. Set this to high to turn on the backlight.
- E is Enable. Transistion of this pin from HIGH to LOW trigger reading of the command/data.
- RW is Read/Write. Set this to LOW when writing to the LCD.
- RS is Register Select. Set this to LOW when writing command, set this to HIGH when writing data.
Since only 4-bit will be sent at a time, each command and data is divided into high and low nibbles for data transmission in bits 4-7 along with settings in bits 0-3.
As a result, the I2C transmission you'll see is:
Data Description 0x27 << 1 7-bit address + Write 0x8X High nibble of first command "0x80" + X settings depending on your code 0x27 << 1 7-bit address + Write 0x0X Low nibble of first command "0x80" + X settings depending on your code 0x27 << 1 7-bit address + Write 0x5X High nibble of first data "S" + X settings depending on your code 0x27 << 1 7-bit address + Write 0x3X High nibble of first data "S" + X settings depending on your code 0x27 << 1 7-bit address + Write 0x8X High nibble of second command "0x81" + X settings depending on your code 0x27 << 1 7-bit address + Write 0x1X Low nibble of second command "0x81" + X settings depending on your code 0x27 << 1 7-bit address + Write 0x4X High nibble of second data "E" + X settings depending on your code 0x27 << 1 7-bit address + Write 0x5X High nibble of second data "E" + X settings depending on your code Same pattern for the remaining command and data Lab Question: Based on your message, compare the I2C signal and see if you can find the "Hel" from "Hello World!".
(Optional) If you want to take a screenshot of your oscilloscope for analysis, an easy way to do this is to connect to its web interface using the computer at your workstation. Press Utility > I/O > LAN on the oscilloscope to find it's IP address then navigate to this IP address using a browser on the workstation computer.
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Let's display some longer message.
Lab Question: Modify your code to display your name and student number on row 1, and your lab partner's name and student number on row 2 (or be creative, like "SEP600 Embedded Systems is Awesome"). Since the message will be too wide for the LCD, display the text as a horizontal scrolling message at a reasonable rate.
Hint: There are many ways to do this. Refer to the library documentation on how to move the print cursor.
Part 2: Interrupt
An interrupt is a way for the microcontroller to listen for events without continuously polling the input. Most of the GPIO pins on the Freedom K64F or K66F board can be attached to an interrupt.
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Connect a pull-up or pull-down button to any digital pin of your choosing.
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Add the following code before
main()to create an interrupt object.InterruptIn button(PTXX); -
Add the following interrupt routine before
main()and include the appropriate code for displaying a message on the LCD when the interrupt is triggered. Display the message for a few seconds, then return to displaying the previous message.void button_isr(){ // Display an interrupt message on the LCD // Use wait_us for delay // Do NOT use ThisThread::sleep_for }Hint: You can try triggering a flag in the interrupt and then display the message in the
main()loop or in a separate thread. -
Within
main(), attach the interrupt routine to the button and adjust for the rising or falling edge, depending on your circuit configuration.button.rise(&button_isr); -
Upload and test your interrupt.
Lab Question: What will happen if you put a wait() function in the interrupt?
Once you've completed all the steps above (and ONLY when you are ready, as you'll only have one opportunity to demo), ask the lab professor or instructor to come over and demonstrate that you've completed the lab. You may be asked to explain some of the concepts you've learned in this lab.