Hello World

Background

       “Hello World” is a C program written by Dennis Ritchie(The Inventor of C programming Language) in his book The C Programming Language to showcase the capabilities of the C language to print a string “Hello World”. In this blog we will tweak this famous program a bit and display “Hello World” message on a 20x4 LCD display.

The IO Problem with MCUs!!!

       As explained in keypad article, each new input or output addition to MCU requires usage of an IO pin. If we need to display numbers 0 to 9 then we need to interface a seven segment display! including the dot LED, we need a total of 8 IO pins to control a single digit display from MCU. imagine if we need to display a sentence as output in our application. That would require more than 300 IO pins from MCU. This is not a feasible design as MCU’s will have very few IO pins.

HD44780 based LCD display

       This is where HD44780 based character displays come to the rescue. They have a built in LCD controller which can control a variaty of LCD configurations as listed below. Datasheet for HD44780 controller is availble online here.

• 16x1
• 16x2
• 20x4

LCD interface pins

       HD44870 LCD controller based displays have 16 IO pins as listed below.

SLNO	LCD	Meaning	Connection
1	VSS	Ground pin	GND
2	VDD	Supply pin	VCC
3	VEE	Contrast set pin	0.18V
4	RS	Register select pin	HIGH/LOW based on need.
5	R/W	Read/Write pin	GND
6	E	Execute pin	/Execute control line
7	D0	LSB data pin	Parallel data D0 pin LSB
8	D1	data pin	Data D1 pin
9	D2	data pin	Data D2 pin
10	D3	data pin	Data D3 pin
11	D4	data pin	Data D4 pin
12	D5	data pin	Data D5 pin
13	D6	data pin	Data D6 pin
14	D7	MSB data pin	Parallel data D7 pin MSB
15	LED+	data pin	4.3V
16	LED-	MSB data pin	GND

HD44780 Interface v1: 8-bit mode.

       Pin configurations for 8-bit LCD iunterfacing to atmega328p is shown below.

Schematic

Connections

SLNO	LCD	Nano	328p	Meaning	Connection
1	VSS	VSS	-	Ground pin	GND
2	VDD	VDD	-	Supply pin	VCC
3	VEE	-	-	Contrast set pin	0.18V
4	RS	D8	PB0	Register select pin	HIGH/LOW based on need.
5	R/W	GND	-	Read/Write pin	GND
6	E	D9	PB1	Execute pin	/Execute control line
7	D0	D0	PD0	LSB data pin	Parallel data D0 pin LSB
8	D1	D1	PD1	data pin	Data D1 pin
9	D2	D2	PD2	data pin	Data D2 pin
10	D3	D3	PD3	data pin	Data D3 pin
11	D4	D4	PD4	data pin	Data D4 pin
12	D5	D5	PD5	data pin	Data D5 pin
13	D6	D6	PD6	data pin	Data D6 pin
14	D7	D7	PD7	MSB data pin	Parallel data D7 pin MSB
15	LED+	-	-	data pin	4.3V
16	LED-	-	-	MSB data pin	GND

HD44780 Interface v2: 4-bit mode.

       Pin configurations for 4-bit LCD iunterfacing to atmega328p is shown below.

Schematic

Connections

SLNO	LCD	Nano	328p	Meaning	Connection
1	VSS	VSS	-	Ground pin	GND
2	VDD	VDD	-	Supply pin	VCC
3	VEE	-	-	Contrast set pin	0.18V
4	RS	D8	PB0	Register select pin	HIGH/LOW based on need.
5	R/W	GND	-	Read/Write pin	GND
6	E	D9	PB1	Execute pin	/Execute control line
7	D4	D4	PD4	data pin	Nibble data D4 pin LSB
8	D5	D5	PD5	data pin	Data D5 pin
9	D6	D6	PD6	data pin	Data D6 pin
10	D7	D7	PD7	MSB data pin	Nibble data D7 pin MSB
11	LED+	-	-	data pin	4.3V
12	LED-	-	-	MSB data pin	GND

HD44780 I2C Interface v3: 4-bit mode.

       Pin configurations for I2C 4-bit LCD iunterfacing to atmega328p is shown below.

Schematic

Connections

SLNO	LCD	Nano	328p	Meaning	Connection
1	SCL	A5	-	I2C clock pin	SCL
2	SDA	A4	-	I2C data pin	SDA

       The above schematic is derived from referring to multiple online sources related to PCF8574 I2C LCD modules.

Software

       Famous word’s from Linus Torvalds, the father of Linux operating system, comes to my mind.

“Talk is Cheap, Show me the code!” – Linus Torvalds

       Let’s evaluate how the code can be constructed to decode the key pressed in matrix configuration. Row’s and column’s of 4x4 matrix keypad is connected to MCU as per below list.

MCU port    –>   Keypad connections

• Row_0    –>    D0    –>    PD0
• Row_1    –>    D1    –>    PD1
• Row_2    –>    D2    –>    PD2
• Row_3    –>    D3    –>    PD3
• Col_3      –>    D4    –>    PD4
• Col_2      –>    D5    –>    PD5
• Col_1      –>    D6    –>    PD6
• Col_0      –>    D7    –>    PD7

       We are using port D of atmega328p microcontroller for our interfacing of 4x4 keypad. Lower 4 bits(PD0-PD3) are used for Row connections and Upper 4 bits(PD4-PD7) are used for Column connections of keypad. This particular connection scheme makes the SW design a bit easier :)

Software design

       We will be using bitwise operators to scan the keypad and decode the key pressed in keypad.

#define ROW_COUNT         (4)
#define COL_COUNT         (4)
#define ROW_PORT          (PORTD)
#define COL_PORT          (PORTD) 
#define ROW_MASK          (0b00000001)
#define COL_MASK          (0b10000000)
#define NO_KEY_PRESSED    (255)

uint8 key_scan(void)
{
    uint8 col_mask = COL_MASK;
    uint8 row_mask;
    uint8 key_pressed = 0;

    for(row_counter = 0; row_counter < ROW_COUNT; row_counter++)
    {
        row_mask = ROW_MASK;
        for(col_counter = 0; col_counter < COL_COUNT; col_counter++)
        {
            ROW_PORT = row_mask;
            if(COL_PORT & col_mask)
            {
                return (key_pressed);
            }
            else
            {
                key_pressed++;
            }
            col_mask = col_mask >> 1;
        }
        row_mask = row_mask << 1;
    }
    return (NO_KEY_PRESSED);
}

Switch Debouncing

       Switch is a mechanical device and when a switch is pressed the switch plate gets closed. The switch closure doesnt produce a step response as we expect in an ideal switch. There are some spikes which are produces whenever a key is pressed or released. These spikes needs to filtered(debounced) to get proper key readings. key_scan() function can be called once in a ms and a debounce of 10 to 20 times counts can be done to rule out errors due to mechanical debounce.

Build images

      

Closing notes

       !!!Happy tinkering!!! :D

References