Software Development for a 2-Line Calculator

The implementation involves low-level interactions with hardware peripherals such as buttons, displays, and timers. Interrupts are critical for responsive button presses, while arithmetic operations must account for overflow, division by zero, and invalid inputs. Optimizing display refresh rates ensures minimal flickering and efficient use of CPU cycles.

Step-by-Step Firmware Development Process

Hardware Abstraction Layer (HAL) Setup
The HAL provides a standardized interface between the microcontroller and hardware components. This layer includes:

Interrupt-Driven Input Handling
Button presses are managed via interrupts to minimize CPU load. The ISR captures raw input, applies debouncing, and triggers state transitions. A state machine ensures sequential processing of multi-digit inputs and operator precedence.

Arithmetic Core Implementation
The core logic processes operations using a stack-based or direct evaluation approach. Key considerations include:

Display Refresh Optimization
Efficient display updates reduce CPU overhead. Techniques include:

Code Snippet: Managing Multiple Input Lines and Real-Time Display Updates

#include #include #include

// Display buffer for two lines (16 chars each)
volatile char display_buffer[2][17] = {0};

// Current input state
typedef enum { IDLE, INPUT_LINE1, INPUT_LINE2, OPERATOR } InputState;
volatile InputState current_state = IDLE;
volatile char line1_input[16] = {0};
volatile char line2_input[16] = {0};
volatile char operator_symbol = '\0';

// Button matrix rows and columns
#define ROWS 4
#define COLS 4
volatile uint8_t button_matrix[ROWS][COLS] = {0};

// ISR for button interrupts
ISR(EXT_INT0_vect) {
// Debounce and read button press
_delay_ms(20);
if (!PINB & (1 << PB0)) { // Example: Check if button is still pressed
// Parse button press (e.g., '7' pressed)
update_input_buffer();
}
}

void update_input_buffer() {
switch (current_state) {
case IDLE:
// Start first line input
current_state = INPUT_LINE1;
break;
case INPUT_LINE1:
// Append digit to line1_input
append_to_line(line1_input, get_pressed_button());
break;
case INPUT_LINE2:
// Append digit to line2_input
append_to_line(line2_input, get_pressed_button());
break;
case OPERATOR:
// Set operator and transition to second line
operator_symbol = get_pressed_button();
current_state = INPUT_LINE2;
break;
}
update_display();
}

void append_to_line(char *line, char digit) {
static uint8_t index = 0;
if (index < 15) {
line[index++] = digit;
line[index] = '\0';
}
}

void update_display() {
// Clear display
lcd_clear();

// Update line 1 (e.g., "123+")
lcd_set_cursor(0, 0);
lcd_print(line1_input);

// Update line 2 (e.g., "456=")
lcd_set_cursor(0, 1);
lcd_print(line2_input);

// Display result if in OPERATOR state
if (current_state == OPERATOR && line2_input[0] != '\0') {
char result[16];
compute_result(line1_input, operator_symbol, line2_input, result);
lcd_set_cursor(8, 1); // Right-align result
lcd_print(result);
}
}

Key Features of the Snippet:

Error Handling for Division by Zero, Overflow, and Invalid Inputs

Division by Zero Detection

void safe_divide(char num1, char op, char num2, char result) {
if (atof(num2) == 0.0f) {
strcpy(result, "ERR");
return;
}
// Proceed with division
}

Arithmetic Overflow Handling

; Check for overflow after addition
BRCS overflow_error ; Branch if carry set

Invalid Input Validation

if (button_code < '0' || button_code > '9') {
if (button_code != '+' && button_code != '-' && button_code != '*') {
// Ignore invalid input
return;
}
}

Error Display Protocol:

Flowchart: Decision-Making for Operation Prioritization in Limited Memory

+-------------------+
| Start Evaluation |
+----------+---------+
|
v
+----------+---------+ +-------------------+
| Is Stack Empty? |------>| Yes: Push Operand |
+----------+---------+ +----------+---------+
| |
v v
+----------+---------+ +-------------------+
| Top of Stack is |------>| No: Compare Precedence |
| Operator? | +----------+---------+
+----------+---------+ |
| |
v v
+----------+---------+ +-------------------+
| Precedence of |------>| Higher: Pop & Eval |
| New Operator > | +----------+---------+
| Current Top? | |
+----------+---------+ |
| |
v v
+----------+---------+ +-------------------+
| Yes: Push Operator|------>| Lower: Push New Op |
+-------------------+ +----------+---------+
|
v
+-------------------+
| Equal: Evaluate |
| Current Operator |
+-------------------+
|
v
+-------------------+
| Push Result |
+-------------------+

Key Decision Points:
1. Stack Management: Operands are pushed directly; operators are evaluated only when precedence allows.
2. Precedence Rules: Multiplication/division take priority over addition/subtraction.
3. Memory Efficiency: Avoids recursive descent; uses iterative evaluation.

Algorithms for Optimizing Display Refresh Rates