nitish-r-g/skills-i-use/embedded-systems

/* Flag shared between ISR and task — must be volatile */
static volatile uint8_t g_uart_rx_flag = 0;
static volatile uint8_t g_uart_rx_byte = 0;
 
/* Keep ISR short: read hardware, set flag, exit */
void USART2_IRQHandler(void) {
    if (USART2->SR & USART_SR_RXNE) {
        g_uart_rx_byte = (uint8_t)(USART2->DR & 0xFF); /* clears RXNE */
        g_uart_rx_flag = 1;
    }
}
 
/* Main loop or RTOS task processes the flag */
void process_uart(void) {
    if (g_uart_rx_flag) {
        __disable_irq();                   /* enter critical section */
        uint8_t byte = g_uart_rx_byte;
        g_uart_rx_flag = 0;
        __enable_irq();                    /* exit critical section  */
        handle_byte(byte);
    }
}

#include "FreeRTOS.h"
#include "task.h"
#include "queue.h"
 
#define SENSOR_TASK_STACK  256   /* words */
#define SENSOR_TASK_PRIO   2
 
static QueueHandle_t xSensorQueue;
 
static void vSensorTask(void *pvParameters) {
    TickType_t xLastWakeTime = xTaskGetTickCount();
    const TickType_t xPeriod  = pdMS_TO_TICKS(10); /* 10 ms period */
 
    for (;;) {
        /* Periodic, deadline-driven read */
        uint16_t raw = adc_read_channel(ADC_CH0);
        xQueueSend(xSensorQueue, &raw, 0); /* non-blocking send */
 
        /* Check stack headroom in debug builds */
        configASSERT(uxTaskGetStackHighWaterMark(NULL) > 32);
 
        vTaskDelayUntil(&xLastWakeTime, xPeriod);
    }
}
 
void app_init(void) {
    xSensorQueue = xQueueCreate(8, sizeof(uint16_t));
    configASSERT(xSensorQueue != NULL);
 
    xTaskCreate(vSensorTask, "Sensor", SENSOR_TASK_STACK,
                NULL, SENSOR_TASK_PRIO, NULL);
    vTaskStartScheduler();
}

/* Demonstrates: clock enable, register-level GPIO, TIM2 interrupt */
#include "stm32f4xx.h"
 
void TIM2_IRQHandler(void) {
    if (TIM2->SR & TIM_SR_UIF) {
        TIM2->SR &= ~TIM_SR_UIF;           /* clear update flag */
        GPIOA->ODR ^= GPIO_ODR_OD5;        /* toggle LED on PA5  */
    }
}
 
void blink_init(void) {
    /* GPIO */
    RCC->AHB1ENR |= RCC_AHB1ENR_GPIOAEN;
    GPIOA->MODER |= GPIO_MODER_MODER5_0;  /* PA5 output */
 
    /* TIM2 @ ~1 Hz (84 MHz APB1 × 2 = 84 MHz timer clock) */
    RCC->APB1ENR |= RCC_APB1ENR_TIM2EN;
    TIM2->PSC  = 8399;   /* /8400  → 10 kHz  */
    TIM2->ARR  = 9999;   /* /10000 → 1 Hz    */
    TIM2->DIER |= TIM_DIER_UIE;
    TIM2->CR1  |= TIM_CR1_CEN;
 
    NVIC_SetPriority(TIM2_IRQn, 6);
    NVIC_EnableIRQ(TIM2_IRQn);
}