add interrupt read example

1_Basic_GPIO/digitalIn.md

@@ -64,6 +64,46 @@ Small delay is added to mitigate switch bouncing, as explained in [Circuit Basic
 
  - insert gif of fan toggling.
 
-## EfficientToggle
+## EfficientRead
 
-This example reads the changes in digital states using the interrupt model. Above examples use the polling method which constantly checks the state of the pin. Interrupt only mode only triggers a particular code to run only if a particular state is observed.
+This example reads the changes in digital states using the interrupt read model. All the above examples use polling method which constantly checks the state of the pin. Interrupt mode triggers a particular code to run only if a particular state is observed on a given pin.
+
+```c
+#define BUTTON_PIN 15
+#define LED_PIN 14
+
+void ReadLED(int pin, uint32_t event)
+{
+    if (pin == BUTTON_PIN && event & GPIO_IRQ_EDGE_RISE)
+    {
+        gpio_put(LED_PIN, 1);
+    }
+    else if (pin == BUTTON_PIN && event & GPIO_IRQ_EDGE_FALL)
+    {
+        gpio_put(LED_PIN, 0);
+    }
+}
+
+int main()
+{
+    stdio_init_all();
+    gpio_init(BUTTON_PIN);
+    gpio_init(LED_PIN);
+
+    gpio_set_dir(BUTTON_PIN, GPIO_IN);
+    gpio_set_dir(LED_PIN, GPIO_OUT);
+
+    gpio_set_drive_strength(LED_PIN, GPIO_DRIVE_STRENGTH_8MA);
+    gpio_pull_down(BUTTON_PIN);
+
+    gpio_set_irq_enabled_with_callback(BUTTON_PIN, GPIO_IRQ_EDGE_RISE | GPIO_IRQ_EDGE_FALL, true, ReadLED);
+    while (1)
+        sleep_ms(1e10);
+}
+```
+
+In the interrupt model, the code to run when an interrupt in triggered must be stored in a function, `ReadLED` in this case. The `ReadLED` function takes 2 arguments, one of them is the pin on which the interrupt is triggered, and `event` contains the what kind of interrupt is triggered. If the logic level changes from low to high, the LED pin is set to ON, else it is set to OFF.
+
+Most of the code in `main` is the same from older examples, to initialize, set the direction and supplementary settings like drive strength and pull. The new function here, `gpio_set_irq_enabled_with_callback` attaches the `ReadLED` function to a GPIO Pin. The first argument is the gpio pin to attach the interrupt, it is the same as the gpio pin use everywhere. Second argument is for providing the incidents when the function should run. Third argument is a to mention if this interrupt is enabled of not and the fourth argument is the function to call when the mentioned incident happens on the mentioned pin.
+
+Since this is detached from the main loop, the main code can move on to run other functions, which is nothing in this case.