feat: Calculate and display RPM every 100ms

tach/tach.ino

@@ -5,11 +5,15 @@
 #include <Adafruit_GFX.h>
 #include "Adafruit_LEDBackpack.h"  // "Adafruit LED Backpack Library" v1.5.0
 
+#define MEASUREMENT_PERIOD_MS 100
+#define PULSES_PER_REVOLUTION 8
+
 Adafruit_7segment matrix = Adafruit_7segment();
 
 // --- Tachometer variables ---
 const int TACH_INTERRUPT_PIN = A0; // Or any other digital pin capable of interrupts
 volatile unsigned long pulse_count = 0;
+unsigned long last_measurement_time = 0;
 
 // --- Interrupt Service Routine (ISR) ---
 void count_pulse() {
@@ -43,5 +47,29 @@ void setup() {
 }
 
 void loop() {
-;
+  unsigned long current_time = millis();
+
+  if (current_time - last_measurement_time >= MEASUREMENT_PERIOD_MS) {
+    detachInterrupt(digitalPinToInterrupt(TACH_INTERRUPT_PIN));
+    unsigned long collected_pulses = pulse_count;
+    pulse_count = 0;
+    // It's important to re-attach the interrupt as soon as possible
+    // to minimize missed pulses.
+    attachInterrupt(digitalPinToInterrupt(TACH_INTERRUPT_PIN), count_pulse, RISING);
+
+    // Calculate RPM
+    // RPM = (pulses / pulses_per_revolution) / (measurement_period_ms / 1000 / 60)
+    // RPM = (pulses / pulses_per_revolution) * (60000 / measurement_period_ms)
+    // Ensure floating point division for accuracy before converting to int for display
+    float rpm_float = ( (float)collected_pulses / PULSES_PER_REVOLUTION ) * (60000.0 / MEASUREMENT_PERIOD_MS);
+    int rpm = (int)rpm_float;
+
+    show(rpm);
+    Serial.print("Pulses: ");
+    Serial.print(collected_pulses);
+    Serial.print(", RPM: ");
+    Serial.println(rpm);
+
+    last_measurement_time = current_time;
+  }
 }