Fix semantics of HYSTERESIS constant and calculate error for dewpoint

src/main.c

@@ -6,7 +6,9 @@
 
 #include <avr/interrupt.h>
 
-#define HYSTERESIS 1.0f
+#define DEADBAND      2.0f
+#define HYSTERESIS_C  0.5f
+#define HYSTERESIS_H  0.5f
 
 // https://en.wikipedia.org/wiki/Bang%E2%80%93bang_control
 // https://support.75f.io/hc/en-us/articles/360044956794-Deadband-and-Hysteresis
@@ -133,6 +135,7 @@ static void Update(void)
 	char ch;
 	cmd_t cmd;
 	float terr;
+	float derr;
 
 	// Parse serial commands
 	while ((ch = UART_Getc()) >= 0) {
@@ -153,6 +156,7 @@ static void Update(void)
 
 	// TODO: Sanity check setpoint!
 	// TODO: Check thermistor for overheating!
+	// TODO: Implement control stages based on hysteresis
 	// TODO: PID control for fan pulse-width modulation?
 
 	// The dead band represents the lower and upper limits
@@ -161,12 +165,13 @@ static void Update(void)
 
 	state = S_IDLE;
 	terr = temp - temp_target;
+	derr = dewp - dewp_target;
 
-	if (terr < -HYSTERESIS) {
+	if (terr < -DEADBAND / 2) {
 		state = S_HEAT_UP;
 	}
 
-	if (terr > HYSTERESIS) {
+	if (terr > DEADBAND / 2) {
 		state = S_COOL_DOWN;
 	}
 
@@ -193,6 +198,7 @@ static void Update(void)
 		PWM_SetValue(FAN03, 60);
 		break;
 	case S_DEHUMIDIFY:
+		UNUSED(derr);
 		break;
 	}
 }