added function to linearize temperature

firmware/cup.c

@@ -18,9 +18,11 @@ uint8_t setting_timeout = 0;
 
 uint8_t adc_pos = 0;
 uint16_t adc_sum = 0;
-uint16_t temperature = 0;
+uint16_t temperature_adc = 0;
 uint16_t target = 0;
 
+uint16_t temperature = 0;
+
 uint8_t heat_pwm = 0;
 uint8_t heat_on[2] = {0, 0};
 uint8_t heat_off[2] = {0, 0};
@@ -65,6 +67,51 @@ ISR(PCINT0_vect)
 	input_state = new_state;
 }
 
+// Measure: 5V --- 10k --- --- PTY81-121 -- GND
+//                        |
+//                    uC ADC pin (with 1.11V reference)
+uint16_t linearize_temp(uint16_t temp_in)
+{
+	static const uint16_t one_point_one_volt = 65472; //2^16-2^6
+	uint16_t temp_out = ~0; //burning
+	// made from datasheet by:
+	// for i in 677 740 807 877 951 1029 1111 1196 1286 1378 1475 1575 1679 1786 1896 2003 2103 2189; do calc 5*$i/9860*65472/1.11; done
+	static const uint16_t coeffs[] =
+	{
+		// in, out
+		20250, 0, // -20 celsius
+		22134, 0, // -10 celsius
+		24138, 0, // 0 celsius
+		26232, 100, // 10 celsius
+		28445, 200,
+		30778, 300,
+		33231, 400, // 50 celsius
+		35773, 500, // ..
+		38465, 600,
+		41216, 700,
+		44118, 800,
+		47109, 900,
+		50220, 1000,
+		53420, 1100,
+		56711, 1200,
+		59911, 1300,
+		62902, 1400,
+		65474, 1500, // 150 celsius
+		65535, 1520, // made-up, not from datasheet
+	};
+        uint8_t i=2;
+	for (; i < (sizeof(coeffs)>>1)-2; i+=2)
+	{
+        	if ( temp_in < coeffs[i] )
+		{
+			temp_out = (uint16_t) ( coeffs[i-1] + (uint32_t)
+			( (uint32_t) (temp_in-coeffs[i-2]) * (coeffs[i+1]-coeffs[i-1]) ) / (coeffs[i]-coeffs[i-2]));
+			break;
+		}
+	}
+	return temp_out;
+}
+
 void io_init(void)
 {
 	PORTA = (1 << TAST_PIN) | (1 << IMPULS1_PIN) | (1 << IMPULS0_PIN);/* enable pullups on inputs */
@@ -127,14 +174,15 @@ int main(void)
 			adc_sum += ADC;
 			if(++adc_pos == 64)
 			{
-				temperature = adc_sum;
+				temperature_adc = adc_sum;
 				adc_sum = 0;
 				adc_pos = 0;
+				temperature = linearize_temp(temperature_adc);
 			}
 		}
 		
 		if(setting_timeout) {led_set(target);}
-		else {led_set(temperature);}
+		else {led_set(temperature);}OB
 	}
 	
 	return 0;