[SOFT-504] Change front PD adc read scale (#420)

For front PD, we've currently got a linear scale with 0 to the ADC reference voltage mapping to 0-100% fan speed. Per Aashmika and onsite testing, we need 0-1.65V to map to 0-100% fan speed. (1.65V or 1650mV should be a configurable parameter in a #define at the top of the file.) Also,
> if it is 3.3V the fans should be 0% or off (because the spst (switch) is open)
But the ADC can only detect up to its reference voltage, which is ~2V or so usually. So instead say that if the voltage is >1.65V (by some epsilon, maybe 50mV) and within an epsilon (again maybe 50mV) of min(reference voltage, 3.3V) then say that the switch is open and shut the fans off.

projects/power_distribution/src/pd_fan_ctrl.c

@@ -20,6 +20,7 @@ static FanCtrlStorage s_fan_storage;
 
 // If adc reading is within this margin of max, interpret it as max reading
 #define ADC_EPSILON_MV 50
+#define FRONT_FAN_CTRL_MAX_VALUE_MV 1650
 
 // Transmit fan error message if overtemp
 static void prv_fan_overtemp_callback(void) {
@@ -56,8 +57,13 @@ static void prv_front_temp_to_fan_percent(uint16_t v_measured, uint8_t *fan_spee
     *fan_speed = 100;
     return;
   }
-  double ratio = s_fan_storage.ref_reading / 100.0;
-  *fan_speed = v_measured / ratio;
+  if ((v_measured == 0) || (v_measured > (FRONT_FAN_CTRL_MAX_VALUE_MV + ADC_EPSILON_MV))) {
+    *fan_speed = 0;
+    return;
+  }
+
+  double ratio = FRONT_FAN_CTRL_MAX_VALUE_MV / 100.0;
+  *fan_speed = MIN(v_measured / ratio, 100);
 }
 
 // Converts rear pd adc reading of thermistor to fan speed percent

projects/power_distribution/test/test_pd_fan_ctrl.c

@@ -41,6 +41,7 @@
 #define FAN_ERR_FLAGS (FAN1_STATUS | FAN2_STATUS | FAN3_STATUS | FAN4_STATUS)
 
 #define ADC_MAX_VAL 3300
+#define FRONT_FAN_CTRL_MAX_VALUE_MV 1650
 
 #define FAN_UNDERTEMP_VOLTAGE 1163
 #define FAN_OVERTEMP_VOLTAGE 758
@@ -201,22 +202,28 @@ void test_front_pd_fan_ctrl_pot(void) {
   // Check max, min, and 50% potentiometer values transmit correct
   // values and execute properly
 
-  // set adc to return max value
+  // set adc to open value
   adc_ret_val = ADC_MAX_VAL;
   TEST_ASSERT_EQUAL(STATUS_CODE_OK, pd_fan_ctrl_init(&s_fan_settings, true));
   delay_ms(REAR_FAN_CONTROL_REFRESH_PERIOD_MILLIS + 10);
+  TEST_ASSERT_EQUAL(FAN_MIN_I2C_WRITE, i2c_buf1[1]);
+  TEST_ASSERT_EQUAL(FAN_MIN_I2C_WRITE, i2c_buf2[1]);
+
+  // set adc to return max value
+  adc_ret_val = FRONT_FAN_CTRL_MAX_VALUE_MV;
+  delay_ms(REAR_FAN_CONTROL_REFRESH_PERIOD_MILLIS + 10);
   TEST_ASSERT_EQUAL(FAN_MAX_I2C_WRITE, i2c_buf1[1]);
   TEST_ASSERT_EQUAL(FAN_MAX_I2C_WRITE, i2c_buf2[1]);
 
   // set adc to return mid value
-  adc_ret_val = ADC_MAX_VAL / 2;
-  delay_ms(REAR_FAN_CONTROL_REFRESH_PERIOD_MILLIS);
+  adc_ret_val = FRONT_FAN_CTRL_MAX_VALUE_MV / 2;
+  delay_ms(REAR_FAN_CONTROL_REFRESH_PERIOD_MILLIS + 10);
   TEST_ASSERT_EQUAL(FAN_50_PERC_I2C_WRITE, i2c_buf1[1]);
   TEST_ASSERT_EQUAL(FAN_50_PERC_I2C_WRITE, i2c_buf2[1]);
 
   // set adc to return min value
   adc_ret_val = 0;
-  delay_ms(REAR_FAN_CONTROL_REFRESH_PERIOD_MILLIS);
+  delay_ms(REAR_FAN_CONTROL_REFRESH_PERIOD_MILLIS + 10);
   TEST_ASSERT_EQUAL(FAN_MIN_I2C_WRITE, i2c_buf1[1]);
   TEST_ASSERT_EQUAL(FAN_MIN_I2C_WRITE, i2c_buf2[1]);
 }