drv/battery/battery_adc: improve accuracy

ADC scaling is tweaked based on experimental measurements. Also current
is now taken into account in the voltage measurement as it has been
shown to contribute significantly (more than 0.1 V) due to the fact that
the voltage is measured after the resettable fuses on the hubs.

Issue: pybricks/pybricks-micropython#5

Author: dlech

lib/pbio/drv/battery/battery_adc.c

@@ -3,6 +3,32 @@
 
 // Battery driver that uses an ADC to read battery voltage and current
 
+// Configuration parameters:
+//
+// PBDRV_CONFIG_BATTERY_ADC:
+//      enable/disable driver.
+// PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_CH:
+//      ADC channel that measures battery voltage.
+// PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_RAW_MAX:
+//      The max size of the raw value, e.g. 4096 for 12-bit ADC. Prefer power
+//      of 2 for smaller code size, e.g. use 4096 instead of 4095.
+// PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_SCALED_MAX:
+//      The voltage in mV that corresponds that would result in the raw measured
+//      value PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_RAW_MAX being read on the ADC.
+// PBDRV_CONFIG_BATTERY_ADC_CURRENT_CORRECTION:
+//      Current correction factor applied to measured battery voltage. Units are
+//      1/16 Ω, e.g. 12 => 12/16 Ω = 0.75 Ω.
+// PBDRV_CONFIG_BATTERY_ADC_CURRENT_CH:
+//      ADC channel that measures battery current.
+// PBDRV_CONFIG_BATTERY_ADC_CURRENT_RAW_OFFSET:
+//      An offset to apply to the raw value before scaling it.
+// PBDRV_CONFIG_BATTERY_ADC_CURRENT_RAW_MAX:
+//      The max size of the raw value, e.g. 4096 for 12-bit ADC. Prefer power
+//      of 2 for smaller code size, e.g. use 4096 instead of 4095.
+// PBDRV_CONFIG_BATTERY_ADC_CURRENT_SCALED_MAX:
+//      The current in mA that corresponds that would result in the raw measured
+//      value PBDRV_CONFIG_BATTERY_ADC_CURRENT_RAW_MAX being read on the ADC.
+
 #include <pbdrv/config.h>
 
 #if PBDRV_CONFIG_BATTERY_ADC
@@ -17,36 +43,63 @@
 void pbdrv_battery_init() {
 }
 
-pbio_error_t pbdrv_battery_get_voltage_now(uint16_t *value) {
+pbio_error_t pbdrv_battery_get_current_now(uint16_t *value) {
     uint16_t raw;
     pbio_error_t err;
 
-    err = pbdrv_adc_get_ch(PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_CH, &raw);
+    // this is measuring the voltage across a 0.05 ohm shunt resistor probably
+    // via an op amp with unknown gain.
+    err = pbdrv_adc_get_ch(PBDRV_CONFIG_BATTERY_ADC_CURRENT_CH, &raw);
     if (err != PBIO_SUCCESS) {
         return err;
     }
 
-    // REVISIT: do we want to take into account shunt resistor voltage drop
-    // like on EV3? Probably only makes a difference of ~10mV at the most.
-    *value = raw * PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_SCALED_MAX /
-        PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_RAW_MAX;
+    // REVISIT: Hubs, especial Prime and Inventor hubs seem to have an offset
+    // that varies greatly by hub (20 or 40 mA). We could introduce a calibration
+    // step to try to determine this offset at boot (before lights, ports, etc.
+    // are powered on) if we find that we need more accurate current measurement.
+
+    // NOTE: On Move hub, City hub and Technic hub, current measurement is
+    // non-linear at low currents (< ~100 mA) so the reported battery current
+    // is not accurate at low currents.
+
+    *value = (raw + PBDRV_CONFIG_BATTERY_ADC_CURRENT_RAW_OFFSET) *
+        PBDRV_CONFIG_BATTERY_ADC_CURRENT_SCALED_MAX /
+        PBDRV_CONFIG_BATTERY_ADC_CURRENT_RAW_MAX;
 
     return PBIO_SUCCESS;
 }
 
-pbio_error_t pbdrv_battery_get_current_now(uint16_t *value) {
+pbio_error_t pbdrv_battery_get_voltage_now(uint16_t *value) {
     uint16_t raw;
     pbio_error_t err;
 
-    // this is measuring the voltage across a 0.05 ohm shunt resistor probably
-    // via an op amp with unknown gain.
-    err = pbdrv_adc_get_ch(PBDRV_CONFIG_BATTERY_ADC_CURRENT_CH, &raw);
+    err = pbdrv_adc_get_ch(PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_CH, &raw);
     if (err != PBIO_SUCCESS) {
         return err;
     }
 
-    *value = raw * PBDRV_CONFIG_BATTERY_ADC_CURRENT_SCALED_MAX /
-        PBDRV_CONFIG_BATTERY_ADC_CURRENT_RAW_MAX;
+    uint16_t current;
+    err = pbdrv_battery_get_current_now(&current);
+    if (err != PBIO_SUCCESS) {
+        return err;
+    }
+
+    // REVISIT: On Technic hub, only the current to ports A/C affect the voltage
+    // measurement since the voltage measurement is after the resettable fuse
+    // for those ports. So currently, the battery voltage will be reported as
+    // up to several tenths of a volt higher than it actually is if there is
+    // high current on ports B/D.
+    // NOTE: On Move hub, City hub and Technic hub, current measurement is
+    // non-linear at low currents (< ~100 mA) so the reported battery voltage
+    // will be about 0.1V lower than it actually is when the current is low.
+    // NOTE: On Prime and Inventor hubs, the voltage is measured on the same
+    // resettable fuse as the lights. So when all of the lights are on at full
+    // brightness, the battery voltage will be reported as a few hunderdts of
+    // a volt lower than it actually is, which is neglegable.
+    *value = raw * PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_SCALED_MAX /
+        PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_RAW_MAX + current *
+        PBDRV_CONFIG_BATTERY_ADC_CURRENT_CORRECTION / 16;
 
     return PBIO_SUCCESS;
 }

lib/pbio/platform/city_hub/pbdrvconfig.h

@@ -12,12 +12,13 @@
 #define PBDRV_CONFIG_BATTERY                        (1)
 #define PBDRV_CONFIG_BATTERY_ADC                    (1)
 #define PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_CH         11
-#define PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_RAW_MAX    3893
-#define PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_SCALED_MAX 9600
+#define PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_RAW_MAX    4096
+#define PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_SCALED_MAX 10100
+#define PBDRV_CONFIG_BATTERY_ADC_CURRENT_CORRECTION 12
 #define PBDRV_CONFIG_BATTERY_ADC_CURRENT_CH         10
-// FIXME: these values come from LEGO firmware, but seem to be 2x current
-#define PBDRV_CONFIG_BATTERY_ADC_CURRENT_RAW_MAX    4095
-#define PBDRV_CONFIG_BATTERY_ADC_CURRENT_SCALED_MAX 2444
+#define PBDRV_CONFIG_BATTERY_ADC_CURRENT_RAW_OFFSET 130
+#define PBDRV_CONFIG_BATTERY_ADC_CURRENT_RAW_MAX    4096
+#define PBDRV_CONFIG_BATTERY_ADC_CURRENT_SCALED_MAX 2448
 
 #define PBDRV_CONFIG_BUTTON                         (1)
 #define PBDRV_CONFIG_BUTTON_GPIO                    (1)

lib/pbio/platform/move_hub/pbdrvconfig.h

@@ -12,12 +12,13 @@
 #define PBDRV_CONFIG_BATTERY                        (1)
 #define PBDRV_CONFIG_BATTERY_ADC                    (1)
 #define PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_CH         11
-#define PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_RAW_MAX    3893
-#define PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_SCALED_MAX 9600
+#define PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_RAW_MAX    4096
+#define PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_SCALED_MAX 10100
+#define PBDRV_CONFIG_BATTERY_ADC_CURRENT_CORRECTION 12
 #define PBDRV_CONFIG_BATTERY_ADC_CURRENT_CH         10
-// FIXME: these values come from LEGO firmware, but seem to be 2x current
-#define PBDRV_CONFIG_BATTERY_ADC_CURRENT_RAW_MAX    4095
-#define PBDRV_CONFIG_BATTERY_ADC_CURRENT_SCALED_MAX 2444
+#define PBDRV_CONFIG_BATTERY_ADC_CURRENT_RAW_OFFSET 0
+#define PBDRV_CONFIG_BATTERY_ADC_CURRENT_RAW_MAX    4096
+#define PBDRV_CONFIG_BATTERY_ADC_CURRENT_SCALED_MAX 2448
 
 #define PBDRV_CONFIG_BLUETOOTH                      (1)
 

lib/pbio/platform/prime_hub/pbdrvconfig.h

@@ -19,11 +19,13 @@
 #define PBDRV_CONFIG_BATTERY                        (1)
 #define PBDRV_CONFIG_BATTERY_ADC                    (1)
 #define PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_CH         1
-#define PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_RAW_MAX    4095
-#define PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_SCALED_MAX 9615
+#define PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_RAW_MAX    4096
+#define PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_SCALED_MAX 9900
+#define PBDRV_CONFIG_BATTERY_ADC_CURRENT_CORRECTION 3
 #define PBDRV_CONFIG_BATTERY_ADC_CURRENT_CH         0
-#define PBDRV_CONFIG_BATTERY_ADC_CURRENT_RAW_MAX    4095
-#define PBDRV_CONFIG_BATTERY_ADC_CURRENT_SCALED_MAX 4175
+#define PBDRV_CONFIG_BATTERY_ADC_CURRENT_RAW_OFFSET 0
+#define PBDRV_CONFIG_BATTERY_ADC_CURRENT_RAW_MAX    4096
+#define PBDRV_CONFIG_BATTERY_ADC_CURRENT_SCALED_MAX 7300
 
 #define PBDRV_CONFIG_BUTTON                         (1)
 #define PBDRV_CONFIG_BUTTON_ADC                     (1)

lib/pbio/platform/technic_hub/pbdrvconfig.h

@@ -17,11 +17,13 @@
 #define PBDRV_CONFIG_BATTERY                        (1)
 #define PBDRV_CONFIG_BATTERY_ADC                    (1)
 #define PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_CH         0
-#define PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_RAW_MAX    4095
-#define PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_SCALED_MAX 9615
+#define PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_RAW_MAX    4096
+#define PBDRV_CONFIG_BATTERY_ADC_VOLTAGE_SCALED_MAX 9618
+#define PBDRV_CONFIG_BATTERY_ADC_CURRENT_CORRECTION 12
 #define PBDRV_CONFIG_BATTERY_ADC_CURRENT_CH         1
-#define PBDRV_CONFIG_BATTERY_ADC_CURRENT_RAW_MAX    4095
-#define PBDRV_CONFIG_BATTERY_ADC_CURRENT_SCALED_MAX 4175
+#define PBDRV_CONFIG_BATTERY_ADC_CURRENT_RAW_OFFSET 20
+#define PBDRV_CONFIG_BATTERY_ADC_CURRENT_RAW_MAX    4096
+#define PBDRV_CONFIG_BATTERY_ADC_CURRENT_SCALED_MAX 4178
 
 #define PBDRV_CONFIG_BLUETOOTH                      (1)