qtouch: subtract min sensor value per slider

This improves touch position accuracy in noisy environments
and may reduce touch issues reported in the wild.

Touch position is calculated with a weighted average of the sensor
readings. If properly calibrated, sensors on the opposite end of a
finger touch would be zero and thus make no contribution to the
weighted average. If the baseline sensor readings are elevated, the
sensors on the opposite edge DO contribute to the weighted average
making a positional artifact (i.e. the position is more central than
it should be in reality). This artifact is higher when the finger is
a bit distant while approaching and lower/negligible when the finger
is fully touching the device. This can cause the position to move
enough to enter "slide" mode and disable "tap" events being emitted.

In addition, defines unnamed constant DEF_SENSOR_CEILING.

Author: douglasbakkum

CHANGELOG.md

@@ -10,6 +10,7 @@ customers cannot upgrade their bootloader, its changes are recorded separately.
 - Increased performance when signing Bitcoin transactions
 - Warn if the transaction fee is higher than 10% of the coins sent
 - ETH Testnets: add Goerli and remove deprecated Rinkeby and Ropsten
+- Improved touch button positional accuracy in noisy environments
 
 ### 9.13.1
 - Fix bug introduced in 9.13.0: remove double cancel confirmation in the 'Restore from recovery words' workflow

src/qtouch/qtouch.c

@@ -1,4 +1,3 @@
-
 /*============================================================================
 Filename : touch.c
 Project : QTouch Modular Library
@@ -570,7 +569,7 @@ uint16_t qtouch_get_sensor_node_signal_filtered(uint16_t sensor_node)
 {
     // Filter the sensor signal.
     //
-    // Smooth it out and saturate it so that values never go beyond 50.
+    // Smooth it out and saturate it so that values never go beyond DEF_SENSOR_CEILING.
     // This helps to mitigate 'jumpy' channels that exist at higher sensor readings when
     // in noisy environments.
     //
@@ -592,7 +591,7 @@ uint16_t qtouch_get_sensor_node_signal_filtered(uint16_t sensor_node)
         X = (uint16_t)((double)X * (1 + DEF_SENSOR_EDGE_WEIGHT));
     }
     // Saturate out-of-range readings.
-    X = (X > 50) ? 50 : X;
+    X = (X > DEF_SENSOR_CEILING) ? DEF_SENSOR_CEILING : X;
 
     // Calculate sensor readout using a moving average
     // The moving average wieghts previous N readings twice current reading
@@ -630,21 +629,38 @@ void qtouch_process_scroller_positions(void)
         uint8_t i, j;
         uint16_t sum = 0;
         uint16_t max_sensor_reading = 0;
+        uint16_t min_sensor_reading = DEF_SENSOR_CEILING;
         uint16_t weighted_sum = 0;
+        uint16_t filtered_readings[DEF_SCROLLER_NUM_CHANNELS] = {0};
         uint16_t sensor_location[DEF_SCROLLER_NUM_CHANNELS] = {
             1, // Offset by `1` because a `0` location cannot be weight-averaged
             DEF_SCROLLER_RESOLUTION / 3,
             DEF_SCROLLER_RESOLUTION / 3 * 2,
             DEF_SCROLLER_RESOLUTION};
 
-        // Read filterd data and weight by sensor physical location
         for (i = 0; i < DEF_SCROLLER_NUM_CHANNELS; i++) {
-            uint16_t value;
-            value = qtouch_get_sensor_node_signal_filtered(
+            filtered_readings[i] = qtouch_get_sensor_node_signal_filtered(
                 i + (scroller ? DEF_SCROLLER_OFFSET_1 : DEF_SCROLLER_OFFSET_0));
-            sum += value;
-            weighted_sum += value * sensor_location[i];
-            max_sensor_reading = (value > max_sensor_reading) ? value : max_sensor_reading;
+            min_sensor_reading = (filtered_readings[i] < min_sensor_reading) ? filtered_readings[i]
+                                                                             : min_sensor_reading;
+            max_sensor_reading = (filtered_readings[i] > max_sensor_reading) ? filtered_readings[i]
+                                                                             : max_sensor_reading;
+        }
+
+        // Read filterd data and weight by sensor physical location
+        // Reduce the value by the min_sensor_reading to improve positional accuracy.
+        // Touch position is calculated with a weighted average of the sensor readings.
+        // If properly calibrated, sensors on the opposite end of a finger touch would
+        // be zero and thus make no contribution to the weighted average. If the baseline
+        // sensor readings are elevated, the sensors on the opposite edge DO contribute
+        // to the weighted average making a positional artifact (i.e. the position is more
+        // central than it should be in reality). This artifact is higher when the finger
+        // is a bit distant while approaching and lower/negligible when the finger is
+        // fully touching the device. This can cause the position to move enough to enter
+        // "slide" mode and disable "tap" events being emitted.
+        for (i = 0; i < DEF_SCROLLER_NUM_CHANNELS; i++) {
+            sum += filtered_readings[i] - min_sensor_reading;
+            weighted_sum += (filtered_readings[i] - min_sensor_reading) * sensor_location[i];
         }
 
         // Compensate for deadband (i.e. when only a single edge button gives a reading and

src/qtouch/qtouch.h

@@ -262,6 +262,9 @@ extern "C" {
     0.15 // Percent added weight to edge sensors, which are physically smaller
 #define DEF_SENSOR_NUM_PREV_POS \
     4 // Number of previous sensor positions to remember; used in a simple filter
+#define DEF_SENSOR_CEILING \
+    50 // Maximum sensor reading. Mitigates 'jumpy' channels that exist at higher
+       // sensor readings when in noisy environments.
 
 #ifdef __cplusplus
 }