SignalBase: Adjust Schmitt-trigger thresholds

Currently, the thresholds are determined by the minimum
and maximum values of a signal. From those, we derive the
high and low thresholds by using a 10% margin to min/max.

However, this approach doesn't work very well when the
measurement includes reset conditions or similar, causing
spikes that raise the min/max significantly.

Example:
sigrok-dumps/i2c/eeprom_24xx/microchip_24lc64/sainsmart_dds120_powerup_scl_sda_analog.sr

This patch changes the thresholds margins to 35%. However,
they are expressed differently: (max-min)/2 is used as the
center line, from which 15% of the amplitude (max-min) is
used as the margin. This way seems a little more intuitive
for me since the percentage given (15) is directly proportional
to the hysteresis.

diff --git a/pv/data/signalbase.cpp b/pv/data/signalbase.cpp
index ee1112d5d5260a67b745a8656590ca023fbeffdb..b43e5bb07f4db8e92c9c797aeb6daa1efc16ce55 100644 (file)
--- a/pv/data/signalbase.cpp
+++ b/pv/data/signalbase.cpp
@@ -310,8 +310,9 @@ void SignalBase::conversion_thread_proc(QObject* segment)
 
                                if (conversion_type_ == A2LConversionBySchmittTrigger) {
                                        const float amplitude = max_v - min_v;
-                                       const float lo_thr = min_v + (amplitude * 0.1);  // 10% above min
-                                       const float hi_thr = max_v - (amplitude * 0.1);  // 10% below max
+                                       const float center = min_v + (amplitude / 2);
+                                       const float lo_thr = center - (amplitude * 0.15); // 15% margin
+                                       const float hi_thr = center + (amplitude * 0.15); // 15% margin
                                        uint8_t state = 0;  // TODO Use value of logic sample n-1 instead of 0
 
                                        // Convert as many sample blocks as we can