--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/vbat-adc/crude-cal-test	Fri Jun 04 03:02:18 2021 +0000
@@ -0,0 +1,81 @@
+The data table that follows captures the results of this experiment:
+
+* An FCDEV3B modem board was powered from a bench PSU whose output voltage
+  is controlled by an analog rotary knob;
+
+* The PSU voltage knob was turned up and down to produce 0.1 V steps
+  from 3.2 V to 4.2 V;
+
+* At each PSU voltage step, VBAT MADC reading was captured with an
+  mpr 30 command in fc-tmsh, and the actual voltage was measured with a
+  digital multimeter.
+
+PSU voltage	MADC reading	Voltmeter reading
+-------------------------------------------------
+3.2 V		473		3207 mV
+3.3 V		488		3306 mV
+3.4 V		502		3403 mV
+3.5 V		517		3500 mV
+3.6 V		532		3597 mV
+3.7 V		546		3696 mV
+3.8 V		562		3804 mV
+3.9 V		576		3897 mV
+4.0 V		592-593 [1]	4003 mV
+4.1 V		606		4.08-4.09 V [2]
+4.2 V		621		4.18-4.19 V [2]
+
+[1] on the cusp of MADC step, got alternating readings
+
+[2] My cheap multimeter's 4 V range was exceeded at this point, and I had
+to switch to the next higher range in which only two digits after the
+decimal point were displayed.
+
+If we apply the firmware's default VBAT calibration values of A=7000 B=0,
+we see that VBAT numbers computed from MADC readings are higher than the
+true VBAT by 24 mV at the low 3.2 V end and by 45 mV at the high 4.2 V end.
+
+Calibration math attempt from 3207 to 4003 mV, using MADC=592 at high end
+=========================================================================
+
+A=(4003-3207)/(592-473)*1024 = 6849
+
+B parameter computed from each calibration point:
+
+MADC reading	Voltmeter reading	Computed VBAT w/o B	B parameter
+---------------------------------------------------------------------------
+473		3207 mV			3163			44
+488		3306 mV			3263			43
+502		3403 mV			3357			46
+517		3500 mV			3457			43
+532		3597 mV			3558			39
+546		3696 mV			3651			45
+562		3804 mV			3758			46
+576		3897 mV			3852			45
+592		4003 mV			3959			44
+
+B values range from 39 to 46, a very small variation - thus the behaviour of
+VBAT MADC really does fit the linear model, and linear calibration really does
+work well.
+
+Calibration math attempt from 3207 to 4003 mV, using MADC=593 at high end
+=========================================================================
+
+A=(4003-3207)/(593-473)*1024 = 6792
+
+B parameter computed from each calibration point:
+
+MADC reading	Voltmeter reading	Computed VBAT w/o B	B parameter
+---------------------------------------------------------------------------
+473		3207 mV			3137			70
+488		3306 mV			3236			70
+502		3403 mV			3329			74
+517		3500 mV			3429			71
+532		3597 mV			3528			69
+546		3696 mV			3621			75
+562		3804 mV			3727			77
+576		3897 mV			3820			77
+593		4003 mV			3933			70
+
+B values range from 69 to 77, still a very small variation - thus we are not
+too terribly sensitive to errors in the A parameter resulting from MADC values
+being on the cusp.