targets/sg48key, sgh60: fix joystick center drift (#78)

ADCDevice.Get2 assigned the raw value to a 2048-wide zone by flooring,
so zone 0 covered only [0, 2047] - entirely on the positive side of true
center - and the dead zone extended one zone (2048 counts) further in the
positive direction than in the negative one. The stick therefore
registered movement much earlier in one direction, observed as a slight
drift near center. Add half a zone before shifting so the assignment
rounds to the nearest zone and the dead zone becomes symmetric.

Both targets carry the same copy of ADCDevice; confirmed on sg48key
hardware.

Author: sago35

targets/sg48key/adcdevice.go

@@ -62,6 +62,13 @@ func (a *ADCDevice) Get2() int16 {
 
 	ret := int(a.Value)
 	ret += 0x4000
+	// Round to the nearest 2048-wide zone instead of always flooring. Without
+	// this, zone 0 (the dead zone's center bucket) covers only [0, 2047] -
+	// entirely on the positive side of true center - so the negative side of
+	// the dead zone is one zone (2048 counts) narrower than the positive
+	// side, and the stick registers movement much more easily to one side
+	// (observed as a slight drift) than the other.
+	ret += 1024
 	ret >>= 11
 	ret -= 8
 

targets/sgh60/adcdevice.go

@@ -62,6 +62,13 @@ func (a *ADCDevice) Get2() int16 {
 
 	ret := int(a.Value)
 	ret += 0x4000
+	// Round to the nearest 2048-wide zone instead of always flooring. Without
+	// this, zone 0 (the dead zone's center bucket) covers only [0, 2047] -
+	// entirely on the positive side of true center - so the negative side of
+	// the dead zone is one zone (2048 counts) narrower than the positive
+	// side, and the stick registers movement much more easily to one side
+	// (observed as a slight drift) than the other.
+	ret += 1024
 	ret >>= 11
 	ret -= 8