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Ford BF to JVC Radio (KDR-90BT) steering wheel remote code for ATTINY85

This code can be used on any vehicle that uses a 5v (or lower) output from the vehicle or a resistor divider bridge
to supply the steering wheel controls through to the radio.  

The ATTINY85 ADC pin must not receive a voltage above VCC (5v) i.e. 12V would be bad!  Use a high impedance resistor 
divider to get the correct voltage if required.

On the Ford BF tap the line under the steering column (Red/White stripe) and that will be a 5v signal that changes with the button presses.
Do NOT cut this wire as it also operates other functionality on the steering wheel such as overspeed alarm control (mode button).

The whole circuit can be powered from the amplifier line on the back of the JVC which has power when the unit is on.

I/O and connection notes:
PORTB0 is the line to connect to the JVC remote control input.
PORTB4 is the analogue input connected to the steering wheel control line (Red/White stripe)
RESET: short to VCC
VCC: 10uF Capacitor across VCC/GND, power with a 7805 regulator from the Amplifier control wire in the JVC connector.
GND: connect to GND in radio wiring harness

The internal 8Mhz oscillator is fast and accurate enough for our purposes.
Fuses for programming the ATTINY85:
SELFPRGEN = [ ]
RSTDISBL = [ ]
DWEN = [ ]
SPIEN = [X]
WDTON = [ ]
EESAVE = [ ]
BODLEVEL = 4V3
CKDIV8 = [ ]
CKOUT = [ ]
SUT_CKSEL = INTRCOSC_8MHZ_6CK_14CK_64MS
EXTENDED = 0xFF (valid)
HIGH = 0xDC (valid)
LOW = 0xE2 (valid)

The processing steps are:
1. Read ADC values
2. Translate ADC values using tolerances to a command
3. Debounce the command for 5ms (done in the ISR to ensure consistent timing)
4. Process the debounced command in a state machine in main() to allow sequenced codes and delays as required

See https://www.avforums.com/threads/jvc-stalk-adapter-diy.248455/ for the raw protocol details.

Ford BF values and mappings as implemented:
Button	ADC Value	JVC Mapping	JVC Code
fwd		442			Forwards	0x12/0x14
-		0			VolDn		0x05
+		216			VolUp		0x04