CP calibration

Control pilot calibration is performed by setting the right PILOT_DOWN_THRESHOLD_X values in board.cfg. These values are used for detection EVSE states and diode short error.

Next table descibe J1772 specification pilot states. (source)

State	High voltage	Low voltage	Frequency	Resistance
A	12 V	N/A	N/A	N/A
B	9 V	- 12 V	1 kHz	2.74 kΩ
C	6 V	- 12V	1 kHz	882 Ω
D	3 V	- 12V	1 kHz	246 Ω
E	0 V	0 V	N/A
F	N/A	-12 V	N/A

On following example circuit, sensing CP voltage is provided by voltage divider with shift (R2, R4, R6). Wire CP_OUT is connected to the EV, CP_SENS is connected to ESP32 adc.

For this circuit there is simulation with EV side. Because state detection is performed only on high voltage, a DC power supply can be used in the simulation.

State A

State B

State C

State D

And negative voltage

For diode short detection, just detect -6V in the lower voltage.

In next table is measured values for EV states.

State	ADC voltage
A	2560 mV
B	2251 mV
C	1948 mV
D	1636 mV
Low voltage	728 mV

Next, calculate midpoint voltage between states, that will be used as down threshold. For state D, the value is calculated as a sequence of the difference B-C and C-D.

State	Calculation	Down threshold
A-B	(2560 + 2251) / 2	2406 mV
B-C	(2251 + 1948) / 2	2100 mV
C-D	(1948 + 1636) / 2	1792 mV
D	1792 - (2100 - 1792)	1484 mV

When the calculated values are displayed on the graph, they should have a linear course.

From these values board.cfg will look like this:

PILOT_DOWN_THRESHOLD_12=2406
PILOT_DOWN_THRESHOLD_9=2100
PILOT_DOWN_THRESHOLD_6=1792
PILOT_DOWN_THRESHOLD_3=1484
PILOT_DOWN_THRESHOLD_N12=728

Note When designing a new voltage divider, remember the ESP32 adc suggested range. (adc is configured for attenuation 11dB)