I've spent the past few hours arguing with ChatGPT because it doesn't know how to convert nibbles into bytes. I guess there's not enough modern books or websites on the subject, so it doesn't know much.
Yes. I'm trying to create a prompt for you guys. Once someone hands you the right prompt, coding with ChatGPT is effortless.
It's extremely common for it to invoke nonexistent methods. If you tell it that the method doesn't exist it apologizes and rewrites the solution using classes that don't exist. If you tell it about that it again apologies and gives a third solution. Today's fourth solution was basically identical to the first.
I agree! Or when (ChatGPT 3.5 in particular) doesn't know, so it starts hallucinating a response.
Giving it some more background information seems to have helped just as well.
After that, I'd hit a problem here and there, but as I encountered each problem, I added a clarification to the writing prompt:
The problem I'm running into now is that the explanation for how to handle working with a 12-bit variable is equal in size to the prompt above. It simply lacks the skill. I need to find a smarter way of explaining the solution that it understands, rather than telling it step-by-step.
CAN-C and CAN-IHS Message IDs for the 2018+ Jeep Wrangler JL,,,,,,,,,,,,,,
"MAY ALSO APPLY TO OTHER LATE-MODEL CHRYSLER, DODGE, JEEP, AND FIAT VEHICLES.",,,,,,,,,,,,,,
"Here are ChatGPT's instructions.
Always follow the data table for extracting values from the payload, including the PAYLOAD START NIBBLE and PAYLOAD END NIBBLE values, and double-check the data table for units and conversion factors when processing variables. Check the data table for the encoding format of the variable, including its size (number of bytes) and whether it is signed or unsigned.
In the table that follows, the fields are described as follows:
CAN IHS: Whether the CAN ID is part of the IHS network or not. Values can be 'Y' (Yes) or 'N' (No).
CAN C: Whether the CAN ID belongs to the CAN C network. Values can be 'Y' (Yes) or 'N' (No).
MSG ID: The Message Identifier in hexadecimal format for each CAN message.
VARIABLE NAME: The name of the variable that is stored in the payload of the CAN message.
UNITS: In what measurement units VARIABLE NAME is given in. If blank, then the there are no units.
FORMULA: What formula (if any) to apply to the value before using.
PAYLOAD START NIBBLE: This field represents the starting offset (in nibbles) that VARIABLE NAME is stored in the payload.
PAYLOAD END NIBBLE: This field represents the ending offset (in nibbles) that VARIABLE NAME is stored in the payload.
NEEDS ENGINE RUNNING: Whether the engine needs to be running to receive the message. Values can be 'Y' (Yes) or 'N' (No).
CAN MESSAGE BYTE LENGTH: The total length of the CAN message in bytes.
UPDATE RATE: The rate at which the CAN message is updated, typically in seconds or milliseconds.
ENCODING FORMAT: The format used for encoding the data in the CAN message.
USAGE NOTES: Additional comments on how the variable should be used.",,,,,,,,,,,,,,
CAN IHS,CAN C,MSG ID,VARIABLE NAME,UNITS,FORMULA TO APPLY BEFORE USING,PAYLOAD START NIBBLE,PAYLOAD END NIBBLE,NEEDS ENGINE RUNNING,CAN MESSAGE BYTE LENGTH,UPDATE RATE,ENCODING FORMAT,USAGE NOTES,,
N,Y,$023,STEERING WHEEL ANGLE,degrees,*2,0,3,Y,8,0.01 seconds,word,"After multiplying by 2, subtract by 0x2000 in order to make 0x0000 become the center value.",,1
N,Y,$023,STEERING WHEEL DRIVER TORQUE,raw value,,4,7,Y,8,0.01 seconds,word,,,1
N,Y,$079,VEHICLE IS STOPPED,raw value,,0,0,N,8,0.02 seconds,array of bits,,,9
N,Y,$079,BRAKE SYSTEM PRESSURE,raw value,,1,3,N,8,0.02 seconds,12-bit word,,,9
N,Y,$079,BRAKE PEDAL PRESSURE,raw value,,5,7,N,8,0.02 seconds,12-bit word,,,9
N,Y,$079,VEHICLE SPEED,MPH,*200,8,11,N,8,0.02 seconds,word,,,9
N,Y,$08B,FRONT DRIVER TIRE SPEED,MPH,*20,0,3,N,8,0.02 seconds,word,,,18
N,Y,$08B,FRONT PASSENGER TIRE SPEED,MPH,*20,4,7,N,8,0.02 seconds,word,,,18
N,Y,$08B,REAR DRIVER TIRE SPEED,MPH,*20,8,11,N,8,0.02 seconds,word,,,18
N,Y,$08B,REAR PASSENGER TIRE SPEED,MPH,*20,12,15,N,8,0.02 seconds,word,,,18
N,Y,$127,INTAKE AIR TEMPERATURE,Celsius,-40,0,1,Y,8,0.10 seconds,signed byte,,,35
N,Y,$127,ENGINE COOLANT TEMPERATURE,Celsius,-40,2,3,Y,8,0.10 seconds,signed byte,,,35
N,Y,$127,BAROMETRIC PRESSURE,absolute kPa,,4,5,Y,8,0.10 seconds,byte,,,35
N,Y,$127,UNKNOWN TEMPERATURE,raw value,,8,9,Y,8,0.10 seconds,byte,,,35
N,Y,$127,UNKNOWN STATUS,raw value,,10,11,Y,8,0.10 seconds,byte,,,35
N,Y,$127,UNKNOWN,raw value,,12,13,Y,8,0.10 seconds,byte,,,35def extract_by_nibble(nibble_start,ending_nibble,payload):
if starting_nibble > ending_nibble:
return 'N/A'
if ending_nibble >= len(payload) * 2:
return 'N/A'
value = 0
for nibble in range(starting_nibble, ending_nibble+1):
byte_idx = nibble // 2
byte = payload[byte_idx]
shift = (1 - (nibble % 2)) * 4
nibble_value = (byte >> shift) & 0x0F
value = (value << 4) | nibble_value
return value#!/usr/bin/python3
import can
import time
def extract_by_nibble(nibble_start,ending_nibble,payload):
if starting_nibble > ending_nibble:
return 'N/A'
if ending_nibble >= len(payload) * 2:
return 'N/A'
value = 0
for nibble in range(starting_nibble, ending_nibble+1):
byte_idx = nibble // 2
byte = payload[byte_idx]
shift = (1 - (nibble % 2)) * 4
nibble_value = (byte >> shift) & 0x0F
value = (value << 4) | nibble_value
return value
def process_messages(bus, current_values):
msg = bus.recv(timeout=0.001)
if msg is not None:
if msg.arbitration_id == 0x023:
steering_wheel_angle = extract_by_nibble(0, 3, msg.data) * 2
steering_wheel_angle = steering_wheel_angle - 0x2000
current_values["steering_wheel_angle"] = steering_wheel_angle
elif msg.arbitration_id == 0x079:
vehicle_speed = extract_by_nibble(8, 11, msg.data) * 200
current_values["vehicle_speed"] = vehicle_speed
elif msg.arbitration_id == 0x08B:
front_driver_tire_speed = extract_by_nibble(0, 3, msg.data) * 20
current_values["front_driver_tire_speed"] = front_driver_tire_speed
elif msg.arbitration_id == 0x127:
intake_air_temperature = extract_by_nibble(0, 1, msg.data) - 40
current_values["intake_air_temperature"] = intake_air_temperature
def print_values_every_second(current_values):
print(f"Steering Wheel Angle: {current_values['steering_wheel_angle']} degrees")
print(f"Vehicle Speed: {current_values['vehicle_speed']} MPH")
print(f"Front Driver Tire Speed: {current_values['front_driver_tire_speed']} MPH")
print(f"Intake Air Temperature: {current_values['intake_air_temperature']} Celsius")
print("\n")
if __name__ == "__main__":
can_filters = [{"can_id": 0x023, "can_mask": 0x7FF},
{"can_id": 0x079, "can_mask": 0x7FF},
{"can_id": 0x08B, "can_mask": 0x7FF},
{"can_id": 0x127, "can_mask": 0x7FF}]
bus = can.interface.Bus(channel="can1", bustype="socketcan", can_filters=can_filters)
current_values = {"steering_wheel_angle": 0,
"vehicle_speed": 0,
"front_driver_tire_speed": 0,
"intake_air_temperature": 0}
start_time = time.time()
while True:
process_messages(bus, current_values)
elapsed_time = time.time() - start_time
if elapsed_time >= 1.0:
print_values_every_second(current_values)
start_time = time.time()For safety it would probably go into limp mode and disable the throttle as the ecu would be receiving contradicting messages between your messages and the throttle's actual messages.
