E46 CAN mapping Lua script question

[jaytee]

I was going through the E46 CAN mapping Lua script and it's a great reference as I work through the mappings for my car. Had a question with the handling of the data in a couple cases.

Why is the data bitwise AND in the examples below (bit.band):

Code: Select all

function processWheel(id, data, offset)
  --wheel speed is 13 bits long, little endian
  --low byte high byte
  --76543210 76543210
  --11111111 11111XXX
  local highByte = bit.band(data[offset + 2], 0x1F)
  local lowByte = data[offset + 1]
  local value = highByte * 256 + lowByte
  value = value * 0.0625
  --convert to MPH. comment to keep KPH
  value = value * 0.621371
  setChannel(id, value)
end

Code: Select all

function fuelFilter(value)
  --adjust for 7 bit value
  value = bit.band(value, 0x7F)
  --convert liters to %. tank holds 62.83 liters
  return value / 0.6283
end

Guessing the data is signed?


Thanks!

Jeff

[zandr]

The E46 has a ton of weird bit-stealing going on, where there's a 12 or 13-bit value, and the remaining bits are flags for some related (or unrelated) value.

I don't remember what's in the rest of the wheel speed bytes off the top of my head. I do know that they stole the high bit of the fuel value for the low-fuel light. (so somewhere around 10L remaining the value will suddenly jump by 128)

BMW likes to use the high bit for sign, rather than something sensible like twos complement.

[brentp]

The steering angle sensor for E46 - and later as we found out - Porsche as well, uses that uncommon sign-magnitude format - so we added that to the direct mapping capabilities as a supported data type.
https://wiki.autosportlabs.com/CAN_Bus_Integration