Inside the Volkswagen emissions cheating

Posted Jan 8, 2016 11:17 UTC (Fri) by branden (guest, #7029)
Parent article: Inside the Volkswagen emissions cheating

Fascinating piece! Thanks, LWN!

Minor correction: kelvins are not degrees; they're just kelvins. So one would write, e.g., "273 K", not "273°K".

And I love the use of a signed 16-bit integer to represent decikelvins. :D

Why signed?

Posted Jan 8, 2016 16:59 UTC (Fri) by pr1268 (guest, #24648) [Link] (2 responses)

use of a signed 16-bit integer to represent decikelvins

Why signed? Recent scientific research notwithstanding, I don't think temperatures below 0 [d]K would ever be measured by this system.

Unless they were using negative values as error codes, etc. (Disclosure: I haven't [yet] read the documents linked above.)

Why signed?

Posted Jan 8, 2016 22:07 UTC (Fri) by pbonzini (subscriber, #60935) [Link]

In the slides there is a "> -3276.8" condition that is always true, so the only plausible interpretation is that temperature is stored in a signed 16-bit integer. Presumably the format (signed 16-bit integer) and unit of measure (decikelvin) is also documented in the A2L files.

Why signed?

Posted Jan 9, 2016 22:53 UTC (Sat) by mrkrad (guest, #106224) [Link]

It's likely the ecu represented the unplugging of a sensor as 0xffff or 0x0000 which in unsigned 16 bit would meet the guidelines of a impossible to reach degrees <-32768 degrees.

back in the days of old 8-bit ecu's a linear look-up-table's 1st result would be used as a failsafe when a sensor was unplugged to lock in a value (for say coolant to timing adjustment), allowing one to unplug the coolant temp sensor and manually set the distributor timing! Talking about 1st generation VW ecu's here. but bosch had their hand in the design as well back then!