"I believe we fixed the cause several years ago, so no new ones should be going bad now, at least not with a modern Hijack installed".

The story was, that while clocking commands out to read temperatures, the routine used to be interrupted once in a while by the "DMA" interrupt (moving sound bits to the DSP). This causes the ds1821 to misinterpret those pauses as phantom bits. Once in a while it would "get lucky", and those "phantom bits" would tell it to switch to thermostat mode. Permanently.

We fixed that by masking all interrupts (including the "DMA" interrupt, aka. FIRQ) during any bit-banging operations for the ds1821 chip. Problem solved.

Cheers