Meet EMU2.2 32/64, my EPROM Emulator, built around an 80C51.
It was a commercial grade design, built for both fun and for commercial purposes.
Even though the design was pretty cool, the market management was bad, so it was a financial failure.
Here are the schematics (PDF, 25 KB) and the documentation (PDF, 91 KB).
Quartz crystals have a low, but non-zero temperature coefficient. On the other hand, quartz has superb quality factor and time stability. By measuring the thermal drift of the oscillation frequency of a quartz crystal, one could build a thermometer with amazingly good characteristics (resolution, accuracy, repetability/stability).
The only impediment is the need for a reference frequency generator. In fact, the reference needs to be so good, that it raises the cost and complexity of the initial design with a few orders of magnitude, compromising the idea.
My solution is rather funny: the "reference" frequency can be done with another quartz crystal, having similar, but not identical thermal behaviour, thermally coupled with the first crystal. One can then build a frequency meter that has the time base generated by one of the crystals, and the input generated by the other one. The output of this frequency meter is, with a very good approximation, only a function of temperature. Only a calibration is needed initially.
Most importantly, no internal voltage/current/frequency/etc references are needed. Thermometer's output is only a function of the material contants of the quartz, which are notably stable in time. Another nice feature is its (almost) all-digital design. No sensitive analog components are needed, except for the oscillators themselves.
I was routinely measuring temperature differences of 1/1000oCelsius.
While the thermometer was sitting on my desk, it was able to thermally detect whether or not I was sitting at my desk, at a distance of ~25cm from the sensor, by measuring variations in temperature due to the thermal radiation emitted by my body.
The hardware implementation consisted of two microcontroller-based systems (slaves), and a PC (master). The two slaves were based on 80C51 and 80C552. The 80C51 slave was actually an EPROM emulator providing the program memory for the 80C552 slave. The 80C552 slave was a complex analog interface module, with analog current/voltage inputs/outputs. The master application (on a PC) was uploading 80C552 code in the 80C51 slave, then making analog measurements through the 80C552 slave. The test application was measuring transistors characteristics (both FETs and bipolars).