Temperature Logging with the W1209

A project log for W1209 Data Logging Thermostat

The W1209 thermostat is cheap, but it's about time that it learns new tricks!

ThomasThomas 12/10/2017 at 09:450 Comments

By default, the firmware exploits the fact that the Value Line STM8S003F3 µC is just a STM8S103F3 with relaxed specs, and with only 128 bytes of EEPROM memory specified. All the chips I tested hat the full 640 bytes of the "Access Line - Low Density Devices" available, and I decided to use all but 64 bytes as a ring buffer, enough for more than 24 hours at a rate of 10 samples per second.

The ring buffer requires a single head pointer, which, for practical reasons, must be in the EEPROM. The number of EEPROM write cycles of Value Line devices, however, is specified with 1x10^5, in the full temperature range with nominal data retention, that is. At a rate of 10 log entries per hour a 20ct µC is still good for 10.000 hours, more than a year.

I have little doubt (not "little doubts", like my Indian colleagues usually say when the strongly disagree) that the number of write cycles at room temperature, and with random data, is more in the range of 1x10^6, more than 10 years. The "electrical life" of a Chinese low-end relay in a thermostat application is most likely much shorter.