In an unprecedented move I’ve released another version of the firmware for the AVR Butterfly Logger within a few months of a previous release. This was a minor change just adding MAX6675 based J and K thermocouple support.
You can download the source code from sourceforge, with the changes from the previous version are listed below.
Changes summary: - MAX6675 K-Type Thermocouple logging (adjustable averaging) - MAX6675 available on 'T' via the USART. - MAX6675 available on LCD via 'THERMOCOUPLE' Menu.
The new library has configurable support for averaging readings but each individual reading takes 200ms so an 8 point average will take 1.6s to complete. If you use this averaging feature please make sure that your logging interval is long enough to ensure enough time for the complete reading to finish. While thermocouples are capable of measuring wide ranges of temperature both positive and negative the MAX6675 is limited to returning readings between 0°C and +1024°C. If the chip detects an open thermocouple then the system will log ‘-1’ as the temperature.
The MAX6675 connects to the AVR Butterfly via the SPI bus. This bus is also used by the onboard Dataflash and the Kionix acceleration sensors (when in use). As this bus is already available the only additional pins required is a single chip select (CS). The library currently only supports a single thermocouple input but the code could easily be expanded to suport multiple MAX6675s requiring a single CS line for each additional chip and thermocouple.
The SPI bus is available on PORTB and also via the ISP connector. An example breakout PCB for the MAX6675 was published here. Default wiring from the this PCB to the AVR Butterfly Logger is given below:
|PCB||MAX6675||Butterfly||J403 (ISP)||J405 (USI)|
More information about the data logger project can be found at the project website.
Any questions can be asked in the project forums.