After a too long break we finally got back to debugging the problems in the radio boards.
There was’t actually anything seriously wrong, mainly smaller issues that together made things look worse than they were. Version 1 did have some hardware design bugs but they were quite quickly spotted.
The first prototypes don’t work together with the ITead boards because there seems to be 50kHz offset in the radio frequency. That might be because the first prototypes are single sided while the reference design assumes double sided. We are not RF engineers so it might be anything else as well
It also turned out that soldering 0.5mm pitch VQFN chips without proper tools or experience can easily leave some pads unconnected. Combining that with initial software and interoperability problems with the old boards, things were not looking good. After figuring out these deficiencies we started to make good progress again.
Now we have couple of reworked v1 boards with the TMP275 temperature sensor and they are working reliably. The software is measuring the battery voltage using ADC and getting the temperature over I2C from the TMP275 and sending both over the air.
We have done also some tests with the low power modes. The whole board consumes 7uA in a sleep mode:
The low consumption is in LPM3 with ACLK running so that the chip can wake up using a timer interrupt. It should be possible to get down to 1uA in LPM4 but it’s still a bit unclear to me if that needs an external signal to wake up. Even with 7uA we are quite confident that long operating times with small batteries are achievable
Now that the boards are working, we fixed the design bugs and ordered v2 boards from ITead. Hardware-wise next step is testing different antennas to achieve good range with small antennas and software-wise we’ll combine the low power modes with temperature measuring.