Design review: Memory

[krook]

Evaluate alternatives to the SD card currently in use to optimize for robustness and storage needed.

The non-volatile memory storage medium in this design is a standard SD card. While this can be utilized in a final design, SD cards typically are not rated for high temperatures and tend to fail much more commonly than more standard embedded memory interfaces.

In order to better understand the requirements for memory, an evaluation regarding exactly what is stored in non-volatile memory needs to be conducted. Once this is done, an appropriate module can be selected.

All sensor samples are captured to the SD card as a 55-byte string being written every 10 seconds. However, it should be considered that the data does not need to be stored as a string, and instead, it can be packed into a binary format with as little as 32 bytes, or less depending on the resolution needed for each sensor value. If the device should be expected to store 100 hours of data, then, in the worst case, as little as 2Mb, but perhaps as low as 1Mb. This means that a simple SPI memory interface that is widely available, lower cost, and more reliable, and more manufacturable than an SD card can be used.

Further, it should be evaluated if all data requires logging from the sensor since it will usually be sent directly to the smartphone. Perhaps only data that was not sent to the phone needs to be written to a temporary cache file, which can be sent when the connection is re-established.

An additional aspect to address is how time is kept without the phone connected. It is possible an RTC could be needed, and it is possible a separate battery to power the RTC would need to be included as well.