Hello,
I am making an open source privacy-first fitness band for myself and I am writing the firmware now as someone relatively inexperienced at firmware development (I am an electronics engineer by trade). I get it done but sometimes I run into concept issues, especially when I start overthinking, like now that I need help with.
I have a macronix SPI NOR flash on-board that I want to use as offline activity saving, backup at low battery, etc... I am dreaming up the data structure for it. Here is the values I need to save to not lose information and what will be required for my supported features in the Bluetooth Physical Activity Monitor Service:
struct memory_map_nor {
time_t timestamp;
uint16_t sub_sess_id;
uint32_t steps: 24;
uint8_t bpm;
float16_t spo2;
uint16_t pulse_inter_beat_interval;
uint16_t cadence;
uint16_t speed;
uint16_t activity_level;
uint16_t activity_type;
uint16_t temp;
};
So from this datastructure, it has a total of 28 bytes of data. This has to fit on a 256 byte page, which means 9 "rows" of data can be written per page, 144 per sector, 2304 per 64 bit block, and 147456 in total for a 32Mbit NOR.
But, I am getting confused while reading about memory structures in "normal" processors that need to read everything in 4/8-byte words via the parallel interfaces. This means that conventionally, everything has to be padded to neat structures that are divisible by 4 (32-bit) for QSPI reading. In that case, I would either have to add another 32 bits of data or pad 32 bits to every "row", making a neat 8 data "rows" per page.
OR, because I am only using single lane SPI, would this not matter and I could shove an extra datapoint in each page. The difference is 147456 data rows vs 131072 data rows. At 3s polling rate, that is 5.12 days vs 4.55 days. For my application, the difference might be useless anyway, but the band goal battery life is 2 weeks or so.
Again, maybe I am overthinking this and can just pad the data to make everything neat and fit well. Anyone have any opinions? Thanks!
The way I have done it in the past. Rearrange all struct members from the largest to smallest data size. Do a sizeof the struct to make sure it is what you expect. You can look at the memory map too if your compiler provides it. Point an unsigned char (uint8_t) pointer to the first member of the struct or the variable name of the struct casted as a uint8_t. Then move the data into whatever the EEPROM or flash pages the data needs using the sizeof the struct and the size of the page for the non volatile storage. Sometimes I make a temporary array of the structs if I'm going to do several on a page. Then either copy directly into my ram overlay or just return the data if it is like a lookup or whatever.