samd: Size-optimize the temperature code with an 0.5 to 2.0 degree relative additional error #7045
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Perform most arithmetic with scaled integer values.
For my calibration values
```
const uint32_t NVMCTRL_TEMP_LOG[]={0xfc05511e, 0xcc7ac0f7};
```
the maximum difference between the old and new calculation is 0.50°C.
The difference is smallest (0.13°) at 25.87°C in the old scale.
This reduces mcu_processor_get_temperature from 568 bytes to 348 bytes
(-220 bytes)
Another reduction of -48 bytes can be had if the fine calculation step is skipped. The worst difference compared to the old reference code with my calibration values in the 0° to 60° was 2°C, and the difference at 25°C is 1°C. The final size decrease for non-full builds like Trinket M0 is 268 bytes.
dhalbert
approved these changes
Oct 13, 2022
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Convert most of the math to integer, and don't be too prissy about rounding.
I used a standalone program together with my particular device's calibration values of
and found the "errors" at 0..60°C, meaning the difference relative to the existing reference code.
At 25°C the first commit introduces an 0.5°C relative change. The second commit introduces a 1°C relative change, while the biggest change over the whole 0°C—60°C range was 2°C.
The total size savings appears to be 268 bytes, based on
nm --print-sizeof themcu_processor_get_temperaturefunction in the trinket m0 build.I also tested that the temperature value remains plausible-looking on a PyRuler.