Project: measurement history

A single temperature value says little. Is it rising right now? Is that spike just an outlier? For that the chip remembers the last values in a ring buffer: ten fixed slots in the stack that are overwritten in turn.

The ring#

A pointer pos says where the next value goes. After every write it moves on by one and jumps back to 0 after the last slot:

poke pos temp
add pos pos 1
mod pos pos SIZE         # 9 -> 0

So the newest value sits at pos − 1, the oldest at pos once the ring has been full. Until then the oldest sits at 0. A counter count remembers how many slots are already filled, at most SIZE.

Milestones#

1. Average of the first values: the LED “Mittel” shows the average of the temperatures stored so far in °C, even if the ring is not full yet.
2. Only the last ten: once the ring is full, every new value overwrites the oldest. The average always refers to the last ten values.
3. Trend: the LED “Trend” shows newest minus oldest value in K. The light “Steigt” lights up if the trend is above 0.5 K.
4. Sensor failure: if the gas sensor “Halle” is missing, nothing is stored, the flashing light “Sensorfehler” lights up and the displays keep their last value.

Average over the filled slots#

move total 0
move i 0
summing:
get value db i
add total total value
add i i 1
blt i count summing
div mean total count

Because the ring is filled from 0, the first count values always sit at addresses 0 to count − 1. Once it is full, count equals SIZE, and the loop runs over all slots.