process_pid() was advancing pid_rate and pid_cool unconditionally on
every tick regardless of which one actually drove y. While COOL had
pid_cool driving, pid_rate kept silently integrating against the same
(very negative) heatrate_err with no way to act on it, building a
large stale windup. The moment the FSM returned to HOLD (which only
reset pid_hold), y read straight from that saturated pid_rate instead
of a fresh value, so the heater stayed off well past the target
before the windup finally unwound - an undershoot of about 1 degree
in scripts/demos/pid/demo_temp_controller.py's last (cooling) step.
Now only the PID currently driving y is process()'d each tick, and
pid_rate is also reset on IDLE->HOLD and COOL->HOLD (it was already
reset on the transitions into HEAT/COOL). Undershoot drops to ~0.06
degrees, in line with normal PID settling.
jens
55020d9f38