# K13 — EventGroup and coordinated state

## Position

- Series: FreeRTOS C++
- Lesson: L12, card K13
- Duration: 30 minutes
- Typed bits: UART=0x1, GPIO=0x2, APP=0x4
- Static control block; no heap delta

## Outcome

The student designs a typed readiness mask, distinguishes any/all and
keep/clear-on-exit, explains timeout snapshots, and rejects the interpretation
of an event bit as a counter or payload.

## Lesson plan

| Time | Mode | Evidence |
| --- | --- | --- |
| 0–5 | typed masks | enum bits combine to all=0x7 |
| 5–10 | publishers | coordinator Blocked; workers set 1,2,4 |
| 10–15 | wait all | snapshot/current=0x7, satisfied |
| 15–20 | clear one | clear GPIO returns 0x7, current=0x5 |
| 20–25 | timeout | all unsatisfied for 3 ticks, snapshot=0x5 |
| 25–30 | any + clear | snapshot=0x5, final=0; discuss races/counters |

## Acceptance

- type is an enum and masks remain typed at the wrapper boundary;
- event-group handle equals caller static control and heap delta is zero;
- coordinator is eBlocked before the first worker publishes;
- repeated UART set is idempotent at mask 1;
- all/keep gives 7 and explicit GPIO clear gives 5;
- second all wait times out after at least 3 ticks with snapshot 5;
- any/clear-on-exit succeeds with snapshot 5 and final mask 0;
- target exits with PASS.

## Main traps

1. A bit is state, not an event count.
2. A bit mask is not a payload record.
3. Clear-on-exit affects all selected bits and can race with other waiters.
4. Timeout return is a snapshot; test satisfaction explicitly.
5. EventGroup FromISR set is deferred through the timer service task.