Edge-detection function blocks

Edge detectors convert a level signal into a single-cycle pulse on a transition. Both take a Boolean CLK input and produce a Boolean Q output.

FB	Detects	I/O
R_TRIG	Rising edge of CLK	In: CLK (BOOL); Out: Q (BOOL)
F_TRIG	Falling edge of CLK	In: CLK (BOOL); Out: Q (BOOL)

Signature (both R_TRIG and F_TRIG):

FUNCTION_BLOCK R_TRIG   // and F_TRIG
VAR_INPUT
    CLK : BOOL;
END_VAR
VAR_OUTPUT
    Q : BOOL;
END_VAR
END_FUNCTION_BLOCK

Timing — R_TRIG:

• Q is TRUE for exactly one cycle when CLK changes from FALSE to TRUE.
• On every other cycle Q is FALSE.
• F_TRIG is the mirror image, pulsing on the TRUE→FALSE transition.

VAR
    rise : R_TRIG;
    fall : F_TRIG;
END_VAR

rise(CLK := button);
IF rise.Q THEN
    presses := presses + 1;   // increment once per button press
END_IF

fall(CLK := button);
IF fall.Q THEN
    // button released
END_IF

Caution

Declare one edge-detector instance per signal you want to monitor. Sharing a single instance across two different CLK sources will produce incorrect edges, because the instance only remembers the previous value of its last call.