qmk-firmware/docs/feature_debounce_type.md

Debounce algorithm

QMK supports multiple debounce algorithms through its debounce API.

The logic for which debounce method called is below. It checks various defines that you have set in rules.mk

DEBOUNCE_DIR:= $(QUANTUM_DIR)/debounce
DEBOUNCE_TYPE?= sym_g
ifneq ($(strip $(DEBOUNCE_TYPE)), custom)
    QUANTUM_SRC += $(DEBOUNCE_DIR)/$(strip $(DEBOUNCE_TYPE)).c
endif

Debounce selection

DEBOUNCE_TYPE	Description	What else is needed
Not defined	Use the default algorithm, currently sym_g	Nothing
custom	Use your own debounce.c	SRC += debounce.c add your own debounce.c and implement necessary functions
anything_else	Use another algorithm from quantum/debounce/*	Nothing

Regarding split keyboards: The debounce code is compatible with split keyboards.

Use your own debouncing code

• Set DEBOUNCE_TYPE = custom .
• Add SRC += debounce.c
• Add your own debounce.c. Look at current implementations in quantum/debounce for examples.
• Debouncing occurs after every raw matrix scan.
• Use num_rows rather than MATRIX_ROWS, so that split keyboards are supported correctly.

Changing between included debouncing methods

You can either use your own code, by including your own debounce.c, or switch to another included one. Included debounce methods are:

• eager_pr - debouncing per row. On any state change, response is immediate, followed by locking the row DEBOUNCE_DELAY milliseconds of no further input for that row. For use in keyboards where refreshing NUM_KEYS 8-bit counters is computationally expensive / low scan rate, and fingers usually only hit one row at a time. This could be appropriate for the ErgoDox models; the matrix is rotated 90°, and hence its "rows" are really columns, and each finger only hits a single "row" at a time in normal use.
• eager_pk - debouncing per key. On any state change, response is immediate, followed by DEBOUNCE_DELAY milliseconds of no further input for that key
• sym_g - debouncing per keyboard. On any state change, a global timer is set. When DEBOUNCE_DELAY milliseconds of no changes has occured, all input changes are pushed.