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diff --git a/docs/feature_debounce_type.md b/docs/feature_debounce_type.md index 65b4ea1e5..83ebafe60 100644 --- a/docs/feature_debounce_type.md +++ b/docs/feature_debounce_type.md @@ -1,43 +1,151 @@ -# Debounce algorithm +# Contact bounce / contact chatter -QMK supports multiple debounce algorithms through its debounce API. +Mechanical switches often don't have a clean single transition between pressed and unpressed states. + +In an ideal world, when you press a switch, you would expect the digital pin to see something like this: +(X axis showing time +``` +voltage +---------------------- + ^ | + | | + | ------------------+ + ----> time +``` + +However in the real world you will actually see contact bounce, which will look like multiple 1->0 and 0->1 transitions, +until the value finally settles. +``` + +-+ +--+ +------------- + | | | | | + | | | | | ++-----------------+ +-+ +-+ +``` +The time it takes for the switch to settle might vary with switch type, age, and even pressing technique. + +If the device chooses not to mitigate contact bounce, then often actions that happen when the switch is pressed are repeated +multiple times. + +There are many ways to handle contact bounce ("Debouncing"). Some include employing additional hardware, for example an RC filter, +while there are various ways to do debouncing in software too, often called debounce algorithms. This page discusses software +debouncing methods available in QMK. + +While technically not considered contact bounce/contact chatter, some switch technologies are susceptible to noise, meaning, +while the key is not changing state, sometimes short random 0->1 or 1->0 transitions might be read by the digital circuit, for example: +``` + +-+ + | | + | | ++-----------------+ +-------------------- +``` + +Many debounce methods (but not all) will also make the device resistant to noise. If you are working with a technology that is +susceptible to noise, you must choose a debounce method that will also mitigate noise for you. + +## Types of debounce algorithms -The logic for which debounce method called is below. It checks various defines that you have set in rules.mk +1) Unit of time: Timestamp (milliseconds) vs Cycles (scans) + * Debounce algorithms often have a 'debounce time' parameter, that specifies the maximum settling time of the switch contacts. + This time might be measured in various units: + * Cycles-based debouncing waits n cycles (scans), decreasing count by one each matrix_scan + * Timestamp-based debouncing stores the millisecond timestamp a change occurred, and does substraction to figure out time elapsed. + * Timestamp-based debouncing is usually superior, especially in the case of noise-resistant devices because settling times of physical + switches is specified in units of time, and should not depend on the matrix scan-rate of the keyboard. + * Cycles-based debouncing is sometimes considered inferior, because the settling time that it is able to compensate for depends on the + performance of the matrix scanning code. If you use cycles-based debouncing, and you significantly improve the performance of your scanning + code, you might end up with less effective debouncing. A situation in which cycles-based debouncing might be preferable is when + noise is present, and the scanning algorithm is slow, or variable speed. Even if your debounce algorithm is fundamentally noise-resistant, + if the scanning is slow, and you are using a timestamp-based algorithm, you might end up making a debouncing decision based on only two + sampled values, which will limit the noise-resistance of the algorithm. + * Currently all built-in debounce algorithms support timestamp-based debouncing only. In the future we might + implement cycles-based debouncing, and it will be selectable via a ```config.h``` macro. + +2) Symmetric vs Asymmetric + * Symmetric - apply the same debouncing algorithm, to both key-up and key-down events. + * Recommended naming convention: ```sym_*``` + * Asymmetric - apply different debouncing algorithms to key-down and key-up events. E.g. Eager key-down, Defer key-up. + * Recommended naming convention: ```asym_*``` followed by details of the type of algorithm in use, in order, for key-down and then key-up + +3) Eager vs Defer + * Eager - any key change is reported immediately. All further inputs for DEBOUNCE ms are ignored. + * Eager algorithms are not noise-resistant. + * Recommended naming conventions: + * ```sym_eager_*``` + * ```asym_eager_*_*```: key-down is using eager algorithm + * ```asym_*_eager_*```: key-up is using eager algorithm + * Defer - wait for no changes for DEBOUNCE ms before reporting change. + * Defer algorithms are noise-resistant + * Recommended naming conventions: + * ```sym_defer_*``` + * ```asym_defer_*_*```: key-down is using eager algorithm + * ```asym_*_defer_*```: key-up is using eager algorithm + +4) Global vs Per-Key vs Per-Row + * Global - one timer for all keys. Any key change state affects global timer + * Recommended naming convention: ```*_g``` + * Per-key - one timer per key + * Recommended naming convention: ```*_pk``` + * Per-row - one timer per row + * Recommended naming convention: ```*_pr``` + * Per-key and per-row algorithms consume more resources (in terms of performance, + and ram usage), but fast typists might prefer them over global. + +## Debounce algorithms supported by QMK + +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:= $(QUANTUM_DIR)/debounce -DEBOUNCE_TYPE?= sym_g +DEBOUNCE_TYPE?= sym_defer_g ifneq ($(strip $(DEBOUNCE_TYPE)), custom) QUANTUM_SRC += $(DEBOUNCE_DIR)/$(strip $(DEBOUNCE_TYPE)).c endif ``` -# Debounce selection +### Debounce selection | DEBOUNCE_TYPE | Description | What else is needed | | ------------- | --------------------------------------------------- | ----------------------------- | -| Not defined | Use the default algorithm, currently sym_g | Nothing | +| Not defined | Use the default algorithm, currently sym_defer_g | Nothing | | custom | Use your own debounce code | ```SRC += debounce.c``` add your own debounce.c and implement necessary functions | -| anything_else | Use another algorithm from quantum/debounce/* | Nothing | +| 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``` +### Selecting an included debouncing method +Keyboards may select one of the already implemented debounce methods, by adding to ```rules.mk``` the following line: +``` +DEBOUNCE_TYPE = <name of algorithm> +``` +Where name of algorithm is one of: +* ```sym_defer_g``` - debouncing per keyboard. On any state change, a global timer is set. When ```DEBOUNCE``` milliseconds of no changes has occurred, all input changes are pushed. + * This is the current default algorithm. This is the highest performance algorithm with lowest memory usage, and it's also noise-resistant. +* ```sym_eager_pr``` - debouncing per row. On any state change, response is immediate, followed by locking the row ```DEBOUNCE``` 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. +* ```sym_eager_pk``` - debouncing per key. On any state change, response is immediate, followed by ```DEBOUNCE``` milliseconds of no further input for that key +* ```sym_defer_pk``` - debouncing per key. On any state change, a per-key timer is set. When ```DEBOUNCE``` milliseconds of no changes have occurred on that key, the key status change is pushed. + +### A couple algorithms that could be implemented in the future: +* ```sym_defer_pr``` +* ```sym_eager_g``` +* ```asym_eager_defer_pk``` + +### Use your own debouncing code +You have the option to implement you own debouncing algorithm. To do this: +* Set ```DEBOUNCE_TYPE = custom``` in ```rules.mk```. +* Add ```SRC += debounce.c``` in ```rules.mk``` * 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. +* If the algorithm might be applicable to other keyboards, please consider adding it to ```quantum/debounce``` -# 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``` 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``` milliseconds of no further input for that key -* sym_g - debouncing per keyboard. On any state change, a global timer is set. When ```DEBOUNCE``` milliseconds of no changes has occured, all input changes are pushed. -* sym_pk - debouncing per key. On any state change, a per-key timer is set. When ```DEBOUNCE``` milliseconds of no changes have occured on that key, the key status change is pushed. +### Old names +The following old names for existing algorithms will continue to be supported, however it is recommended to use the new names instead. +* sym_g - old name for sym_defer_g +* eager_pk - old name for sym_eager_pk +* sym_pk - old name for sym_defer_pk +* eager_pr - old name for sym_eager_pr |