Debouncing

The switch is polled at a rate of 1kHz. At each tick, the state of the switch is read as a bit, and shifted into a switch register storing the history of the switch state, as an 8-bit value.

This value is:

• 0b00000000 for a switch in a stable, pressed state.
• 0b11111111 for a switch in a stable, released state.
• 0b10000000 for a switch that has just reached a stable "pressed" state.

Code examples from Tides 2018:

• Switch reading
• Switch state enquiry

The internal pull-up of the GPIO is used.

Timing

Code example from Tides 2018

Explanations:

• When a switch has been recently pressed (1), we store the time (2) at which it has been pressed, and we generate a “switch pressed” event. If the button doesn’t have to respond to long presses, this event is usually immediately handled!
• If a switch is pressed, and the elapsed time exceeds a certain threshold (say 2 seconds), we generate a “switch released” event, and prevent the generation of this event when the switch is released.
• If a switch is released, and is not inhibited, we generate a “switch released” event.

(1) the switch debouncing code stores the state of the switch over the past 8 scans, with a scan rate of 1kHz. So if the state is 0b11111110, we know that the switch has been recently pressed.

(2) The 1kHz timer responsible for UI management counts the number of seconds elapsed since the module is powered on.