pimoroni-pico/drivers/encoder/encoder.pio

; --------------------------------------------------
;        Quadrature Encoder reader using PIO
;      by Christopher (@ZodiusInfuser) Parrott
; --------------------------------------------------
;
; Watches any two pins (i.e. do not need to be
; consecutive) for when their state changes, and
; pushes that new state along with the old state,
; and time since the last change.
;
; - X is used for storing the last state
; - Y is used as a general scratch register
;     and for storing the current state
; - OSR is used for storing the state-change timer
;
; After data is pushed into the system, a long delay
; takes place as a form of switch debounce to deal
; with rotary encoder dials. This is currently set
; to 500 cycles, but can be changed using the debounce
; constants below, as well as adjusting the frequency
; the PIO state machine runs at. E.g. a freq_divider
; of 250 gives a 1ms debounce.


; Debounce Constants
; --------------------------------------------------
.define SET_CYCLES                  10
.define ITERATIONS                  30
.define JMP_CYCLES                  16
.define public ENC_DEBOUNCE_CYCLES  (SET_CYCLES + (JMP_CYCLES * ITERATIONS))

; Ensure that ENC_DEBOUNCE_CYCLES is a multiple of the
; number of cycles the wrap takes, which is currently
; 10 cycles, otherwise timing may be inaccurate


; Encoder Program
; --------------------------------------------------
.program encoder

.wrap_target
loop:
    ; Copy the state-change timer from OSR,
    ; decrement it, and save it back
    mov y, osr
    jmp y-- osr_dec
osr_dec:
    mov osr, y
    ; takes 3 cycles

    ; Read the state of both encoder pins and check
    ; if they are different from the last state
    jmp pin enc_a_was_high
    mov isr, null
    jmp read_enc_b
enc_a_was_high:
    set y, 1
    mov isr, y
read_enc_b:
    in pins, 1
    mov y, isr
    jmp x!=y state_changed [1]
    ; takes 7 cycles on both paths
.wrap

state_changed:
    ; Put the last state and the timer value into
    ; ISR alongside the current state, and push that
    ; state to the system. Then override the last
    ; state with the current state
    in x, 2
    mov x, ~osr     ; invert the timer value to give
                    ; a sensible value to the system
    in x, 28
    push noblock    ; this also clears isr
    mov x, y

    ; Perform a delay to debounce switch inputs
    set y, (ITERATIONS - 1) [SET_CYCLES - 1]
debounce_loop:
    jmp y-- debounce_loop [JMP_CYCLES - 1]

    ; Initialise the timer, as an inverse, and decrement
    ; it to account for the time this setup takes
    mov y, ~null
    jmp y-- y_dec
y_dec:
    mov osr, y
    jmp loop [1]
    ;takes 10 cycles, not counting whatever the debounce adds


; Initialisation Code
; --------------------------------------------------
% c-sdk {
static const uint8_t ENC_LOOP_CYCLES = encoder_wrap - encoder_wrap_target;

// The time that the debounce takes, as the number of wrap loops that the debounce is equivalent to
static const uint8_t ENC_DEBOUNCE_TIME = ENC_DEBOUNCE_CYCLES / ENC_LOOP_CYCLES;
%}