uart_rx.pio

;
; Copyright (c) 2020 Raspberry Pi (Trading) Ltd.
;
; SPDX-License-Identifier: BSD-3-Clause
;

.program uart_rx_mini

; Minimum viable 8n1 UART receiver. Wait for the start bit, then sample 8 bits
; with the correct timing.
; IN pin 0 is mapped to the GPIO used as UART RX.
; Autopush must be enabled, with a threshold of 8.

    wait 0 pin 0        ; Wait for start bit
    set x, 7 [10]       ; Preload bit counter, delay until eye of first data bit
bitloop:                ; Loop 8 times
    in pins, 1          ; Sample data
    jmp x-- bitloop [6] ; Each iteration is 8 cycles

% c-sdk {
#include "hardware/clocks.h"
#include "hardware/gpio.h"

static inline void uart_rx_mini_program_init(PIO pio, uint sm, uint offset, uint pin, uint baud) {
    pio_sm_set_consecutive_pindirs(pio, sm, pin, 1, false);
    pio_gpio_init(pio, pin);
    gpio_pull_up(pin);

    pio_sm_config c = uart_rx_mini_program_get_default_config(offset);
    sm_config_set_in_pins(&c, pin); // for WAIT, IN
    // Shift to right, autopush enabled
    sm_config_set_in_shift(&c, true, true, 8);
    sm_config_set_fifo_join(&c, PIO_FIFO_JOIN_RX);
    // SM transmits 1 bit per 8 execution cycles.
    float div = (float)clock_get_hz(clk_sys) / (8 * baud);
    sm_config_set_clkdiv(&c, div);
    
    pio_sm_init(pio, sm, offset, &c);
    pio_sm_set_enabled(pio, sm, true);
}
%}

.program uart_rx

; Slightly more fleshed-out 8n1 UART receiver which handles framing errors and
; break conditions more gracefully.
; IN pin 0 and JMP pin are both mapped to the GPIO used as UART RX.

start:
    wait 0 pin 0        ; Stall until start bit is asserted
    set x, 7    [10]    ; Preload bit counter, then delay until halfway through
bitloop:                ; the first data bit (12 cycles incl wait, set).
    in pins, 1          ; Shift data bit into ISR
    jmp x-- bitloop [6] ; Loop 8 times, each loop iteration is 8 cycles
    jmp pin good_stop   ; Check stop bit (should be high)

    irq 4 rel           ; Either a framing error or a break. Set a sticky flag,
    wait 1 pin 0        ; and wait for line to return to idle state.
    jmp start           ; Don't push data if we didn't see good framing.

good_stop:              ; No delay before returning to start; a little slack is
    push                ; important in case the TX clock is slightly too fast.


% c-sdk {
static inline void uart_rx_program_init(PIO pio, uint sm, uint offset, uint pin, uint baud) {
    pio_sm_set_consecutive_pindirs(pio, sm, pin, 1, false);
    pio_gpio_init(pio, pin);
    gpio_pull_up(pin);

    pio_sm_config c = uart_rx_program_get_default_config(offset);
    sm_config_set_in_pins(&c, pin); // for WAIT, IN
    sm_config_set_jmp_pin(&c, pin); // for JMP
    // Shift to right, autopush disabled
    sm_config_set_in_shift(&c, true, false, 32);
    // Deeper FIFO as we're not doing any TX
    sm_config_set_fifo_join(&c, PIO_FIFO_JOIN_RX);
    // SM transmits 1 bit per 8 execution cycles.
    float div = (float)clock_get_hz(clk_sys) / (8 * baud);
    sm_config_set_clkdiv(&c, div);
    
    pio_sm_init(pio, sm, offset, &c);
    pio_sm_set_enabled(pio, sm, true);
}

static inline char uart_rx_program_getc(PIO pio, uint sm) {
    // 8-bit read from the uppermost byte of the FIFO, as data is left-justified
    io_rw_8 *rxfifo_shift = (io_rw_8*)&pio->rxf[sm] + 3;
    while (pio_sm_is_rx_fifo_empty(pio, sm))
        tight_loop_contents();
    return (char)*rxfifo_shift;
}

%}