uart_tx.pio

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

.program uart_tx
.side_set 1 opt

; An 8n1 UART transmit program.
; OUT pin 0 and side-set pin 0 are both mapped to UART TX pin.

    pull       side 1 [7]  ; Assert stop bit, or stall with line in idle state
    set x, 7   side 0 [7]  ; Preload bit counter, assert start bit for 8 clocks
bitloop:                   ; This loop will run 8 times (8n1 UART)
    out pins, 1            ; Shift 1 bit from OSR to the first OUT pin
    jmp x-- bitloop   [6]  ; Each loop iteration is 8 cycles.


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

static inline void uart_tx_program_init(PIO pio, uint sm, uint offset, uint pin_tx, uint baud) {
    // Tell PIO to initially drive output-high on the selected pin, then map PIO
    // onto that pin with the IO muxes.
    pio_sm_set_pins_with_mask(pio, sm, 1u << pin_tx, 1u << pin_tx);
    pio_sm_set_pindirs_with_mask(pio, sm, 1u << pin_tx, 1u << pin_tx);
    pio_gpio_init(pio, pin_tx);

    pio_sm_config c = uart_tx_program_get_default_config(offset);

    // OUT shifts to right, no autopull
    sm_config_set_out_shift(&c, true, false, 32);

    // We are mapping both OUT and side-set to the same pin, because sometimes
    // we need to assert user data onto the pin (with OUT) and sometimes
    // assert constant values (start/stop bit)
    sm_config_set_out_pins(&c, pin_tx, 1);
    sm_config_set_sideset_pins(&c, pin_tx);

    // We only need TX, so get an 8-deep FIFO!
    sm_config_set_fifo_join(&c, PIO_FIFO_JOIN_TX);

    // 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 void uart_tx_program_putc(PIO pio, uint sm, char c) {
    pio_sm_put_blocking(pio, sm, (uint32_t)c);
}

static inline void uart_tx_program_puts(PIO pio, uint sm, const char *s) {
    while (*s)
        uart_tx_program_putc(pio, sm, *s++);
}

%}