Add an example to trasmit RS422 signal

README.md

@@ -329,6 +329,7 @@ App|Description
 [pio_ws2812](pio/ws2812) | Example of driving a string of WS2812 addressable RGB LEDs.
 [pio_ws2812_parallel](pio/ws2812) | Examples of driving multiple strings of WS2812 addressable RGB LEDs efficiently.
 [pio_addition](pio/addition) | Add two integers together using PIO. Only around 8 billion times slower than Cortex-M0+.
+[pio_rs422_tx](pio/rs422_tx) | Modifies the pio_uart_tx sample to send "Hello, world!" with RS-422 transmit functionality without an external IC.
 
 ### PWM
 

pio/CMakeLists.txt

@@ -14,6 +14,7 @@ if (TARGET hardware_pio)
     add_subdirectory_exclude_platforms(pwm)
     add_subdirectory_exclude_platforms(quadrature_encoder)
     add_subdirectory_exclude_platforms(quadrature_encoder_substep)
+    add_subdirectory_exclude_platforms(rs422_tx)
     add_subdirectory_exclude_platforms(spi)
     add_subdirectory_exclude_platforms(squarewave)
     add_subdirectory_exclude_platforms(st7789_lcd)

pio/rs422_tx/CMakeLists.txt

@@ -0,0 +1,11 @@
+add_executable(pio_rs422_tx)
+
+pico_generate_pio_header(pio_rs422_tx ${CMAKE_CURRENT_LIST_DIR}/rs422_tx.pio)
+
+target_sources(pio_rs422_tx PRIVATE rs422_tx.c)
+
+target_link_libraries(pio_rs422_tx PRIVATE pico_stdlib hardware_pio)
+pico_add_extra_outputs(pio_rs422_tx)
+
+# add url via pico_set_program_url
+example_auto_set_url(pio_rs422_tx)

pio/rs422_tx/rs422_tx.c

@@ -0,0 +1,48 @@
+/**
+ * Copyright (c) 2020 Raspberry Pi (Trading) Ltd.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include "pico/stdlib.h"
+#include "hardware/pio.h"
+#include "rs422_tx.pio.h"
+
+// Defines the GPIO pins for PIO communication.
+// PIO_TXP_PIN is the positive pin.
+// PIO_TXM_PIN is the negative pin and must be the next pin in sequence.
+#define PIO_TXP_PIN 0
+#define PIO_TXM_PIN 1
+
+// Check the pin is compatible with the platform
+#if (PIO_TXM_PIN) != (PIO_TXP_PIN + 1)
+#error "PIO_TXM_PIN must be the next pin after PIO_TXP_PIN."
+#endif
+#if (PIO_TXM_PIN) >= NUM_BANK0_GPIOS
+#error "Attempting to use a pin>=32 on a platform that does not support it"
+#endif
+
+int main() {
+    // This is the same as the default UART baud rate on Pico
+    const uint SERIAL_BAUD = 115200;
+
+    PIO pio;
+    uint sm;
+    uint offset;
+
+    // This will find a free pio and state machine for our program and load it for us
+    // We use pio_claim_free_sm_and_add_program_for_gpio_range (for_gpio_range variant)
+    // so we will get a PIO instance suitable for addressing gpios >= 32 if needed and supported by the hardware
+    bool success = pio_claim_free_sm_and_add_program_for_gpio_range(&rs422_tx_program, &pio, &sm, &offset, PIO_TXP_PIN, 2, true);
+    hard_assert(success);
+
+    rs422_tx_program_init(pio, sm, offset, PIO_TXP_PIN, SERIAL_BAUD);
+
+    while (true) {
+        rs422_tx_program_puts(pio, sm, "Hello, world! (from PIO!)\r\n");
+        sleep_ms(1000);
+    }
+
+    // This will free resources and unload our program
+    pio_remove_program_and_unclaim_sm(&rs422_tx_program, pio, sm, offset);
+}

pio/rs422_tx/rs422_tx.pio

@@ -0,0 +1,77 @@
+;
+; Copyright (c) 2020 Raspberry Pi (Trading) Ltd.
+;
+; SPDX-License-Identifier: BSD-3-Clause
+;
+.pio_version 0 // only requires PIO version 0
+
+.program rs422_tx
+.side_set 2 opt
+
+; An 8n1 UART transmit program.
+; OUT pin 0-1 are mapped to RS422 TX+ and TX- pin.
+
+    pull        side 0b01 [3]  ; Assert stop bit, or stall with line in idle state
+    nop
+    nop
+    nop
+    nop
+    set x, 7    side 0b10 [3]  ; Preload bit counter, assert start bit
+    nop
+    nop
+    nop
+bitloop:                   ; This loop will run 8 times (8n1)
+    out y, 1               ; Shift 1 bit from OSR to the Y register
+    jmp !y put0
+put1:
+    jmp next    side 0b01 [3]
+put0:
+    nop         side 0b10 [3]
+next:
+    nop
+    jmp x-- bitloop
+
+
+% c-sdk {
+#include "hardware/clocks.h"
+
+static inline void rs422_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_consecutive_pindirs(pio, sm, pin_tx, 2, true);
+    for ( uint i = 0;i<2;i++ ) {
+        pio_gpio_init(pio, pin_tx + i);
+    }
+
+    pio_sm_config c = rs422_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, 2);
+    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 rs422_tx_program_putc(PIO pio, uint sm, char c) {
+    pio_sm_put_blocking(pio, sm, (uint32_t)c);
+}
+
+static inline void rs422_tx_program_puts(PIO pio, uint sm, const char *s) {
+    while (*s)
+        rs422_tx_program_putc(pio, sm, *s++);
+}
+
+%}