Looking for help with PIO (no output on pins) #843

XFer012 · 2022-09-07T13:36:06Z

XFer012
Sep 7, 2022

Hello, I am taking my first steps in PIO territory.

Using the online pioasm tool at https://wokwi.com/tools/pioasm , I tried to write a simple square wave generator, by setting GP15 on and off.

My PIO source:

.program clockgen

set pindirs 1

.wrap_target
    set pins, 1
    set pins, 0
.wrap

% c-sdk {
static inline void clockgen_program_init(PIO pio, uint sm, uint offset, uint pin) {
    pio_sm_config c = clockgen_program_get_default_config(offset);

    // Map the state machine's OUT pin group to one pin, namely the `pin`
    // parameter to this function.
    sm_config_set_out_pins(&c, pin, 1);
    // Set this pin's GPIO function (connect PIO to the pad)
    pio_gpio_init(pio, pin);
    // Set the pin direction to output at the PIO
    pio_sm_set_consecutive_pindirs(pio, sm, pin, 1, true);

    // Load our configuration, and jump to the start of the program
    pio_sm_init(pio, sm, offset, &c);
    // Set the state machine running
    pio_sm_set_enabled(pio, sm, true);
}
%}

Assembled header, "squarewave_pio.h". I added the clock setting with pio_sm_set_clkdiv():

#pragma once

#include <hardware/pio.h>

#define SM_CLOCK_DIVISOR 1000UL   // SM runs at CPU clock / SM_CLOCK_DIVISOR

// -------- //
// clockgen //
// -------- //
#define clockgen_wrap_target 1
#define clockgen_wrap 2

static const uint16_t clockgen_program_instructions[] = {
    0xe081, //  0: set    pindirs, 1                 
            //     .wrap_target
    0xe001, //  1: set    pins, 1                    
    0xe000, //  2: set    pins, 0                    
            //     .wrap
};

static const struct pio_program clockgen_program = {
    .instructions = clockgen_program_instructions,
    .length = 3,
    .origin = -1,
};

static inline pio_sm_config clockgen_program_get_default_config(uint offset) {
    pio_sm_config c = pio_get_default_sm_config();
    sm_config_set_wrap(&c, offset + clockgen_wrap_target, offset + clockgen_wrap);
    return c;
}

static inline void clockgen_program_init(PIO pio, uint sm, uint offset, uint pin) 
{   // Initializes PIO environment
    pio_sm_config c = clockgen_program_get_default_config(offset);
    // Map the state machine's OUT pin group to one pin, namely the `pin`
    // parameter to this function.
    sm_config_set_out_pins(&c, pin, 1); // pin = start pin# , 1 = how many pins from there
    // Set this pin's GPIO function (connect PIO to the pad)
    pio_gpio_init(pio, pin);
    // Set the pin direction to output at the PIO
    pio_sm_set_consecutive_pindirs(pio, sm, pin, 1, true);  // As before, plus "out"="true"
    // Set PIO clock. Here we assume cpu clk = 133 MHz and we want to generate a 67 KHz square wave
    // so we need 133 KHz clock for PIO SM
    pio_sm_set_clkdiv(pio, sm, SM_CLOCK_DIVISOR);
    // Load our configuration, and jump to the start of the program
    pio_sm_init(pio, sm, offset, &c);

    pio_sm_set_enabled (pio, sm , true); // Starts the SM
}

My .ino main program:

#include "squarewave_pio.h" // Assembled PIO program with wrapper function and configuration. Program name = "clockgen"

 
#define WAVE_OUTPUT_PIN 15         // lower-leftmost pin according to chart 
#define ONBOARD_LED 25
#define WAVE_START_AFTER_MS 10000UL
#define LED_BLINK_MS 1000UL
static const PIO pio = pio0; // pio0 or pio1


void setup() 
{
  uint32_t prgmOffset = 0;
  const uint32_t outputBasePin = WAVE_OUTPUT_PIN;
  uint32_t stateMachine;

  pinMode(ONBOARD_LED, OUTPUT);
  
  for (uint8_t i = 0; i < WAVE_START_AFTER_MS / (2*LED_BLINK_MS); i++)
  {
    digitalWrite(ONBOARD_LED, HIGH);
    delay(LED_BLINK_MS);
    digitalWrite(ONBOARD_LED, LOW);
    delay(LED_BLINK_MS);
  }
  
  // From SDK
  stateMachine = pio_claim_unused_sm(pio, true);
  prgmOffset = pio_add_program (pio, &clockgen_program);

  // Defined in included PIO program header "squarewave_pio.h"
  clockgen_program_init (pio, stateMachine, prgmOffset, outputBasePin);
}


void loop() 
{

  // Nothing to do

}

Unfortunately, I get nothing at pin 15.
I tried 2 different scopes (with ample bandwidth: we are generating a 67 KHz square wave after all) and a logic analyzer; pin 15 remains silent at 0V.

Am I missing something obvious?
The program is quite simple after all... :(

Thanks for looking!

Answered by XFer012

Sep 7, 2022

Ok, solved.

First of all, working example for future reference.

squarewave1.pio

; square wave

.program squarewave1

set pindirs, 1


.wrap_target
	set pins, 1
	set pins, 0
.wrap	

% c-sdk {
static inline void squarewave1_program_init(PIO pio, uint sm, uint offset, uint pin) 
{
    pio_sm_config c = squarewave1_program_get_default_config(offset);

    // Map the state machine's SET pin group to one pin, namely the `pin`
    // parameter to this function.
    sm_config_set_set_pins(&c, pin, 1);
	// Set the SM clock, by dividing the cpu clock
    sm_config_set_clkdiv(&c, SM_CLOCK_DIVISOR);
    // Set this pin's GPIO function (connect PIO to the pad)
    pio_gpio_init(pio, pin);
    // Set t…

View full answer

earlephilhower · 2022-09-07T17:25:15Z

earlephilhower
Sep 7, 2022
Maintainer

I'm going from memory, but I think you're setting a register value, not the actual outputs with set pins

You want something like

nop side_set 0
nop side_set 1

as your PIO program. side_set XXXX is for setting pin values not from the shift registers.

Check the datasheet and SDK docs, they have good details on this.

Also, FWIW, the Tone generator here should be perfectly capable of generating this frequency accurate to 1/133MHz....

1 reply

XFer012 Sep 7, 2022
Author

Thank you for chiming in.

It seems
set pins 1
should work, if SET pins are properly configured, as per datasheet.

This tutorial (for C/C++ SDK, but PIO ASM is the same) suggests set pins, 1 followed by set pins, 0 should actually produce a square wave https://medium.com/geekculture/raspberry-pico-programming-with-pio-state-machines-e4610e6b0f29

I also tried the ASM code with the PIO Emulator https://rp2040pio-docs.readthedocs.io/en/latest/introduction.html and it actually produces a nice square wave:

In the original program I did a mistake, using sm_config_set_out_pins(&c, pin, 1) instead of sm_config_set_set_pins(&c, pin, 1), but correcting it did not change anything.

And yes, the Tone Generator would work for this frequency, but I aim for a 20 MHz square wave (as a clock for an external device).
I cannot even use PWM as it outputs to 2 pins and I have only 1 pin left for this output signal.

XFer012 · 2022-09-07T21:51:10Z

XFer012
Sep 7, 2022
Author

Ok, solved.

First of all, working example for future reference.

squarewave1.pio

; square wave

.program squarewave1

set pindirs, 1


.wrap_target
	set pins, 1
	set pins, 0
.wrap	

% c-sdk {
static inline void squarewave1_program_init(PIO pio, uint sm, uint offset, uint pin) 
{
    pio_sm_config c = squarewave1_program_get_default_config(offset);

    // Map the state machine's SET pin group to one pin, namely the `pin`
    // parameter to this function.
    sm_config_set_set_pins(&c, pin, 1);
	// Set the SM clock, by dividing the cpu clock
    sm_config_set_clkdiv(&c, SM_CLOCK_DIVISOR);
    // Set this pin's GPIO function (connect PIO to the pad)
    pio_gpio_init(pio, pin);
    // Set the pin direction to output at the PIO
    pio_sm_set_consecutive_pindirs(pio, sm, pin, 1, true);
    // Load our configuration, and jump to the start of the program
    pio_sm_init(pio, sm, offset, &c);
    // Set the state machine running
    pio_sm_set_enabled(pio, sm, true);
}
%}

Assembled header squarewave_pio.h:

#define SM_CLOCK_DIVISOR 10000.0f

// -------------------------------------------------- //
// This file is autogenerated by pioasm; do not edit! //
// -------------------------------------------------- //

#pragma once

#if !PICO_NO_HARDWARE
#include "hardware/pio.h"
#endif

// ----------- //
// squarewave1 //
// ----------- //

#define squarewave1_wrap_target 1
#define squarewave1_wrap 2

static const uint16_t squarewave1_program_instructions[] = {
    0xe081, //  0: set    pindirs, 1                 
            //     .wrap_target
    0xe001, //  1: set    pins, 1                    
    0xe000, //  2: set    pins, 0                    
            //     .wrap
};

#if !PICO_NO_HARDWARE
static const struct pio_program squarewave1_program = {
    .instructions = squarewave1_program_instructions,
    .length = 3,
    .origin = -1,
};

static inline pio_sm_config squarewave1_program_get_default_config(uint offset) {
    pio_sm_config c = pio_get_default_sm_config();
    sm_config_set_wrap(&c, offset + squarewave1_wrap_target, offset + squarewave1_wrap);
    return c;
}

static inline void squarewave1_program_init(PIO pio, uint sm, uint offset, uint pin) 
{
    pio_sm_config c = squarewave1_program_get_default_config(offset);
    // Map the state machine's SET pin group to one pin, namely the `pin`
    // parameter to this function.
    sm_config_set_set_pins(&c, pin, 1);
	// Set the SM clock, by dividing the cpu clock
    sm_config_set_clkdiv(&c, SM_CLOCK_DIVISOR);
    // Set this pin's GPIO function (connect PIO to the pad)
    pio_gpio_init(pio, pin);
    // Set the pin direction to output at the PIO
    pio_sm_set_consecutive_pindirs(pio, sm, pin, 1, true);
    // Load our configuration, and jump to the start of the program
    pio_sm_init(pio, sm, offset, &c);
    // Set the state machine running
    pio_sm_set_enabled(pio, sm, true);
}

#endif

Main .ino file:

// First try at a square wave (clock) generated with PIO

#include "squarewave_pio.h" // Assembled PIO program with wrapper function and configuration. Program name = "squarewave1"

 
#define WAVE_OUTPUT_PIN 15         // lower-leftmost pin according to chart 
#define ONBOARD_LED 25
#define WAVE_START_AFTER_MS 10000UL
#define LED_BLINK_MS 1000UL
static const PIO pio = pio0; // pio0 or pio1


void setup() 
{
  uint32_t prgmOffset = 0;
  const uint32_t outputBasePin = WAVE_OUTPUT_PIN;
  uint32_t stateMachine;

  pinMode(ONBOARD_LED, OUTPUT);
  
  for (uint8_t i = 0; i < WAVE_START_AFTER_MS / (2*LED_BLINK_MS); i++)
  {
    digitalWrite(ONBOARD_LED, HIGH);
    delay(LED_BLINK_MS);
    digitalWrite(ONBOARD_LED, LOW);
    delay(LED_BLINK_MS);
  }
  
  // From SDK
  stateMachine = pio_claim_unused_sm(pio, true);
  prgmOffset = pio_add_program (pio, &squarewave1_program);

  // Defined in included PIO program header "squarewave_pio.h"
  squarewave1_program_init (pio, stateMachine, prgmOffset, outputBasePin);
}


void loop() 
{

  // Nothing to do

}

Which mistakes I made:

using sm_config_set_out_pins(&c, pin, 1) , wrong, instead of sm_config_set_set_pins(&c, pin, 1) , which is right
using pio_sm_set_clkdiv(pio, sm, SM_CLOCK_DIVISOR) , wrong, instead of sm_config_set_clkdiv(&c, SM_CLOCK_DIVISOR)

Now everything works!

Thanks @earlephilhower , great work on arduino-pico and very helpful in general.

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