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  • Author Author: Jan Cumps
  • Date Created: Date Created
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Stepper Motor Control with Raspberry Pico PI and DRV8711 driver- Part 2: SPI

Jan Cumps

The Pico has a set of PIO co-processors. They are real-time controllers that can execute logic with deterministic timing. Ideal to run strict-timed sequences and state machines. And to implement extra peripherals.
In this series, I'm trying to use the PIO to control a TI DRV8711 stepper motor controller.

image

Follow up of  Stepper Motor Control with Raspberry Pico PI and DRV8711 driver- Part 1: Hardware Provisioning 

SPI requirements and setup

The TI DRV8711 stepper motor driver, that we use in this project, relies on pre-configuration via SPI.

These are the Pico resources I reserve:

signal DRV8711 Pico
SPI CLK SCLK default SCLK
SPI CS SCS default CS*
SPI MISO sdato default MISO**
SPI MOSI sdati default MOSI


* DRV8711 uses a chip select that's active high. Pico's SPI library doesn't support that, so I will bit bang that pin.
** DRV8711 uses open collector for outputs. Pico will have to provide the rail via a pull-up.

The setup code isn't complex. You 'll recognise the special cases mentioned below:

void init_spi_hw() {
    // Enable SPI 0 at 1 MHz and connect to GPIOs
    spi_init(spi_default, 1000 * 1000);
    spi_set_format(spi_default, 16, SPI_CPOL_0, SPI_CPHA_0, SPI_MSB_FIRST); // 16 bit registers
    gpio_set_function(PICO_DEFAULT_SPI_RX_PIN, GPIO_FUNC_SPI);
    gpio_set_pulls(PICO_DEFAULT_SPI_RX_PIN, true, false); // drv8711 outputs are open drain
    gpio_set_function(PICO_DEFAULT_SPI_SCK_PIN, GPIO_FUNC_SPI);
    gpio_set_function(PICO_DEFAULT_SPI_TX_PIN, GPIO_FUNC_SPI);
    // Chip select is active-high, so we'll initialise it to a driven-low state
    gpio_init(PICO_DEFAULT_SPI_CSN_PIN);
    gpio_put(PICO_DEFAULT_SPI_CSN_PIN, 0);
    gpio_set_dir(PICO_DEFAULT_SPI_CSN_PIN, GPIO_OUT);
}

I provided helpers for CS and 16 bit write (the width of the DRV8711 registers):

static inline void cs_drive(bool high) {
    asm volatile("nop \n nop \n nop"); // FIXME
    gpio_put(PICO_DEFAULT_SPI_CSN_PIN, high? 1 : 0);
    asm volatile("nop \n nop \n nop"); // FIXME
}

static void spi_write(const uint16_t &data) {
    cs_drive(true); // drv8711 has CS active high
    spi_write16_blocking(spi_default, &data, 1);
    cs_drive(false);
}

Here's an example of writing to a register (the structure used in the example uses Texas Instruments' registers.h file that comes with their DRV8711 examples).:

    uint16_t data;

    // CTRL Register
    G_CTRL_REG.Address     = 0x0000;
    G_CTRL_REG.DTIME     = 0x0003;
    G_CTRL_REG.ISGAIN     = 0x0003;
    G_CTRL_REG.EXSTALL     = 0x0000;
    // G_CTRL_REG.MODE     = 0x0003;  // in 8th steps
    G_CTRL_REG.MODE     = 0x0000; // in full steps
    G_CTRL_REG.RSTEP     = 0x0000;
    G_CTRL_REG.RDIR     = 0x0000;
    G_CTRL_REG.ENBL     = 0x0001;
    // Write CTRL Register
    data = REGWRITE | (G_CTRL_REG.Address << 12) | (G_CTRL_REG.DTIME << 10) | (G_CTRL_REG.ISGAIN << 8) |(G_CTRL_REG.EXSTALL << 7) | (G_CTRL_REG.MODE << 3) | (G_CTRL_REG.RSTEP << 2) | (G_CTRL_REG.RDIR << 1) | (G_CTRL_REG.ENBL);
    spi_write(data);

The full code will be attached when I'm a bit further into this project. Next post will focus on provisioning the non-SPI pins.

Meanwhile: here's the full conversation from Pico to DRV8711:

image