Has anyone ever used a (queued) SPI port as a PWM/DAC?

[dciliske]

A thought occurred to me this afternoon: Given the right data to send, it should be possible to use an SPI module for PWM output, and using a low pass filter a crude DAC. Has anyone here ever done something as foolhardy as this?

Also, it seems to me that the QSPI on the V2 core Coldfires is perfect for testing this theory, as with the right commands should loop forever..

I think I have a project for this weekend.

[dciliske]

Well, that was easy... It's not quite 100% accurate as a PWM output, but it's fairly close.

Code: Select all

#include <predef.h>
#include <constants.h>
#include <ctype.h>
#include <ucos.h>
#include <system.h>
#include <init.h>
#include <sim.h>
#include <pins.h>
#include <pinconstant.h>

#define PWM_DAT_LEN 16

uint32_t PWM_Dat[PWM_DAT_LEN] __attribute__(( aligned(32) )); // 256 bit data stream, aka 8-bit resolution


const char AppName[] = "SPI_PWM";

extern "C" {
    void UserMain( void * pd );
}

void SetupDSPI()
{
    sim1.dspi3.mcr = 0x80000C01; // clear fifos, halt transfers
    sim1.dspi3.ctar[0] = 0x78000000;
    sim1.dspi3.sr = 0x9A0A0000; // clear flags
    sim1.dspi3.rser = 0x03000000; // enable TFF_RE and dma requests

    // DSPI3 pins are UART1, UART1 slewrate is bits [3:2]
    sim1.gpio.srcr_uart     &= ~0x0C;
    sim1.gpio.srcr_uart     |= 0x08;
}

void SetupDMA()
{
    volatile edma_tcdstruct &tcd = sim2.edma.tcd[45];

    sim2.edma.cr &= ~0x000300FF;

    iprintf("saddr: %p\n", PWM_Dat);
    tcd.attr = 0x0202;
    tcd.saddr = (uint32_t)PWM_Dat;
    tcd.soff = 0x0004;
    tcd.nbytes = 0x0004;

    tcd.daddr = ((uint32_t)&sim1.dspi3.pushr);
    tcd.doff = 0x0000;
    tcd.slast = -(PWM_DAT_LEN * 4); // reset to start PWM_Dat buffer
    tcd.dlast_sga = 0x00000000;
    tcd.citer = PWM_DAT_LEN;
    tcd.biter = PWM_DAT_LEN;
    tcd.csr = 0x0000;
}

uint16_t LeadBitLookup[16] = {
    0x0000, 0x8000, 0xC000, 0xE000,
    0xF000, 0xF800, 0xFC00, 0xFE00,
    0xFF00, 0xFF80, 0xFFC0, 0xFFE0,
    0xFFF0, 0xFFF8, 0xFFFC, 0xFFFE
};

void ReadyPWMDat(uint8_t val)
{
    uint8_t i = 0;
    while (val > 16) {
        PWM_Dat[i] = 0xFFFF;
        i++;
        val -= 16;
    }
    PWM_Dat[i] = LeadBitLookup[val];
    i++;
    for (; i < PWM_DAT_LEN; i++) {
        PWM_Dat[i] = 0x0000;
    }
    for (i = 0; i < PWM_DAT_LEN; i++) {
        PWM_Dat[i] |= 1 << 31;
    }
}

void StartPWM(uint8_t val)
{
    sim1.dspi3.mcr = 0x80000C01; // clear fifos, halt transfers
    sim1.dspi3.sr = 0x9A0A0000; // clear flags
    ReadyPWMDat(val);
    sim2.edma.serq = 45;
    sim1.dspi3.rser = 0x03000000; // enable TFF_RE and dma requests
    sim1.dspi3.mcr = 0xC0000000; // start transfers
}

void StopPWM()
{
    sim1.dspi3.mcr = 0x80000C01; // clear fifos, halt transfers
}

void UserMain( void * pd )
{
    init();

    SetupDSPI();
    SetupDMA();
    J2[22].function(PINJ2_22_DSPI3_SOUT);
    J2[24].function(PINJ2_24_DSPI3_SCK);

    iprintf("Application: %s\r\nNNDK Revision: %s\r\n",AppName,GetReleaseTag());

    StartPWM( 0x2F );
    while(1) {
        iprintf("PWM_DAT:\n  [\n    ");
        for (int i = 0; i < PWM_DAT_LEN; i++) {
            if (i) { putchar(',');
                if ((i%4) == 0) { iprintf("\n    "); }
            }
            iprintf(" 0x%08lX", PWM_Dat[i]);
        }
        iprintf("\n  ]\n\n");
        OSTimeDly(TICKS_PER_SECOND);

    }
}

SPI_PWM.png (61.78 KiB) Viewed 5987 times