rpi4-osdev/part9-sound/kernel.c

#include "fb.h"
#include "io.h"

#define PWM_BASE   (PERIPHERAL_BASE + 0x20C000 + 0x800) /* PWM1 register base address on RPi4 */
#define CLOCK_BASE (PERIPHERAL_BASE + 0x101000)

#define BCM2711_PWMCLK_CNTL 40
#define BCM2711_PWMCLK_DIV  41

#define BCM2711_PWM_CONTROL 0
#define BCM2711_PWM_STATUS  1
#define BCM2711_PWM0_RANGE  4
#define BCM2711_PWM0_DATA   5
#define BCM2711_PWM_FIFO    6
#define BCM2711_PWM1_RANGE  8
#define BCM2711_PWM1_DATA   9

#define BCM2711_PWM1_USEFIFO  0x2000  /* Data from FIFO */
#define BCM2711_PWM1_ENABLE   0x0100  /* Channel enable */

#define BCM2711_PWM0_USEFIFO  0x0020  /* Data from FIFO */
#define BCM2711_PWM0_ENABLE   0x0001  /* Channel enable */

#define BCM2711_GAPO2 0x20
#define BCM2711_GAPO1 0x10
#define BCM2711_RERR1 0x8
#define BCM2711_WERR1 0x4
#define BCM2711_FULL1 0x1

#define PM_PASSWORD 0x5A000000 

#define ERRORMASK (BCM2711_GAPO2 | BCM2711_GAPO1 | BCM2711_RERR1 | BCM2711_WERR1)

volatile unsigned* clk = (void*)CLOCK_BASE;
volatile unsigned* pwm = (void*)PWM_BASE;

static void audio_init(void)
{
    gpio_useAsAlt0(40); // Ensure PWM1 is mapped to GPIO 40/41
    gpio_useAsAlt0(41);

    wait_msec(2);

    *(clk + BCM2711_PWMCLK_CNTL) = PM_PASSWORD | (1 << 5); // Stop clock

    int idiv = 2;
    *(clk + BCM2711_PWMCLK_DIV)  = PM_PASSWORD | (idiv<<12);
    *(clk + BCM2711_PWMCLK_CNTL) = PM_PASSWORD | 16 | 1; // Enable + oscillator

    wait_msec(2);

    // Disable PWM
    *(pwm + BCM2711_PWM_CONTROL) = 0;
    
    wait_msec(2);

    *(pwm+BCM2711_PWM0_RANGE) = 0x264; // 44.1khz, Stereo, 8-bit (54Mhz / 44100 / 2)
    *(pwm+BCM2711_PWM1_RANGE) = 0x264;

    *(pwm+BCM2711_PWM_CONTROL) =
          BCM2711_PWM1_USEFIFO | 
          BCM2711_PWM1_ENABLE | 
          BCM2711_PWM0_USEFIFO | 
          BCM2711_PWM0_ENABLE | 1<<6;

    wait_msec(2);
}

void playaudio_cpu()
{
    int i=0;
    long status;

    extern unsigned char _binary_audio_bin_start[];
    extern unsigned char _binary_audio_bin_size[];
    
    unsigned int size = (long)&_binary_audio_bin_size;
    unsigned char *data = &(_binary_audio_bin_start[0]);

    while (i < size) {
        status = *(pwm + BCM2711_PWM_STATUS);
        if (!(status & BCM2711_FULL1)) {
            *(pwm+BCM2711_PWM_FIFO) = *(data + i);
            i++;
            *(pwm+BCM2711_PWM_FIFO) = *(data + i);
            i++;
        }
        if ((status & ERRORMASK)) {
            debugstr("error: ");
            debughex(status);
            debugcrlf();
            *(pwm+BCM2711_PWM_STATUS) = ERRORMASK;
        }
    }
}

void main()
{
    fb_init();

    debugstr("Initialising audio... ");
    audio_init();
    debugstr("done"); debugcrlf();

    debugstr("Starting to play sample using CPU"); debugcrlf();
    playaudio_cpu();
    debugstr("CPU finished playing sample"); debugcrlf();

    while (1);
}