ESP32的I2S同步时钟BCK最高输出时钟达不到手册标的40MHZ怎么解决？

在idf的例程中复制项目i2s_basic,我尝试修改采样率来提高i2s的频率,当我设置采样率到150000,使用逻辑分析仪测量bck_io的频率为9.7MHz,我接着往大的设置,尝试了250000,350000....625000。发现bck_io的频率最高只能到15MHZ，设置高了也没有效果。
/* I2S Example    This example code will output 100Hz sine wave and triangle wave to 2-channel of I2S driver    Every 5 seconds, it will change bits_per_sample [16, 24, 32] for i2s data    This example code is in the Public Domain (or CC0 licensed, at your option.)    Unless required by applicable law or agreed to in writing, this    software is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR    CONDITIONS OF ANY KIND, either express or implied.*/#include #include "freertos/FreeRTOS.h"#include "freertos/task.h"#include "driver/i2s.h"#include "driver/gpio.h"#include "esp_system.h"#include "esp_log.h"#include #define SAMPLE_RATE     (150000)#define I2S_NUM         (0)#define WAVE_FREQ_HZ    (100)#define PI              (3.14159265)#define I2S_BCK_IO      (GPIO_NUM_4)#define I2S_WS_IO       (GPIO_NUM_5)#define I2S_DO_IO       (-1)#define I2S_DI_IO       (GPIO_NUM_18)#define SAMPLE_PER_CYCLE (SAMPLE_RATE/WAVE_FREQ_HZ)static const char* TAG = "i2s_example";int16_t i2s_readraw_buff[400];static void setup_triangle_sine_waves(int bits){    int *samples_data = malloc(((bits+8)/16)*SAMPLE_PER_CYCLE*4);    unsigned int i, sample_val;    double sin_float, triangle_float, triangle_step = (double) pow(2, bits) / SAMPLE_PER_CYCLE;    size_t i2s_bytes_write = 0;    printf("rnTest bits=%d free mem=%d, written data=%dn", bits, esp_get_free_heap_size(), ((bits+8)/16)*SAMPLE_PER_CYCLE*4);    triangle_float = -(pow(2, bits)/2 - 1);    for(i = 0; i < SAMPLE_PER_CYCLE; i++) {        sin_float = sin(i * 2 * PI / SAMPLE_PER_CYCLE);        if(sin_float >= 0)            triangle_float += triangle_step;        else            triangle_float -= triangle_step;        sin_float *= (pow(2, bits)/2 - 1);        if (bits == 16) {            sample_val = 0;            sample_val += (short)triangle_float;            sample_val = sample_val << 16;            sample_val += (short) sin_float;            samples_data[i] = sample_val;        } else if (bits == 24) { //1-bytes unused            samples_data[i*2] = ((int) triangle_float) << 8;            samples_data[i*2 + 1] = ((int) sin_float) << 8;        } else {            samples_data[i*2] = ((int) triangle_float);            samples_data[i*2 + 1] = ((int) sin_float);        }    }    ESP_LOGI(TAG, "set clock");    i2s_set_clk(I2S_NUM, SAMPLE_RATE, bits, 2);    //Using push    // for(i = 0; i < SAMPLE_PER_CYCLE; i++) {    //     if (bits == 16)    //         i2s_push_sample(0, &samples_data[i], 100);    //     else    //         i2s_push_sample(0, &samples_data[i*2], 100);    // }    // or write    ESP_LOGI(TAG, "write data");    i2s_write(I2S_NUM, samples_data, ((bits+8)/16)*SAMPLE_PER_CYCLE*4, &i2s_bytes_write, 100);    free(samples_data);}void app_main(void){    //for 36Khz sample rates, we create 100Hz sine wave, every cycle need 36000/100 = 360 samples (4-bytes or 8-bytes each sample)    //depend on bits_per_sample    //using 6 buffers, we need 60-samples per buffer    //if 2-channels, 16-bit each channel, total buffer is 360*4 = 1440 bytes    //if 2-channels, 24/32-bit each channel, total buffer is 360*8 = 2880 bytes    i2s_config_t i2s_config = {        .mode = I2S_MODE_MASTER | I2S_MODE_RX,        .sample_rate = SAMPLE_RATE,        .bits_per_sample = I2S_BITS_PER_SAMPLE_32BIT,        .channel_format = I2S_CHANNEL_FMT_ALL_LEFT,        .communication_format = I2S_COMM_FORMAT_STAND_MSB,        .dma_buf_count = 6,        .dma_buf_len = 60,        .use_apll = true,        .intr_alloc_flags = ESP_INTR_FLAG_LEVEL1                                //Interrupt level 1    };    i2s_pin_config_t pin_config = {        .bck_io_num = I2S_BCK_IO,        .ws_io_num = I2S_WS_IO,        .data_out_num = I2S_DO_IO,        .data_in_num = I2S_DI_IO                                               //Not used    };    i2s_driver_install(I2S_NUM, &i2s_config, 0, NULL);    i2s_set_pin(I2S_NUM, &pin_config);    // i2s_set_clk(I2S_NUM, SAMPLE_RATE, I2S_BITS_PER_SAMPLE_16BIT, 2);    size_t bytes_read;     while (1) {        // setup_triangle_sine_waves(test_bits);        i2s_read(I2S_NUM, i2s_readraw_buff, 200, &bytes_read, portMAX_DELAY);        printf("read i2s %d:",bytes_read);        for(int i = 0;i < 200; i++)        {            printf("%x ",i2s_readraw_buff[i]);        }        printf("n");        vTaskDelay(5000/portTICK_RATE_MS);    }}