Frequency Adjusting with STM32 DAC - stm32

Question

I used STM32F407VG to create a 30 khz sine wave. Timer settings are; Prescaler = 2-1, ARR = 1, also the clock is 84 Mhz(the clock which runs DAC).
I wrote a function called generate_sin();
#define SINE_ARY_SIZE (360)
const int MAX_SINE_DEGERI = 4095; // max_sine_value
const double BASLANGIC_NOKTASI = 2047.5; //starting point
uint32_t sine_ary[SINE_ARY_SIZE];
void generate_sine(){
for (int i = 0; i < SINE_ARY_SIZE; i++){
double deger = (sin(i*M_PI*360/180/SINE_ARY_SIZE) * BASLANGIC_NOKTASI) + BASLANGIC_NOKTASI; //double value
sine_ary[i] = (uint32_t)deger; // value
}
This is the function which creates sine wave. I used HAL DMA to send DAC output variables.
HAL_TIM_Base_Start(&htim2);
generate_sine();
HAL_DAC_Start_DMA(&hdac, DAC_CHANNEL_1, sine_ary, SINE_ARY_SIZE, DAC_ALIGN_12B_R);
These are the codes i used to do what i want. But im having a trouble to change frequency without changing prescaler or ARR.
So here is my question. Can i change frequency without changing timer settings ? For example i want to use buttons and whenever i push button i want my frequency to change.

Answer 1

The generate_sine function will give you one period of a sine wave which has SINE_ARY_SIZE of samples.
To increase the frequency you need to make the period shorter (for 2x frequency, you would have half the number of samples per period). So you should calculate the array for smaller SINE_ARY_SIZE (which will fill just part of the original buffer with a shorter sine wave) and also put this smaller value in the HAL_DAC_Start_DMA function.
Decreasing the frequency will require making the array longer.
You should declare the sine_ary with a maximum length that you will need (for lowest frequency). Make sure it fits in RAM.
#define MAXIMUM_ARRAY_LENGTH 360
uint32_t usedArrayLength = 180;
const double amplitude = 2047.5;
uint32_t sine_ary[MAXIMUM_ARRAY_LENGTH];
void generate_sine(){
for (int i = 0; i < usedArrayLength; i++){
double value = (sin(i*M_PI*2/usedArrayLength) * amplitude) + amplitude;
sine_ary[i] = (uint32_t)value; // value
}
This will have two times higher frequency than the original code, because it only has 180 samples per period, compared to 360.
Start it using
HAL_DAC_Start_DMA(&hdac, DAC_CHANNEL_1, sine_ary, usedArrayLength, DAC_ALIGN_12B_R);
To change the frequency, stop DAC, change the value of usedArrayLength (smaller value means higher frequency, must be less or equal to MAXIMUM_ARRAY_LENGTH). Then call the generate_sine function and start the DAC again by the same function (that now uses new usedArrayLength).
Frequency will be: Clock/prescaler/ARR/usedArrayLength
Also, you should use uint16_t for the array (values are from 0 to 4095, the DAC is 12bit I suppose) and DMA should be set to Half-word (2 bytes per value).