stm32f411 sample frequency

[LTut.226]

hi, im experimenting with a guitar string frequency detector,

on Arduino uno I get the correct values in serial monitor,

but the values are approx halved when I use stm32f411,

anyone know why, could it be the sample frequency

 
#define LENGTH 512
 
byte rawData[LENGTH];
int count;
 
// Sample Frequency in kHz
const float sample_freq = 8919;
 
int len = sizeof(rawData);
int i,k;
long sum, sum_old;
int thresh = 0;
float freq_per = 0;
byte pd_state = 0;
 
void setup(){
 //analogReference(EXTERNAL); // Connect to 3.3V
 analogRead(A0);
 Serial.begin(115200);
 count = 0;
}
 
 
void loop(){
 
 if (count < LENGTH) {
 count++;
 rawData[count] = analogRead(A0)>>2;
 }
 else {
 sum = 0;
 pd_state = 0;
 int period = 0;
 for(i=0; i < len; i++)
 {
 // Autocorrelation
 sum_old = sum;
 sum = 0;
 for(k=0; k < len-i; k++) sum += (rawData[k]-128)*(rawData[k+i]-128)/256;
 // Serial.println(sum);
 
 // Peak Detect State Machine
 if (pd_state == 2 && (sum-sum_old) <=0) 
 {
 period = i;
 pd_state = 3;
 }
 if (pd_state == 1 && (sum > thresh) && (sum-sum_old) > 0) pd_state = 2;
 if (!i) {
 thresh = sum * 0.5;
 pd_state = 1;
 }
 }
 // for(i=0; i < len; i++) Serial.println(rawData[i]);
 // Frequency identified in Hz
 if (thresh >100) {
 freq_per = sample_freq/period;
 Serial.println(freq_per);
 }
 count = 0;
 }
}

[TDK]

You mean the frequency is halved? So it gives you 50 Hz instead of 100Hz (for example)?

I'd check your clock settings. Toggle a GPIO pin at every sample and look on a scope to see if it matches your expectations.

You don't show how your calls to loop() are done. Maybe you're calling it half/twice as often as you think you are. Most accurate way to do this would be to trigger the ADC from a timer and transfer values via DMA.

> // Sample Frequency in kHz

> const float sample_freq = 8919;

This is in Hz, not kHz right? Doubt you can sample at 8.9 Msps.

[LTut.226]

sorry I don’t know what a timer or dma or sample frequency is, and I don’t have a scope, also didn’t write the code, the output value in serial monitor is approx halved, so a low E string using uno is correct at 82.41 hz but it’s 40 hz if I use a stm32f411 blackpill board, any ideas?