STM32（使用Mbed在线）在较高的模拟输入频率下显示延迟

Question

I am new to the use of controllers. I am setting up a STM32F769 Controller(Using Mbed online compiler), my target is to get a PWM output which changes its frequency according to an analog input. I did some basic coding but there is a problem. When i check the output on oscilloscope with analog input 1Hz frequency, its working perfectly, but when i check it with 100Hz analog input there is delay in the output, and i get wrong values. I do not understand why, because this board is faster(216 MHZ) and i should not face such issue. (If someone could also explain, is it possible to use the board at 216MHz or other max frequency? and how?)

1st time user

{
 meas_r=0;
 for(int i=1;i<=1024;i++)
 {             
    meas_r = meas_r+analog_value.read(); 
 }
 meas_r=meas_r/1024;
 meas_v = meas_r * 3300;
 out_freq=50000+(meas_v*50);   
 pulse.period( 1.0 / out_freq);  
 }
}

It should be working on 100Hz analog input as it works on 1 Hz.

Answer 1

216MHz might be the maximum clock frequency at which your processor can operate, however it does not mean that it can input/output that much frequency from its ports.

• The delays are caused by time it takes to read analog values and compute required math operations. You are using multiple multiplications and divisions which are more complex than adding and subtracting for almost any hardware device. Obviously, you are using library/libraries as well (pulse.period(), analog_value.read()), there are some hidden computations on top of those multiplications and divisions. Finally, it is possible that your device is working with other stuffs as well (only you know about this). All those computations need time. At lower frequencies you might not be able to notice the delay, however when the frequency is high enough, the delays can be noticed. Also consider time required to read the analog values many times .
• Wrong signal and period is due to the delays and some other uncertainties. If the processor is working with other tasks as well, then it will be hard to predict time it takes to finish them all. As the processor executes the instructions line by line and waits for previous computation to finish before the new one starts, it causes some uncertainty in timing. Data path and frequency of peripheral devices (getting input from peripherals) play crucial role in timing uncertainty and delays.

If timing and accuracy is really important in solving the problem you have, and if you can't solve the problem with DSP, MPU, MCU, CPU, GPU, etc. I would suggest you to use an FPGA to solve the problem.