Generate Noise Using LFSR (Linear Feedback Shift Register)

[Kneepatch]

Hello,

I am trying to generate white noise using the DAC, much like was mentioned in this forum post:

How to generate a white noise with the DAC on STM32L1

However, I don't know if putting values into memory table and using cyclic mode is the only answer. From my research using Audio and waveform generation using the DAC in STM32 microcontrollers - Application note, it looks like you should be able to use the LFSR (Linear Feedback Shift Register) / Random Number Generator in Figure 5 to make white noise as is shown in Figure 6 on page 9:

 

Here is how I have my STMCubeMX set:

And here is my "Software Trigger":

HAL_DAC_Start(&hdac, DAC1_CHANNEL_1);

HAL_DAC_Start(&hdac, DAC1_CHANNEL_2);

Yet, when I look at the oscilloscope, all I get is a DC offset just like my friend in the other forum post observed:

 

Any thoughts on this? Many thanks in advance!

[waclawek.jan · Accepted solution]

Which STM32 are you using?

As the figure in Noise generation subchapter of DAC chapter in RM illustrates, there is one output value generated per one pulse on SWTRIG signal/bit. Thus, you have to set the SWTRIG periodically. The Cube/HAL function probably sets it once, I don't use Cube, you can look it up, it's open source.

While you can try to set SWTRIG periodically, e.g. in a loop, for trial, it's probably more reasonable to set the DAC being triggered from a timer.

JW

[Kneepatch]

Hey JW,

Thanks for the prompt reply, that was it! I had to trigger the SWTRIG repeatedly and it worked. I just ended up putting the HAL_DAC_Start(); function (which contains the SWTRIG) in an infinite loop and it worked like a charm.

I am using the STM32F469 Discovery board right now with the STM32CubeIDE.