Use a single ordinary timer of STM32 to generate 4 square waves of different frequencies-EEWORLD

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The ordinary timer of STM32 has four outputs: TIMx_CH1, TIMx_CH2, TIMx_CH3 and TIMx_CH4. The output comparison method can be used to generate square wave outputs of different frequencies. The simple method is:

1) Set the counter to up-counting mode and set the auto-reload register to 0xFFFF; the counter will then count in a loop.

2) Set each timer channel to output compare mode, and set the corresponding output pin to flip output when compare matches.

3) Calculate a value called Half_Cyc according to the half-wave period of the output waveform. For example: if the clock frequency of the timer is 72MHz and a 3456Hz square wave needs to be generated, then Half_Cyc = 72M/(3456*2) = 41667; if a 200kHz square wave needs to be generated, then Half_Cyc = 72M/(200k*2) = 180.

4) Set each channel to generate an interrupt when the output compare matches. In the interrupt, read the value of the compare register and add the value of Half_Cyc. If the calculated value exceeds 16 bits, discard the excess part and write the new value back to the corresponding compare register. In this way, the next successful comparison will occur exactly after a half-wave cycle, and the corresponding pin will be flipped.

The above method is very effective when the required frequency is not high, but if the frequency is high, frequent interrupts will occur. In this case, DMA can be used to improve it.

The basis of the above method is to generate pin flip output by constantly changing the match point of the output comparison. We can calculate these comparison match points in advance and update the contents of the comparison register one by one through DMA at each match:

Method 1: Use two DMA buffers. When the DMA controller operates one buffer, the program calculates the data of the other buffer, and then switches the buffer for DMA operation in the interrupt processing when the DMA transfer ends.

Method 2: Use a large DMA buffer, calculate the contents of half of the buffer first, start DMA in loop mode and set it to generate interrupts when the DMA transfer is halfway and completed; after starting DMA, continue to calculate the contents of the other half of the buffer. When a DMA interrupt occurs, it means that half of the buffer has become empty. At this time, calculate this half of the buffer in the interrupt processing.

As long as the DMA buffer is large enough, method 2 can ensure that the CPU has sufficient time to process data and ensure continuous waveform output.

Disclaimer: The above explanation has not been verified in practice, it is just a principle explanation, and there may be some things that are not fully considered.

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