Design (principle and procedure) of FSM modem based on stm32

Approximate requirements: Design an FSK modem with a baseband signal rate of 2000 B/s, carrier frequencies of 4 kHz and 8 kHz, and the ability to fully restore the baseband signal after demodulation. There are multiple approaches to implement this. Most modems in the communication field use hardware circuits. However, given my preference for programming (though I still have limited knowledge in circuit design), I decided to use STM32 for a purely software-based implementation. It may sound complex at first, but it's actually manageable once you break it downâ€”though there are several important details to consider.

The overall design concept is as follows:

First, we need to generate the baseband signal, which is the data we want to modulate and demodulate. The baseband signal consists of a sequence of random symbols. To simulate this, I used a timer to create an 8-bit PN code sequence with a symbol rate of 2000 B/s. Timer 3 is configured to trigger an interrupt every 0.5 ms. Each time the interrupt occurs, the variable 'num' is incremented by one, and the IO pin level is updated accordingly. The 8-bit PN code only needs to be set eight times before resetting 'num' to zero.

TIM3_Init(499, 71); // Baseband signal

U8 num = 0;

Void TIM3_IRQHandler(void)

{

If (TIM_GetITStatus(TIM3, TIM_IT_Update) != RESET)

{

Num++;

Switch (num)

{

Case 1: Base_Signal = 1; break;

Case 2: Base_Signal = 0; break;

Case 3: Base_Signal = 0; break;

Case 4: Base_Signal = 0; break;

Case 5: Base_Signal = 1; break;

Case 6: Base_Signal = 0; break;

Case 7: Base_Signal = 1; break;

Case 8: Base_Signal = 0; break; // PN code sequence

}

If(num == 8)

Num = 0;

TIM_ClearITPendingBit(TIM3, TIM_IT_Update);

}

Next, we generate the carrier signal, which is a sine wave. The required carrier frequencies are 4 kHz and 8 kHz. To generate the sine wave, I used the STM32â€™s DMA, DAC, and TIM2. A sine wave with 64 sample points and 12-bit precision was created using a sine wave generator and stored in the Sine12bit[] array. However, the data sent to the DMA isnâ€™t the raw dataâ€”it undergoes some processing before being transmitted.

Uint16_t Sine12bit[64] = {

0x7FF, 0x8C8, 0x98E, 0xA51, 0xB0F, 0xBC4, 0xC71, 0xD12, 0xDA7, 0xE2E, 0xEA5, 0xF0D, 0xF63, 0xFA6, 0xFD7, 0xFF5,

0xFFE, 0xFF5, 0xFD7, 0xFA6, 0xF63, 0xF0D, 0xEA5, 0xE2E, 0xDA7, 0xD12, 0xC71, 0xBC4, 0xB0F, 0xA51, 0x98E, 0x8C8,

0x7FF, 0x736, 0x670, 0x5AD, 0x4EF, 0x43A, 0x38D, 0x2EC, 0x257, 0x1D0, 0x159, 0x0F1, 0x09B, 0x058, 0x027, 0x009,

0x000, 0x009, 0x027, 0x058, 0x09B, 0x0F1, 0x159, 0x1D0, 0x257, 0x2EC, 0x38D, 0x43A, 0x4EF, 0x5AD, 0x670, 0x736

};

Uint32_t Idx = 0;

Int main(void)

{

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