Add 433 MHz transmitter example

[sodoku]

I was playing around with 433Mhz and thought this might help someone. Feel free to reject if this is too basic.

[unicab369]

can you add example how to receive the data?

[sodoku]

can you add example how to receive the data?

This example works with a simple transmit only board. I haven't used a receiver yet, but I can see if I can also make it work. Would you split this in two examples or put both in one?

[unicab369]

if the code is too big, I would split it into 2 if not one is ok i think.

[cnlohr]

Is it possible to do both send and receive from one part? If so, that would be best.

And I am glad you kept things simple, but I do think the minimum viable product would be both send and receive.

[cnlohr]

So definitely not a no, @sodoku but, maybe a "when you get TX/RX going.

Other than that the PR is good.

[sodoku]

@cnlohr I managed to solder my receiver antenna and play around a bit with it. Reading how rc-switch does the receive and looking through the ch32fun examples, it looks like I will need to use interrupts for the input data change detection and for being able to measure time difference (micros() in Arduino). Imho that would make this example more complex.
What do you think?

[cnlohr]

@sodoku on ch32fun, you use SysTick->CNT, and for comparing times you do something like:

uint32_t start_time = SysTick->CNT;
while( 1 )
{
   // Do stuff
   if( TimeElapsed32( SysTick->CNT, start_time ) >= Ticks_from_Ms(150) )
   {
      // 150ms elapsed.
      start_time += Ticks_from_Ms(150);
   }
}

[sodoku]

I spent hours on this and learned a lot, but somehow the timing values didn't make sense. I am not sure what I am doing wrong. I boiled my problem down to a very simple example:

int miliseconds = 15000;

uint32_t start_time = SysTick->CNT;

while ( 1 )
{
  // Do stuff
  uint32_t diff = TimeElapsed32( SysTick->CNT, start_time );
  if ( diff >= Ticks_from_Us( miliseconds ) )
  {
    start_time += Ticks_from_Us( miliseconds );
    printf( "%lu \n", diff / DELAY_US_TIME );
  }
}

This works an outputs:

+15000
+15000
+15000
+15000
+15000
+15000
+15000
+15000

However if I change miliseconds to a smaller value of 1500 since I need to count the timings in small values of microseconds (e.g 650), I get:

+1500
+1575
+1722
+1811
+1974
+2188
+2366
+2428
+2456
+2492
+2498
+2523
+2552
+2668
+2819
+3032
+3198
+3351
+3429
+3453
+3478
+3512
+3683
+3723
+3801

What am I doing wrong?