Attempt at random number generation with noise from last bit of ADC read, for seeding pseudorandom generator. For STM32F103 board.

randomgepruts.ino

#define debug

void setup() {
  // put your setup code here, to run once:
  //analogReadResolution(12); // STM32F103, 12 bits adc resolution // Allwrong, this should work but doesn't. Fortunately, in contrast to documentation, a 12 bit result turns out te be the default on this board.
  if(Serial);
}

void loop() {
  // put your main code here, to run repeatedly:
  #ifdef debug
  //Serial.printf(F("Pitje: "), %d, BittenVoorPitten()); // waarom heeft die stm32 geen printf! Terwijl de IDE het wel een mooi kleurtje geeft. 
  Serial.println(BittenVoorPitten(), BIN);
  #endif
  Serial.println(BittenVoorPitten());// Random seed.
  
  delay(1000);
}

long int BittenVoorPitten(){ // for use as random seed (ADC LSB noise on internal thermal input)
  int temp, pitjes=0;
  for(int i=0; i<16;i++){ 
    
    //ADC_CR2 |= (1<<23); //enable internal temp sensor, ch16 (And VREF, ch 17), hmmz, doesn't work this way. Kinda frustrating...
    ADC1->regs->CR2 |= (1<<23); //enable internal temp sensor, ch16 (And VREF, ch 17); 
    //(Undocumented?, found out by digging around .../sketchbook/hardware/Arduino_STM32-master/STM32F1/system/libmaple/include/libmaple/adc.h)
    
    temp= analogRead(16); // ch 16 is internal temperature sensor. But heating the chip does not change the read value... So something goes wrong.
    // enabling internal VREF and temp sensor did not help. (Despite this, the last bit is still noisy enough for this purpose... But it should not be on an external input...)
     
    #ifdef debug
    //Serial.printf(F("gelezen temperatuur"), %d, F(" binair:"), %b12, temp,temp);
    Serial.print(F("gelezen temperatuur"));
    Serial.print(temp);
    Serial.print(F(" binair:"));
    Serial.println(temp, BIN);   
    #endif
    pitjes |= ((temp&1)<<i);
  }
  return pitjes;
  }