stimmer:
while((ADC->ADC_ISR & 0xC0)==0);// wait for two conversions (pin A0[7] and A1[6])Well spotted - what it should actually be is this:
while((ADC->ADC_ISR & 0xC0)!=0xC0);// wait for two conversions (pin A0[7] and A1[6])I'll go back and change my code in case anyone else copy/pastes it.
For reducing missed samples and allowing more processing time in the main loop, the biggest win comes through using peripheral DMA. This example uses DMA and interrupts - although if you are new to the Due, don't expect to understand it all at once
#undef HID_ENABLED
// Arduino Due ADC->DMA->USB 1MSPS
// by stimmer
// Input: Analog in A0
// Output: Raw stream of uint16_t in range 0-4095 on Native USB Serial/ACM
// on linux, to stop the OS cooking your data:
// stty -F /dev/ttyACM0 raw -iexten -echo -echoe -echok -echoctl -echoke -onlcr
volatile int bufn,obufn;
uint16_t buf[4][256]; // 4 buffers of 256 readings
void ADC_Handler(){ // move DMA pointers to next buffer
int f=ADC->ADC_ISR;
if (f&(1<<27)){
bufn=(bufn+1)&3;
ADC->ADC_RNPR=(uint32_t)buf[bufn];
ADC->ADC_RNCR=256;
}
}
void setup(){
SerialUSB.begin(0);
while(!SerialUSB);
pmc_enable_periph_clk(ID_ADC);
adc_init(ADC, SystemCoreClock, ADC_FREQ_MAX, ADC_STARTUP_FAST);
ADC->ADC_MR |=0x80; // free running
ADC->ADC_CHER=0x80;
NVIC_EnableIRQ(ADC_IRQn);
ADC->ADC_IDR=~(1<<27);
ADC->ADC_IER=1<<27;
ADC->ADC_RPR=(uint32_t)buf[0]; // DMA buffer
ADC->ADC_RCR=256;
ADC->ADC_RNPR=(uint32_t)buf[1]; // next DMA buffer
ADC->ADC_RNCR=256;
bufn=obufn=1;
ADC->ADC_PTCR=1;
ADC->ADC_CR=2;
}
void loop(){
while(obufn==bufn); // wait for buffer to be full
SerialUSB.write((uint8_t *)buf[obufn],512); // send it - 512 bytes = 256 uint16_t
obufn=(obufn+1)&3;
}
It reads ADC data at 1 million samples/sec and outputs the data to SerialUSB. (I've only tested it on Linux and most of the time it works, sometimes it is unreliable, I don't know why). Using GNU Radio I was then able to analyse the data stream and receive a long-wave radio signal, with an LC tuned circuit into A0 as an aerial.
This code is running smoother and with less jitter.
I am measuring jitter, by setting HIGH and LOW one Digital pin with an oscilloscope and the inline function:
inline void digitalWriteDirect(int pin, boolean val){
if(val) g_APinDescription[pin].pPort -> PIO_SODR = g_APinDescription[pin].ulPin;
else g_APinDescription[pin].pPort -> PIO_CODR = g_APinDescription[pin].ulPin;
}
BTW, I am controlling the sample rate by using the PRESCAL register in:
ADC->ADC_MR |= 0x2680; // these lines set free running mode on adc 7 (pin A0) [PRESCAL at 50kHz]
Is there any other (better) way to do it?
Actually, depending on if I am using one channel or two an the ADC, I need to setup a different PRESCAL, since there is only one ADC for all input pins:
if (channels_2)
{
ADC->ADC_MR |= 0x1280; // these lines set free running mode on adc 7 (pin A0) and adc 6(pin A1) PRESCAL at 50kHz
}
else
{
ADC->ADC_MR |= 0x2680; // these lines set free running mode on adc 7 (pin A0) PRESCAL at 50kHz
}
On the other hand, what would the changes for two channels and 4 buffers? Is it possible to get a buffer for each channel?
Finally, I'm trying to measure performance (the same way as jitter) and for 50kHz sample rate and 512 points, the time in the interrupt is 1.8us and the free time in the loop is 10.23ms (99.98% free!!).
For 1MHz, it is still around 1.8us against 256us (it is still 99.29% free), however more jitter at the beggining of the loop is appreciated.
I think it is because of the while(obufn==bufn); instruction
It is interesting the
SerialUSB.begin(0);
while(!SerialUSB);
Does the computer (or other device) set the speed of the port?
Your spectrogram app looks great, keep up the good work!! Congratulations.