Wow. Thank you jmknapp and oracle.
So if I just include your defines in the top of my code and the function calls in my setup, I should be good to go? As in, analogReads will just return a lot faster (at a bit lower precision)?
Chris
Yes, analogRead() will just return faster if you set the prescale, I've tried it. Here's a little test program that shows the effect:
#define FASTADC 1
// defines for setting and clearing register bits
#ifndef cbi
#define cbi(sfr, bit) (_SFR_BYTE(sfr) &= ~_BV(bit))
#endif
#ifndef sbi
#define sbi(sfr, bit) (_SFR_BYTE(sfr) |= _BV(bit))
#endif
void setup() {
int start ;
int i ;
#if FASTADC
// set prescale to 16
sbi(ADCSRA,ADPS2) ;
cbi(ADCSRA,ADPS1) ;
cbi(ADCSRA,ADPS0) ;
#endif
Serial.begin(9600) ;
Serial.print("ADCTEST: ") ;
start = millis() ;
for (i = 0 ; i < 1000 ; i++)
analogRead(0) ;
Serial.print(millis() - start) ;
Serial.println(" msec (1000 calls)") ;
}
void loop() {
}
As it stands above, with FASTADC defined as 1, the 1000 calls to analogRead() take 16 msec (16 microseconds per call). With FASTADC defined as 0, the default prescale gives 111 microseconds per call.
Personally I've been looking into ways to make ADC processing less expensive in CPU cycles, to avoid calling analogRead() which burns over 100 microseconds sitting in a loop waiting for the conversion to finish. Turns out there's a mode where the conversion finishing can generate an interrupt, so that's the route I'm going, so the processor can be doing other things during the conversion (as long as the result is not needed right away!).
As for how the above code relates to a prescale value of 16, that's from a table in the Atmega data sheet:
