This code aims at using the ADC unit of the ATMega328 more judiciously than the basic analogRead() function.
The need arised when I had serious timing contrains that couldn't have been achieved reliably with Arduino alone.
Channel A0 of the ADC had to be polled 8 times and all that data had to be stored in an SD card after dropping 2 LSBs from the reading.
All this took 1.8 ms, and it wasn't that appreciable. I knew that this could be reduced without getting a better SD card, but I didn't know how I could do it.
This is the attempt I made.
I found in the datasheet that the first conversion takes 25 clock cycles and all the rest take 13, unless the ADC is disturbed and turned on again.
Considering 1 clock cycle = 62.5 ns
Total clock cycles = 25 + 13x7 = 116
Minimum time = 116x62.5 = 7250 ns
- An analogRead always calls up the ADC unit (in case the pins had been defined as digital I/Os earlier), sets up the internal reference at Vcc, then sets the AD channel to A0, because the only info we send to the function is A0.
The code above does that only once.
So it latches to the Vcc reference, A0 analog channel, and the pins are defined as analog input only. Once and for all. - The default setting of storing values in internal registers is left-adjusted. Since the ADC has a resolution of 10-bits, it takes 2 registers (ADCL and ADCH) to store any conversion. Now, keep in mind that ADCH will have the 2 MSBs and rest will be in ADCL. This is what left adjusted values mean, MSB towards left.
Now when reading these values, a 2 byte int has to be used, which concatenates the ADCL bits to ADCH and then stores it in the variable.
Right-adjusting the values fills the ADCH with 8 bits and ADCL with 2 bits, which have to be dropped ultimately, so there's no point reading them.
Therefore we read only one register and directly dump that value to the variable.