fast "analogRead" for multiple flex-sensors. I'm Lost!

Other Hardware Sensors
1

i have been searching all over the internet to find a simple and understandable example of an analogRead in the faster AVR type of approach, by adressing the ADC directly instead of through the arduino firmware.

i need to read 5 flex sensors as fast as possible. they are connected to the analog pins 0-3 and 6.analog pin 7 is used too so practically i need to read every analogpin except 4 and 5 because they are used for i2C
What i don't understand is all that mux and adscr setup stuff.

Any help is welcome!!!

2

Most of any speed gained (unless you reduce resolution) is going to be in elimination of the busy-wait in analogRead.
What are you trying to achieve?

3

you might find this thread of some interest - Experiment - split up analog read in 3 steps - Libraries - Arduino Forum -

4

Your best bet is to use the ADC conversion complete interrupt to change the multiplexer, read the ADC value, save the result in a variable, and start another conversion. The default ADC clock is 125kHz, resulting in a conversion time of about 110us. You can double the ADC clock to 250kHz with almost no loss of precision. Beyond that, you can double it again to 500kHz and get ~9 bits precision, or quadruple it again to 1MHz and get ~8 bits precision.

As an example of what you can do, I'm currently clocking the ADC at 1MHz in an induction balance metal detector project, in order to sample a 7.8kHz signal 8 times per cycle to do phase sensitive detection.

5

dc42:
Your best bet is to use the ADC conversion complete interrupt to change the multiplexer, read the ADC value, save the result in a variable, and start another conversion. The default ADC clock is 125kHz, resulting in a conversion time of about 110us. You can double the ADC clock to 250kHz with almost no loss of precision. Beyond that, you can double it again to 500kHz and get ~9 bits precision, or quadruple it again to 1MHz and get ~8 bits precision.

As an example of what you can do, I'm currently clocking the ADC at 1MHz in an induction balance metal detector project, in order to sample a 7.8kHz signal 8 times per cycle to do phase sensitive detection.

That is really cool! you could do obstacle avoidance like that!

though... i have troubles finding a clear example. do you have one or can post one?

i need an example that contains everything to read one analog pin and instructions on what to change to select/read another pin.

6

iSpider:
i need an example that contains everything to read one analog pin and instructions on what to change to select/read another pin.

I could post the code for my metal detector, but it would need a lot of adaptation to meet your needs. Why not start by looking at file wiring_analog.c in your Arduino installation, and relating it to the datasheet for the microcontroller?

7

i have taken a look at the wiring_analog.c and it hasn't gotten any more clear to me.

That asside, i found something interesting in this file. According to this, i should be able to switch the analog pins from 0-7 to 8-15.

So can i connect analog sensors to both sets of pins and the switch between them by setting the MUX high or low?

#elif defined(ADCSRB) && defined(MUX5)
        // the MUX5 bit of ADCSRB selects whether we're reading from channels
        // 0 to 7 (MUX5 low) or 8 to 15 (MUX5 high).
        ADCSRB = (ADCSRB & ~(1 << MUX5)) | (((pin >> 3) & 0x01) << MUX5);
#endif
8

So can i connect analog sensors to both sets of pins and the switch between them by setting the MUX high or low?

MUX5 is used on the ATmega2560 to specify if A0-A7 or A8-A15 is used. Do you have an Arduino Mega2560?

9

I do...

not using it for my project though, but you can never have enough analog pins...