Arduino Nano ADC input selection

Question

The maximum ADC sample rate for the '328p, using the default prescaler of 128, is given as roughly 10000 samples per second. Maybe I've missed it, but I can't see whether that is the total rate for all inputs being sampled, or if each multiplexed input can sustain that rate?

In any case, if I only intend to use one analog input (say A0), is there a way to disable muxing and select which one input is active, perhaps using the ADMUX register?

• 6v6gt: What is the actual application and how many samples per second do you want? Apart from running the ADC in free running mode, which has already been mentioned below, you can drive the ADC from a timer and you use the ADC ISR to execute your code when a conversion is ready. Multiplexing in this context means the ability to route a chosen pin to the ADC. It is not automatically and actively scanning all pins like say multiplexing a 7 segment display.
• jsotola: select which one input is active ... isn't that what a mux does?
• Jim Mack: @6v6gt - Indeed, I made the unwarranted assumption that ADC muxing was a active, automatic scanning of the analog inputs. I see now that it's not, and that clears everything up for me.

Answer 1

With the default prescaler of 128, the maximum sample rate is 9615 samples/s (16 MHz/128/13). It can be achieved by setting the ADC to "free running mode". Running analogRead() in a tight loop, the maximum rate is 8929 S/s (16 MHz/128/14). As there is only one ADC, this is the total rate for all inputs being sampled.

is there a way to disable muxing and select which one input is active

The MUX is a physical device that sits between the inputs and the ADC. You cannot physically remove it. What do you mean by "disable"? You can, of course, keep the channel selection constant.

I assumed that the muxer looks at each input in turn, but it may be that if I only ever read from channel 0, then the mux will never select another channel?

Indeed. You can think of the MUX as a 1P11T (one pole, eleven throw) switch. Each time you change the MUX setting, it moves to a different channel. If you don't change its setting, it doesn't move.

1 pole, n throw switch

• Jim Mack: Maybe I'm over-thinking it. I assumed that the muxer looks at each input in turn, but it may be that if I only ever read from channel 0, then the mux will never select another channel?
• 6v6gt: @Jim Mack Roughly speaking, multiplexing is setting a static route between a pin and the ADC and there can only be one such route at any one time. If you call say AnalogRead(A1) a route is established for this call. A subsequent call to AnalogRead(A2) will clean out any previous route and establish its own route.
• Edgar Bonet: @JimMack: See edited answer.