I am working with a microcontroller with 8-channel, 10 bit multiplexer A/D converter. In the manual it says: When a sequence consists of multiple samples, the samples may be taken from one channel or multiple channels. An interrupt may be generated on the completion of an A/D conversion. The analog multiplexer selects one of the three internal or one of the external signal sources for conversion.

So the questions are:

1- When we have a multiplexer at the input, does it mean that I can sample 8 different signals simultaneously?

2- I am trying to measure the temperature with the MCU and I am not sure what three internal signals mean. I guess in my case I just have one external signal, right?

You can find the data-sheet here: http://cache.freescale.com/files/microcontrollers/doc/app_note/AN2429.pdf

  • \$\begingroup\$ Seems like a reasonable question to me. Link to datasheet would be useful identifying those "three internal signal sources." \$\endgroup\$
    – Keegan Jay
    Oct 7, 2015 at 17:25

2 Answers 2

  1. No, it means you can sample them in sequence, close together. If they need to be sampled truly simultaneously, you will need external sample-and-hold circuits.
  2. The three internal signals are analog values internal to the MCU; one of these is probably the temperature. Another is probably the MCU's internal bandgap reference voltage.
  • \$\begingroup\$ Sample different signals in sequence or one signal? \$\endgroup\$
    – Jack
    Oct 7, 2015 at 14:14
  • \$\begingroup\$ And why the multiplexer selects the internal signals for conversion? In the case of the need for internal information about ADC? \$\endgroup\$
    – Jack
    Oct 7, 2015 at 14:17
  • 1
    \$\begingroup\$ yes you can cample 8 different signals connected on 8 different pins of the MCU. About the internal signals yes it is information inside the MCU that you may need to convert. \$\endgroup\$
    – damien
    Oct 7, 2015 at 14:24
  • \$\begingroup\$ Ok thank you, so the ADC is applied to one of the inputs of MUX and when an interrupt is generated on the completion of an A/D conversion the system apply the ADC to the input signal of the next pin? \$\endgroup\$
    – Jack
    Oct 7, 2015 at 14:34
  • \$\begingroup\$ @MaryE Different signals or the same - either way, the ADC can only perform one conversion at a time. The multiplexer has internal signals so you can do things like measure temperature, or supply voltage. \$\endgroup\$ Oct 7, 2015 at 14:34

Internally it would look something like this. Eight switches allow you to connect one of the eight analog input pins to the ADC or one of the internal pins.


simulate this circuit – Schematic created using CircuitLab

So you can't read the inputs simultaneously but for many applications you can read them sequentially fast enough that it's good enough.

The sequence would be:

  1. Choose the input to read.
  2. Wait for the switch to close and the signal to stabilise.
  3. Initiate the ADC conversion.
  4. When the conversion complete signal turns on copy the ADC value to wherever you want to store it.
  5. Go to step 1.

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