user63260: I find difficulties to measure correctly the voltage at the AREF pin.
I would measure voltages ‭with a resolution not exceeding 1 mV . nda voltage divider 
In one of my trials, I was using a  9V battery and a voltage divider made with a 1k resistor and a 2k one. 

The battery voltage was about 8890 mV and the reference voltage was about 2960 mV (confirmed by a multimeter).
I  connected the output of the divider to the AREF pin of an Arduino Uno board. The output voltage value dropped to about 1800 mV. Reading the voltage value at AREF pin with a simple sketch:.

the serial monitor printed:

as if the voltage dividere did not exist.
What am I doing wrong? or what I don’t understand?
Thank you for your attention.

user4143: 
I was using a 9V battery and a voltage divider made with a 1k resistor and a 2k one.

Using a battery is not a good way of getting a stable voltage. A battery's voltage will change over time.
Using a voltage divider is not a good way of generating a reliable stable voltage. The output impedance is very high and will be affected by any loading you apply to it causing it to droop.
If you want 1mV per LSB then you need a 1024mV voltage reference (since there are 10 bits of resolution, or 1024 possible divisions, in the ADC). For that you want a 1024mV voltage reference, which is a standard value for precision voltage reference chips such as the LM4140CCM-1.0/NOPB from Texas Instruments. Personally I like to use a precision DAC like the MCP4821 to provide a settable analog reference voltage. 

Reading the voltage value at AREF pin with a simple sketch the serial monitor printed 8890 as if the voltage dividere did not exist. What am I doing wrong?

You cannot read anything from the AREF pin. It is merely a voltage reference, not an active pin you can read. When you use analogRead(EXTERNAL) you are actually doing analogRead(0) since EXTERNAL is a macro that expands to 0. That means you are really reading from pin A0 not the AREF pin.  And then you just map that to 8890 when it's at maximum anyway. Since A0 is (probably, since you don't show anything else in your circuit) floating it will most likely be above the voltage you are feeding in to AREF and will return 1023 all the time anyway.

63260: <p>I find difficulties to measure correctly the voltage at the AREF pin.
I would measure voltages ‭with a resolution not exceeding 1 mV . nda voltage divider 
In one of my trials, I was using a  9V battery and a voltage divider made with a 1k resistor and a 2k one. </p>

<p><a href="https://i.stack.imgur.com/3bvsr.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/3bvsr.jpg" alt="enter image description here"></a></p>

<p>The battery voltage was about 8890 mV and the reference voltage was about 2960 mV (confirmed by a multimeter).
I  connected the output of the divider to the AREF pin of an Arduino Uno board. The output voltage value dropped to about 1800 mV. Reading the voltage value at AREF pin with a simple sketch:.</p>

<p><a href="https://i.stack.imgur.com/ybUtu.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ybUtu.jpg" alt="enter image description here"></a></p>

<p>the serial monitor printed:</p>

<p><a href="https://i.stack.imgur.com/zwQ2C.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/zwQ2C.jpg" alt="enter image description here"></a></p>

<p>as if the voltage dividere did not exist.
What am I doing wrong? or what I don’t understand?
Thank you for your attention.</p>
 4143: <blockquote>
  <p>I was using a 9V battery and a voltage divider made with a 1k resistor and a 2k one.</p>
</blockquote>

<p>Using a battery is not a good way of getting a stable voltage. A battery's voltage will change over time.</p>

<p>Using a voltage divider is not a good way of generating a reliable stable voltage. The output impedance is very high and will be affected by any loading you apply to it causing it to droop.</p>

<p>If you want 1mV per LSB then you need a 1024mV voltage reference (since there are 10 bits of resolution, or 1024 possible divisions, in the ADC). For that you want a 1024mV voltage reference, which is a standard value for precision voltage reference chips such as the LM4140CCM-1.0/NOPB from Texas Instruments. Personally I like to use a precision DAC like the MCP4821 to provide a settable analog reference voltage. </p>

<blockquote>
  <p>Reading the voltage value at AREF pin with a simple sketch the serial monitor printed 8890 as if the voltage dividere did not exist. What am I doing wrong?</p>
</blockquote>

<p>You cannot read anything from the AREF pin. It is merely a voltage reference, not an active pin you can read. When you use <code>analogRead(EXTERNAL)</code> you are actually doing <code>analogRead(0)</code> since <code>EXTERNAL</code> is a macro that expands to <code>0</code>. That means you are really reading from pin A0 not the AREF pin.  And then you just map that to 8890 when it's at maximum anyway. Since A0 is (probably, since you don't show anything else in your circuit) floating it will most likely be above the voltage you are feeding in to AREF and will return 1023 all the time anyway.</p>
