On 20/11/2024 09:40, pozz wrote:

Il 13/11/2024 12:36, pozz ha scritto:

I couldn't understand if the first conversion after ADC is enabled
(ADEN bit) must be discarded or not.

The datasheet suggests to discard the first conversion result after
changing the reference signal, but nothing after enabling ADC or
changing input signal (ADMUX).

The only note is about the conversion time that is longer for the
first conversion after ADC is enabled.

I'm asking this question because I'm noting a different behaviour if I
start conversion and enable ADC at the same time for each conversion
against leaving ADC enabled and starting only the conversion (ADSC bit).

For the future readers, I found my issue. Occasionally I put the MCU in
sleep mode after enabling ADC and starting conversion. Even if ADC clock still runs in this mode, the result is not good.

It is a while since I have used AVRs, but it sounds to me like you are
doing something wrong. You should get /better/ results if you put the
micro to sleep immediately after starting a conversion. This is a well-established technique for getting higher accuracy from small microcontroller ADCs, as you reduce the general digital noise in the
system. But you might have to pick the right kind of sleep mode.

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On 21/11/2024 08:28, pozz wrote:

Il 20/11/2024 17:23, David Brown ha scritto:

On 20/11/2024 09:40, pozz wrote:

Il 13/11/2024 12:36, pozz ha scritto:

I couldn't understand if the first conversion after ADC is enabled
(ADEN bit) must be discarded or not.

The datasheet suggests to discard the first conversion result after
changing the reference signal, but nothing after enabling ADC or
changing input signal (ADMUX).

The only note is about the conversion time that is longer for the
first conversion after ADC is enabled.

I'm asking this question because I'm noting a different behaviour if
I start conversion and enable ADC at the same time for each
conversion against leaving ADC enabled and starting only the
conversion (ADSC bit).

For the future readers, I found my issue. Occasionally I put the MCU
in sleep mode after enabling ADC and starting conversion. Even if ADC
clock still runs in this mode, the result is not good.

It is a while since I have used AVRs, but it sounds to me like you are
doing something wrong.  You should get /better/ results if you put the
micro to sleep immediately after starting a conversion.  This is a
well-established technique for getting higher accuracy from small
microcontroller ADCs, as you reduce the general digital noise in the
system.  But you might have to pick the right kind of sleep mode.

Yes, you're right. However for AVRs the mechanism is different than what
you described.

If you want to start a /better/ ADC conversion, you should enable ADC peripheral and ADC interrupt and put the MCU in sleep mode (one specific sleep mode). This procedure automatically starts the ADC
"super-accurate" conversion.

In my case, I was starting the ADC in my code before entering sleep mode
and this is not the correct procedure.

Just for curiosity, I don't need high accuracy, so it's good to make conversion while the MCU is in active mode.

It's certainly better to keep the MCU active than use the /wrong/ sleep
mode or sequence!

Sleep modes are useful if you need to have low power (for battery use,
or to avoid heating), but they can complicate things a bit. I usually
don't bother with sleep modes unless I have a particularly good reason
for them.

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