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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.arch.embedded
Subject: Re: ATmega328PB and ADC
Date: Thu, 21 Nov 2024 09:00:00 +0100
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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.