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From: Michael Schwingen <news-1513678000@discworld.dascon.de>
Newsgroups: comp.sys.raspberry-pi
Subject: Re: RP2350 and Pico 2 - things missing
Date: 18 Aug 2024 18:10:16 GMT
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On 2024-08-18, James Harris <james.harris.1@gmail.com> wrote:
>>> * There are still no analogue /outputs/ - which I would have thought
>>> would have been cheap and easy to provide.
>> 
>> I suspect the analogue drive circuit is not that simple - would take
>> area and may compromise power, especially idle power.
>
> I would have thought that such a circuit could be gated to reduce/remove 
> power consumption. DAC needs far less circuitry (and no iteration) than ADC.

I suspect it takes (quite) some design effort to get it right (see the ADC
on the RP2040), and requires analogue circuitry which is more difficult to
implement in a given process than digital.

Also, on-chip DACs are not too common in competitor parts - there are some
that have it, but it is not standard.

> I was hoping for analogue output pins initially in order to drive a VGA 
> display.

That requires a *really fast* DAC.  IIRC, standard VGA was ~25MHz pixel
clock.

For comparison, the STM32F407 DAC has a settling time of 3us (typical),
meaning you can't do anything above ~300kHz. 

Getting a DAC to VGA speed (even at reduced resolution) requires quite some
more design effort (and probably also chip area), and aside from analog VGA,
there are probably few use cases.

> Would PWM work for that, do you think? My guess is that it 
> would be too slow or jittery.

I agree.

> I have seen Pico set up to drive VGA through five resistors per colour. 
> It may be a Pico reference design but there's a great version of it at
>
>    https://vanhunteradams.com/Pico/VGA/VGA.html
>
> The problem is that for three colour guns it takes fifteen pins!

Well, the new RP2350B has 48 instead of 30 GPIO pins available, so that
should be anough for 6 bits / colour ;-)

cu
Michael
-- 
Some people have no respect of age unless it is bottled.