Re: RP2350 and Pico 2 - things missing

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.