Re: irrational Spicing

On Wed, 30 Oct 2024 18:12:32 -0400, bitrex <user@example.net> wrote:

On 10/30/2024 5:11 PM, john larkin wrote:

On Wed, 30 Oct 2024 16:33:49 -0400, bitrex <user@example.net> wrote:
 

On 10/30/2024 4:07 PM, john larkin wrote:

Things I've seen, and even done, in Spice sims...
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Bypassing voltage sources  (not me!)
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Worrying about resistor power dissipation
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Using standard parts values, like 4.7K ohms or 33nF, when the control
loop will be mostly code anyhow
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Using +-12 or some such opamp supply voltages, and scaling signal
levels to fit. The LT Spice universal opamps will work with hundreds,
or thousands, of volt supplies.
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Drawing hideously ugly schematics without a title, author, date, or
named nodes.
>
>
So I'm rescaling a power supply sim (I'm waiting for a run to finish
now) to have everything in actual 1:1 units. Then we will write the
control loop code to work in those same real engineering units, not
some goofy scaled integers or anything like that.
>
Keeping everything in true units as floats is ideal, but the RP2040
floating point ops are kinda slow, so we may express things as 32-bit
values, 16 bits of signed integer and 16 bits of fraction, as a sort
of fast and cheap float. But 12.5 volts is still visibly 12.5,  just
as if it were a float. That will be handy for debugging.
>
S16.16 is plenty good to express voltages and currents in a power
supply.
>

>
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Floating point kind of sucks for low frequency systems with relatively
long time constants, anyway. There are applications that need 1800 dB of
dynamic range, audio isn't really one of them

 
What has that much dynamic range?

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I dunno like, stars and stuff? Astrophyiscs stuff, maybe..
>

Most real systems, especially my power supply, will have lots of free
noise dithering. That makes the variables have essentially infinite
resolution.

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All sampled signals have an ENOB, even analog storage scopes and
bucket-brigade delays have an implied resolution...I know audio BBDs
sure are noisy despite their "infinite resolution" without companding.
>
On "big iron" these days like x86 and smartphones all audio
processing-stuff tends to be floating point with full-scale defined as 1
and -1, for either 24 or 53 bits of mantissa, depending, so you can
leverage SIMD/MME instructions.
>
It usually make sense just to use double precision for everything except
long-term storage nowadays unless there's some pressing reason to need
more SIMD processing throughput and/or conserve RAM.

The Pi Pico CPU, the RP2040, has integer math hardware but the floats
are "hardware assisted" subroutines in the rom bios. Single
add/sub/mul floats take around 600 ns, roughly 100 instructions, which
is kinda slow for my application, four power supply control loops.