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From: Klaus Vestergaard Kragelund <klauskvik@hotmail.com>
Newsgroups: sci.electronics.design
Subject: Re: irrational Spicing
Date: Fri, 1 Nov 2024 08:41:35 +0100
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On 31-10-2024 21:50, john larkin wrote:
> On Thu, 31 Oct 2024 12:17:49 -0700, dplatt@coop.radagast.org (Dave
> Platt) wrote:
> 
>> In article <rdg5ijl8fkfqe3u3kjffoa90rupsgdot48@4ax.com>,
>> john larkin  <jl@glen--canyon.com> wrote:
>>
>>> 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.
>>
>> Roger that.  I've recently finished porting a simple software "modem"
>> (ham-radio packet TNC) over from an STM32F411-based prototype, to a
>> Pico.  The STM part has hardware single-precision floating point, and
>> each signal pipeline (a bunch of biquad filters) was taking about 5%
>> of the CPU.  On the Pico, using the generic libc software floating-
>> point libraries (which are pretty poor for the M0 core) each pipeline
>> was taking about 35% of the CPU, and since I wanted three of them running
>> in parallel, it was a no-good situation.  I haven't yet figured out how
>> to get the Zephyr library system to link into the "hardware assisted"
>> floating-point emulation in the ROM, and I'm not sure even that would
>> be fast enough for my needs.
>>
>> I switched over to a simple 1.15.16 fixed-point implementation of the
>> biquad filters.  The performance was about 3x better, even though the
>> M0 doesn't have a 32x32->64 integer multiplier in hardware (the library
>> emulates one using four 16x16->32 multiplies).
> 
> 
> I'm planning to use the same math format, only I call it S16.16. That
> covers +-32 kilovolts with 15 uV resolution, which should be fine.
> 
> The RP2040 manual says it will do a 32x32 mul in one clock, 7 ns. If
> that's not true, we can do hacks, like what you suggest, or round
> parameters up or down enough to use shifts instead of multiplies.
> 
> I'll have four power supplies to service. We'll need to read an
> 8-channel SPI ADC to slurp the voltages and currents, do the voltage
> regulation and current limit loops, and generate four PWMs into the
> DRV8962 quad switcher thing.
> 
> The power supply loop bw will be under 1 KHz, so we should have plenty
> of horsepower to do the math. It's a dual core ARM, so we will
> dedicate one core to just those four control loops.

You would need to sample at least 40 times faster than the crossover, to 
get phase erosion of less than 10 degrees. So in your case 40kHz.

Using fixed point math, this should be doable on a small 
microcontroller, not needing a Pico to do that, but I guess, in your 
application cost is not an issue.

Of course the advantage of the RP2040 is plenty of RAM