Re: irrational Spicing

On a sunny day (Fri, 1 Nov 2024 08:41:35 +0100) it happened Klaus Vestergaard
Kragelund <klauskvik@hotmail.com> wrote in <vg20nf$35bfs$1@dont-email.me>:

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

The Microchip 18F14K22 I use for a lot of things has 4 12 bit ADC inputs,
and a PWM generator
goes for maybe 2 dollars in quantity, and has a hardware comparator that can directly stop the PWM for
cycle by cycle current limiting.
So 4 of those is all you need, with ADC capacity to spare,.
Bit of asm, 64 MHz PLL internal clock..
More than enough for controlling a power supply... can drive voltage and current displays at the same time too.