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From: JM <sunaecoNoChoppedPork@gmail.com>
Newsgroups: sci.electronics.design
Subject: Re: Grounded grid VHF front-end
Date: Sun, 17 Nov 2024 18:28:45 +0000
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On Sun, 17 Nov 2024 09:33:54 -0800, john larkin <JL@gct.com> wrote:

>On Sun, 17 Nov 2024 14:50:58 -0000 (UTC), antispam@fricas.org (Waldek
>Hebisch) wrote:
>
>>USB can do milliseconds, ethernet hundreds of microseconds, small
>>micros can do much better.  Theoretically with a micro connected via USB
>>one can synchronize clocks of the micro and PC with microsecond
>>accuracy, I plan to try this but do not know how this will work.
>
>We're designing some products around the RP2040, the Pi Pico
>processor.
>
>Turns out that in some cases, it's easier to bit-bang an SPI interface
>than program an SPI engine. To fine-tune timings in 7 ns increments,
>we can use no-op instructions.
>
>I wonder what's a safe c-language NOP single-clock operation that no
>compiler is smart enough to optimize out and doesn't add a bunch of
>loads and stores.
>
>We're experimenting with that sort of timing on an oscilloscope. The
>GCC or whatever code timing tools don't work in this case.
>
>Something like
>
>        gpio_put(FIRST_GPIO, 1);
>        gpio_put(FIRST_GPIO, 0);
>        gpio_put(FIRST_GPIO, 1);
>        gpio_put(FIRST_GPIO, 0);
>
>Makes the port pin change every 7 ns. That's astounding. So maybe a
>dummy port bang is my no-op. Just repeat what we just set it to.
>

Code in assembly.  It's trivial to use assembly code with c in gcc.