highest frequency periodic interrupt?

On a sunny day (Fri, 13 Jan 2023 15:46:16 -0800) it happened John Larkin snipped-for-privacy@highlandSNIPMEtechnology.com wrote in snipped-for-privacy@4ax.com:

Well in that sort of thing you need to think in asm, instruction times, but I have no experience with the RP2040, and little with ASM on ARM. Should be simple to test how long the C code takes, do you have an RP2040? Playing with one would be a good starting point. Should I get one? Was thinking just for fun...

I got a few Pi Picos but haven't run them.

I recently got a Pi4B "development system" from Amazon. Add a keyboard and a monitor and a mouse and it will compile and debug programs for the Pico. It runs their OS right out the box.

There's a book too

On a sunny day (Fri, 13 Jan 2023 21:08:08 -0800) it happened John Larkin snipped-for-privacy@highlandSNIPMEtechnology.com wrote in snipped-for-privacy@4ax.com:

That 230 USD is a LOT of money! Amazon is trying to profit from the PI4 shortage it seems. Its not evene a 8GB,

Payed about 100 USD for my Pi4 4 GB and my Pi4 8 GB just 2 years ago December 2020, including SDcard, RapiOS, plastic housing, cables, cooling fins and supply.

No fan, it does run hot, about 70 C. But I use that one for web browsing. The older one with 4 GB memory has an ebay metal housing and a fan. After lubricating that fan with vaseline it now has run quiet for 4 years? The metal housing also stops any WiFi, as that one is part of the security system and no WiFi allowed there. It runs 24/7 recording 6 cameras, 2 audio channels, weather sensors (temp, air pressure, humidity airtraffic, ship traffic, radiation etc (from an even older rRaspberry Pi that works as server) ..

Type this in a terminal to see the current temperature: vcgencmd measure_temp

For more info on that command: man vcgencmd

Maybe be intersting: vcgencmd measure_clock

Here now: raspberrypi: ~ # vcgencmd measure_clock arm frequency(48)=700207040

I like the GPIO I/O, so many things you can do with that,

It isn't always necessary to use interrupts. Periodic i/o can be done using DMA and the programmable state machines can do a lot too. The floating point code is stored in zero wait-state masked rom so it isn't affected by cache misses as it would be if executing from the flash memory. For development it is worth looking at the RP400 which is an RP4 built into a keyboard. It is by default clocked faster than the standard RP4 because there is a large internal heat sink for the cpu. Sometimes the RP400 has had better availability. There are keyboard layouts for various countries. John

Usually try to avoid having fast periodic IRQs in favour of offloading them onto some dedicated hardware. But CPUs were slower then than now.

It might be worth benchmarking how fast the FPU really is on that device (for representative sample code). The Intel i5 & i7 can do all except divide in a single cycle these days - I don't know what Arm is like in this respect. You get some +*- for free close to every divide too.

Beware that what you measure gets controlled but for polynomials up to 5 term or rationals up to about 5,2 call overhead may dominate the execution time (particularly if the stupid compiler puts a 16byte structure across a cache boundary on the stack).

Forcing inlining of small code sections can help. DO it to excess and it will slow things down - there is a sweet spot. Loop unrolling is much less useful these days now that branch prediction is so good.

ISR code is generally very short and best done in assembler if you want it as quick as possible. Examining the code generation of GCC is worthwhile since it sucks compared to Intel(better) and MS (best).

In my tests GCC is between 30% and 3x slower than Intel or MS for C/C++ when generating Intel CPU specific SIMD code with maximum optimisation.

MS compiler still does pretty stupid things like internal compiler generated SIMD objects of 128, 256 or 512 bits (16, 33 or 64 byte) and having them crossing a cache line boundary.

It's nothing compared to setting up a big-box PC and installing an OS and compilers and libraries for some chip that will be EOL soon. $230 is dinner for four around here at a middle-good restaurant.

It comes with a power wart and cables and the OS on an SD card. It's not worth shopping around for all that, if your time is worth anything.

Fingers are easier.

On a sunny day (Sat, 14 Jan 2023 08:31:33 -0800) it happened John Larkin snipped-for-privacy@highlandSNIPMEtechnology.com wrote in snipped-for-privacy@4ax.com:

This is from google: For Raspberry Pi 3+, a 'soft' temperature limit of 60°C has been introduced.] This means that even before reaching the hard limit at 85°C, the clock speed is reduced from 1.4GHz to lower frequencies, reducing the temperatu and That is the so-called throttling. The Raspberry Pi monitors the temperature continuously. Above 82 °C (180 °F), the clock frequency is automatically lowered, regardless of which flag is set. This action will reduce heat

So better use vcgencmd and it saves your finger too from getting fried. I should actually get a better housing with fan for my Pi4 8 GB like I have for my Pi4 4 GB that runs at about 46 Degrees C. Of course maybe bringing your own fried finger to a restaurant ?? ..Discount?

So if you need high speed and have a high processor load then get a decent cooling.

The RP2040 chip has FP routines in the rom, apparently code with some sorts of hardware assist, but it's callable subroutines and not native instructions to a hardware FP engine. When it returns it's done.

Various web sites seem to confuse microseconds and nanoseconds. 150 us does seem slow for a "fast" fp operation. We'll have to do experiments.

I wrote one math package for the 68K, with the format signed 32.32. That behaved just like floating point in real life, but was small and fast and avoided drecky scaled integers.

Without true hardware FP, call locations probably don't matter.

We occasionally use polynomials, but 2nd order and rarely 3rd is enough to get analog i/o close enough.

Nobody has answered my question. Generalizations about software timing abound but hard numbers are rare. Programmers don't seem to use oscilloscopes much.

It might get hotter when it's compiling or something, but it's not very warm. It would be easy to add the fan if it got necessary. The kit did come with three stick-on heat sinks.

There are also LCD monitor things that the 4B mounts on the back of. They have a fan.

My finger is calibrated. I can touch 50C forever and 60C for about half a second. Touching 100C briefly hurts but does no harm. Baking a real pie is more dangerous.

I've had interns that refused to touch chips to see if they are hot. They were afraid of being electrocuted by 3.3 volts.

Am 14.01.23 um 19:21 schrieb John Larkin:

I did it on the BeagleBoneBlack. It has an ARM CPU to run Debian Linux etc and two I/O processors that are 200 MHz RISCs without pipeline stalls and operating system. The TI C compiler is on the BBB. I can do I/O with 5ns resolution & rate. No jitter, and directly from a C program.

volatile int i; myportbit = 0; .... myportbit = 1; i = 0; i = 0; myportbit = 0;

would create a 15 ns wide pulse on myportbit.

Cheers, Gerhard

The Pi Pico executes code out of the 2 Mbyte SPI flash, with a 16 Kbyte cache. Cache misses will be *very* slow. So code will need to be very tight bare-metal. The entire ISR should fit in cache.

When that gets dicey, we'll have to add an FPGA.

Control loops need to run at a constant rate, with a modest amount of jitter maybe.

If I understand that, a floating add would take about 500 ns with a

So, scaled integers or FPGA.

lørdag den 14. januar 2023 kl. 21.09.04 UTC+1 skrev John Larkin:

you can copy some (or all) of the code to ram instead of using execute-in-place from flash

I think you can even turn off the cache to get an additional 16k ram

lørdag den 14. januar 2023 kl. 21.15.02 UTC+1 skrev John Larkin:

or a different MCU with an FPU, "blackpills" are a similar formfactor and has a cortex-M4

That's a good idea. A typical ISR could be pretty small, and let the mainline program thrash all it likes.

Yikes, execute out of SPI flash?

We use STM32F207IGT6 on some existing products, but they are hard to get hence expensive. The Pi Pico for $4 is very appealing.

lørdag den 14. januar 2023 kl. 22.33.29 UTC+1 skrev John Larkin:

no, copy all the code to ram on boot

Ok, OK, the entire app and variables and stacks and buffers would have to fit in 256K. Might work.