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| marcmerlin | howdy. I've been reading up on fomus and FPGAs. I see that fomu has 5000 LUTs, which doesn't feel like a lot, but that's somehow still enough to upload a RISC-V softcore + code to run on top? If so, impressive | 11:46 |
|---|---|---|
| marcmerlin | I understand that with an FPGA, I could upload any softcore that fits, but realistically people mostly use RISC-V since it's simple. | 11:47 |
| marcmerlin | So, for a small FPGA, why would I want to use one over a microcontroller that already has a CPU, and where all the resources are dedicated to user code and RAM? | 11:49 |
| marcmerlin | I can see that it would be cool to be able to patch your CPU and add instructions, which you can't do on a microcontroller, but I'm assuming few people do/use that. Same thing with micropython, how would it differ to run it on an FPGA over let's say an ESP32? | 11:52 |
| xobs | marcmerlin: yep, enough to upload a RISC-V softcore + code. Plus USB core. The bootloader is 8 kB, which is loaded into the softcore's "bootrom". | 11:52 |
| xobs | The advantages are mostly educational, and being able to understand how a CPU works. | 11:53 |
| xobs | People have lamented the lack of a multiply unit, so just yesterday someone compiled a new version of the CPU to include a single-cycle multiplier. | 11:54 |
| marcmerlin | Right, I understood that from the docs (workshop is great, much better than tomu that was hard to learn :) ). Got it, so on that small scale, it's not better in any way than a microcontroller | 11:54 |
| tnt | marcmerlin: it's going to be much slower that fpga, that's one difference :p | 11:54 |
| tnt | marcmerlin: but the huge advantage of a fpga is you get the peripherals you want ... exactly like you want them. | 11:55 |
| marcmerlin | @tnt: yes, I figured an FGPA softcore will be slower than a hardware CPU (although you can add instructions to the latter) | 11:55 |
| xobs | You're correct. It's slower and less full-featured than an MCU (i.e. no multiplier or fancy peripherals). | 11:55 |
| xobs | But there are a few tricks. For example, because we control the USB hardware, there's a debug bridge that's exposed over the USB pins. | 11:56 |
| xobs | That means you can poke at hardware registers without needing a third-party debug cable. | 11:56 |
| marcmerlin | @tnt: I see what you mean. If you want 4 hardware SPI busses, you only have 2 on an ESP32 and then you do software SPI. So the FPGA would let you add extra SPI outputs that would be faster than software SPI on ESP32 | 11:57 |
| marcmerlin | xobs: yes, I saw the USB debug bridge, that's very cool indeed. You need a special cable to do that on other chips | 11:58 |
| marcmerlin | Now, I know that at bigger scale (driving 8 RGBPanel strings in parallel), FPGAs do better, because they can do so much I/O | 11:59 |
| tnt | marcmerlin: yeah .. or stuff that just plain doesn't exist. I have a UP5k interfacing to E1 (telecom interfaces), or to SPDIF ... I would have needed external chips to do that with any MCU at all. | 11:59 |
| marcmerlin | can't you use digital pins and PWM to do that on any MCU? | 12:00 |
| marcmerlin | I've never seen it, but I'm guessing an arduino style chip could output composite video on a pin for instance HDMI would be harder due to lack of required speed | 12:01 |
| tnt | E1, no. SPDIF I guess you could possibly bitbang it maybe although at like 3 MHz that's a pretty high rate to bitbang and you gotta be _spot_on_ timing. | 12:01 |
| tnt | And as soo as you bitbang anything on a MCU ... you lose any CPU speed advantage your hw CPU has because you're spending all your time bitbanging stuff. | 12:02 |
| marcmerlin | ok, that's where I may need to learn more. Yes, bigbanging ties up the CPU and requires a spot on ISR. But isn't that true for an FPGA too? I'm assuming you would generate the output not in C code that runs on the softcore, or you're no better off, correct? | 12:04 |
| xobs | You could generate a hardware block that does it. | 12:28 |
| xobs | For example, a ws2812b "neopixels" driver could be native, where you'd have a framebuffer and hardware would take care of the timing. | 12:29 |
| marcmerlin | gotcha, thanks. I need to do more reading and learning | 13:00 |
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| marcmerlin | l | 22:00 |
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| acathla | Are you the marcmerlin on marmerlins.org ? | 22:12 |
| marcmerlin | oops, sorry for sending that stray character. Yes, that's me ( http://marc.merlins.org/ I assume you meant) | 22:14 |
| acathla | Yep. I used your SA-exim patch and conf files a lot :) | 22:17 |
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| marcmerlin | oh my, that's like a totally previous life, 15 years ago :) | 22:20 |
| acathla | You should update it :) | 22:21 |
| marcmerlin | I'll admit that I've kind of lost interest. I still run my own Email, but I'm done writing exim configs (the exim4 config file in debian is partially mine), and exim code :) | 22:25 |
| marcmerlin | but the cool part is that someone else in debian has been maintaining it for me | 22:26 |
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