6 ms·

RP2350 PicoDVI Preview

194 points by fidotron 2y ago

tony-allan 2y ago
The RP2040 discussion -- https://github.com/Wren6991/PicoDVI https://github.com/Wren6991/PicoDVI
typeiierror 2y ago
The new HSTX interface on the RP2350 seems to be squarely targeted at this use case (video output) and doesn't require the use of PIO or consuming a ton of CPU cycles. There's a nice write up on the capability here: https://www.cnx-software.com/2024/08/15/raspberry-pi-rp2350-hstx-high-speed-serial-transmit-interface/ https://www.cnx-software.com/2024/08/15/raspberry-pi-rp2350-...
- phire 2y ago
  The main advantage of the SIO TMDS encoder, is that it allows you to output TMDS on any GPIO, instead of the eight that the HSTX is restricted to. And they allow an easy upgrade path for projects that were already bit-banging DVI on the rp2040. Other than those two advantages, I don't think there is any advantage to the SIO TMDS encoder over the HSTX TMDS encoder. I get the impression that the SIO TMDS encoder was added to the design first, and there wasn't a good reason to remove them after the introduction of HSTX.
- jauntywundrkind 2y ago
  Isn't hstx mainly good for streaming out? My naive guess would be that it wouldn't have as much transceiver offload capabilities, like what the SIO I naively guess would be good for. The nice part about using SIO seems to be that you can do the tdms encoding there. With hstx you need the output bitstream in the right format already which seems like you might be back to needing the CPU to do the encoding.
  - phire 2y ago
    HSTX has a built in TMDS encoder, and as far as I can tell, it has all the functionality of the SIO TMDS encoder. You can configure it to directly consume any line buffer with any pixel format, from 1 to 8 bits per color channel. It even supports formats with different numbers of bits per channel like 8-bit RGB322 and 16-bit RGB565.
    - onetom 2y ago
      what's the fun in that though? i have the impression, that the main motivation for this project was learning the intricacies of TMDS encoding and providing an open implementation as reference for other ppl, who want to learn too. there are too many black box electronics these days, so it's very much welcome to virtually open them up, by providing software equivalents of their operation.
      - szundi 2y ago
        Fun is to proceed to the next fun faster
Palomides 2y ago
can the RP2350 do a higher resolution? it would be a neat option for some retrocomputing stuff if so
- raphlinus 2y ago
  My understanding is that the HSTX can do twice the bit clock as the frequency of the CPU, as opposed to 1x for RP2040. So 800x480 (60Hz) is possible with no overclocking (295MHz bit clock) and Luke said [1] he got 1280x720 50Hz with overclocking (530MHz using CVT-R according to the video timings calculator [2]). [1]: https://mastodon.online/@wren6991@types.pl/112928551221457525 https://mastodon.online/@wren6991@types.pl/11292855122145752... [2]: https://tomverbeure.github.io/video_timings_calculator https://tomverbeure.github.io/video_timings_calculator
  - jauntywundrkind 2y ago
    I wonder what res/refresh Display Stream Compression (DSC) would bring this up to, if this were DisplayPort (instead of DVI)?
    - tverbeure 2y ago
      The lowest you can go with DSC 1.2 is 8 bits per pixel.
- phire 2y ago
  Probably. Especially with the HSTX block which can apparently output at double the system clock (while the rp2040 was limited to 1/10th of the system) If my rough math is correct, it should be possible to do output 720p 60hz video with an overclock to ~320Mhz. Though, actually generating that many pixels might be hard when you have nowhere near enough memory. I would love to see some solid infomation about how well the rp2350 overclocks, apparently 300mhz is easy. 60hz 1080p is almost certainly out of reach, it would require something like a 700mhz overclock, but 30hz 1080p is probably viable.
  - 2y ago
MBCook 2y ago
I think a link to the repo instead of the change set might be better.
- kelnos 2y ago
  I had no idea what this was, ended up clicking around to the README, and found it quite the enjoyable read.
sitkack 2y ago
This is amazing, not only did they port from the 2040 to the 2350, but they also ported all the functionality they could support over to the RV32 cores as well. PicoDVI is usable no matter what cores one chooses to use on the RP2350. Well done!
- jsheard 2y ago
  The author is also the creator of the RISC-V core found in the RP2350, you don't get your own core design taped out as an ASIC and then not show it off :p https://github.com/Wren6991/Hazard3 https://github.com/Wren6991/Hazard3
  - jakobson14 2y ago
    Guy is an employee of raspi, so of course he's going to write code that ships chips.
transpute 2y ago
Is it theoretically possible to have 2-way HID (keyboard, mouse) and 1-way video output on a single USB-c cable? It would simplify connection to a KVM or software-emulated display/keyboard/mouse. Low FPS would be enough for dashboards and CLI.
- jauntywundrkind 2y ago
  There's no current specs for dual directional USB over any cable, afaik. There very well should be. You could present as a device and offer networking and have usb-ip advertised over multicast on that port. Easy, weekend project at most, would be dead obvious to any practitioner. USB4 as a packetized protocol really should offer something. I do wish there were like a half speed 2.5Gbit usb4 option, that microcontrollers could have some hope of accelerated bit banging.
  - FlyingAvatar 2y ago
    Haven't USB hubs done this since USB 1.1?
    - yonatan8070 2y ago
      I believe that the author is talking about plugging in a USB-C cable between your laptop and some headless system, and having your laptop send HID data, while also capturing a display output from the headless system
      - bonzini 2y ago
        Isn't that exactly what a USB-C docking station does? It receives display and audio data, and acts as a USB device (keyboard, mouse, webcam). You'd need hardware that can receive DisplayPort, but it's doable.
        numpad0 2y ago
        Looks like an Alt Mode host with DP sink capability is technically allowed...? But it also looks like USB host controllers with Alt Mode support exposes internal DP source driver inputs, rather than handing you bunch of bare copper wires in Alt Mode, so I doubt it realistically has an implementation. ref: https://vesa.org/wp-content/uploads/2019/10/USB-DevDays-2019-VESA-DP-Alt-Mode-over-USB-Type-C.pdf https://vesa.org/wp-content/uploads/2019/10/USB-DevDays-2019...
wrycoder 2y ago
Bitbanged DVI on the RP2040 Microcontroller
sylware 2y ago
I wish the RP2350 would include a fuse to disable for good the ARM cores for those who want only the risc-v cores (and maybe not pay ARM royalties on those chips).
- jsheard 2y ago
  See pg.1259 in the datasheet, the ARM_DISABLE fuse does exactly that. Putting that in was a smart move because it means they can always make a RISC-V-only variant of the chip without having to tape out new silicon - they just have to blow that fuse at the factory.
  - londons_explore 2y ago
    I'm not sure "arm-core-disabled-by-fuse" is gonna be sufficient to persuade arms legal department to stop charging royalties...
    - axoltl 2y ago
      That's actually a super common way to handle licensing in hard IP. You don't want to spin different revisions of a chip with and without the IP, that's expensive. So you build in fuses to permanently disable the IP. This happens a lot with hardware video encoder/decoder blocks.
      - mschuster91 2y ago
        Or you do it the other way around in software - remember the Pi 1/2 era where you had to buy separate licenses for MPEG/h264 HW decoders, tied to the Pi's serial number?
        sylware 2y ago
        That's why the fuse, with proper sales accounting, is so much important.
  - sylware 2y ago
kristianp 2y ago
In case anyone missed the blog post about the Raspberry Pi Pico 2, here it is: https://www.raspberrypi.com/news/raspberry-pi-pico-2-our-new-5-microcontroller-board-on-sale-now/ https://www.raspberrypi.com/news/raspberry-pi-pico-2-our-new... It has quite a list of products already created by their partners.
peterburkimsher 2y ago
If anyone wants a PicoDVI sock or 4, I had 5 PCBs manufactured and am only using one. I wanted to put the HDMI connector onto the PCB itself, so I could avoid soldering the very small pin pitch. So I took some measurements and did my first KiCad project! https://forum.kicad.info/t/hdmi-pcb-edge-connector/48358/9 https://forum.kicad.info/t/hdmi-pcb-edge-connector/48358/9 Just send me an email if you're interested in having one, and I'd be happy to post it to you. The resistors are already populated, too, you just need to solder on the Pico using the through-hole connections.
amelius 2y ago
Is there any way to select between two HDMI signals? I have an nVidia Jetson but its boot sequence turns off the HDMI output at some point. It would be nice if I could show a placeholder screen while the Jetson is booting. Would something like that be possible by using this RP2350 board?
- ale42 2y ago
  Not directly because it can't handle retransmitting the existing HDMI signal. You probably need to add a HDMI/DVI switch IC like this one: https://www.analog.com/en/products/ad8196.html https://www.analog.com/en/products/ad8196.html
  - psd1 2y ago
    I've tried a few commercial hdmi switches, to share a monitor between machines. The switch time is long and the monitor doesn't reliably wake up. Perhaps it's related to eDID? Do you know how that chip is for real-world usability?
    - kees99 2y ago
      > switch time is long and the monitor doesn't reliably wake up In my personal experience, obtuse monitor's firmware is often to blame. Using a switch with one extra input port, where all data pins are connected to ground (via series resistors), and switching to that port for a brief moment, before switching again to the real intended input, usually helps some.
      - amelius 2y ago
        Isn't it possible to create a valid and stable HDMI signal based on two inputs? I can imagine that some memory is required to synchronize both signals. Is there an IC that can do that?
- dezgeg 2y ago
  I'd be surprised if that was not fixable in the Jetson with pure software changes.
deleted 2y ago
[deleted]
phkahler 2y ago
It says 60 percent of CPU cycles on one core with the other core free. What does the available 40 percent of the one core look like? I'm assuming its got some high frequency interrupts going that can't be disturbed? What restrictions are on using the remaining capacity of the one core?

194 points by fidotron 2y ago

The RP2040 discussion -- https://github.com/Wren6991/PicoDVI https://github.com/Wren6991/PicoDVI

The new HSTX interface on the RP2350 seems to be squarely targeted at this use case (video output) and doesn't require the use of PIO or consuming a ton of CPU cycles. There's a nice write up on the capability here: https://www.cnx-software.com/2024/08/15/raspberry-pi-rp2350-hstx-high-speed-serial-transmit-interface/ https://www.cnx-software.com/2024/08/15/raspberry-pi-rp2350-...

phire 2y ago
The main advantage of the SIO TMDS encoder, is that it allows you to output TMDS on any GPIO, instead of the eight that the HSTX is restricted to. And they allow an easy upgrade path for projects that were already bit-banging DVI on the rp2040. Other than those two advantages, I don't think there is any advantage to the SIO TMDS encoder over the HSTX TMDS encoder. I get the impression that the SIO TMDS encoder was added to the design first, and there wasn't a good reason to remove them after the introduction of HSTX.
jauntywundrkind 2y ago
Isn't hstx mainly good for streaming out? My naive guess would be that it wouldn't have as much transceiver offload capabilities, like what the SIO I naively guess would be good for. The nice part about using SIO seems to be that you can do the tdms encoding there. With hstx you need the output bitstream in the right format already which seems like you might be back to needing the CPU to do the encoding.
- phire 2y ago
  HSTX has a built in TMDS encoder, and as far as I can tell, it has all the functionality of the SIO TMDS encoder. You can configure it to directly consume any line buffer with any pixel format, from 1 to 8 bits per color channel. It even supports formats with different numbers of bits per channel like 8-bit RGB322 and 16-bit RGB565.
  - onetom 2y ago
    what's the fun in that though? i have the impression, that the main motivation for this project was learning the intricacies of TMDS encoding and providing an open implementation as reference for other ppl, who want to learn too. there are too many black box electronics these days, so it's very much welcome to virtually open them up, by providing software equivalents of their operation.
    - szundi 2y ago
      Fun is to proceed to the next fun faster

Palomides 2y ago

can the RP2350 do a higher resolution? it would be a neat option for some retrocomputing stuff if so

raphlinus 2y ago
My understanding is that the HSTX can do twice the bit clock as the frequency of the CPU, as opposed to 1x for RP2040. So 800x480 (60Hz) is possible with no overclocking (295MHz bit clock) and Luke said [1] he got 1280x720 50Hz with overclocking (530MHz using CVT-R according to the video timings calculator [2]). [1]: https://mastodon.online/@wren6991@types.pl/112928551221457525 https://mastodon.online/@wren6991@types.pl/11292855122145752... [2]: https://tomverbeure.github.io/video_timings_calculator https://tomverbeure.github.io/video_timings_calculator
- jauntywundrkind 2y ago
  I wonder what res/refresh Display Stream Compression (DSC) would bring this up to, if this were DisplayPort (instead of DVI)?
  - tverbeure 2y ago
    The lowest you can go with DSC 1.2 is 8 bits per pixel.
phire 2y ago
Probably. Especially with the HSTX block which can apparently output at double the system clock (while the rp2040 was limited to 1/10th of the system) If my rough math is correct, it should be possible to do output 720p 60hz video with an overclock to ~320Mhz. Though, actually generating that many pixels might be hard when you have nowhere near enough memory. I would love to see some solid infomation about how well the rp2350 overclocks, apparently 300mhz is easy. 60hz 1080p is almost certainly out of reach, it would require something like a 700mhz overclock, but 30hz 1080p is probably viable.
- 2y ago

MBCook 2y ago

I think a link to the repo instead of the change set might be better.

kelnos 2y ago
I had no idea what this was, ended up clicking around to the README, and found it quite the enjoyable read.

sitkack 2y ago

This is amazing, not only did they port from the 2040 to the 2350, but they also ported all the functionality they could support over to the RV32 cores as well. PicoDVI is usable no matter what cores one chooses to use on the RP2350. Well done!

jsheard 2y ago
The author is also the creator of the RISC-V core found in the RP2350, you don't get your own core design taped out as an ASIC and then not show it off :p https://github.com/Wren6991/Hazard3 https://github.com/Wren6991/Hazard3
- jakobson14 2y ago
  Guy is an employee of raspi, so of course he's going to write code that ships chips.

transpute 2y ago

Is it theoretically possible to have 2-way HID (keyboard, mouse) and 1-way video output on a single USB-c cable? It would simplify connection to a KVM or software-emulated display/keyboard/mouse. Low FPS would be enough for dashboards and CLI.

jauntywundrkind 2y ago
There's no current specs for dual directional USB over any cable, afaik. There very well should be. You could present as a device and offer networking and have usb-ip advertised over multicast on that port. Easy, weekend project at most, would be dead obvious to any practitioner. USB4 as a packetized protocol really should offer something. I do wish there were like a half speed 2.5Gbit usb4 option, that microcontrollers could have some hope of accelerated bit banging.
- FlyingAvatar 2y ago
  Haven't USB hubs done this since USB 1.1?
  - yonatan8070 2y ago
    I believe that the author is talking about plugging in a USB-C cable between your laptop and some headless system, and having your laptop send HID data, while also capturing a display output from the headless system
    - bonzini 2y ago
      Isn't that exactly what a USB-C docking station does? It receives display and audio data, and acts as a USB device (keyboard, mouse, webcam). You'd need hardware that can receive DisplayPort, but it's doable.
      - numpad0 2y ago
        Looks like an Alt Mode host with DP sink capability is technically allowed...? But it also looks like USB host controllers with Alt Mode support exposes internal DP source driver inputs, rather than handing you bunch of bare copper wires in Alt Mode, so I doubt it realistically has an implementation. ref: https://vesa.org/wp-content/uploads/2019/10/USB-DevDays-2019-VESA-DP-Alt-Mode-over-USB-Type-C.pdf https://vesa.org/wp-content/uploads/2019/10/USB-DevDays-2019...

wrycoder 2y ago

Bitbanged DVI on the RP2040 Microcontroller

sylware 2y ago

I wish the RP2350 would include a fuse to disable for good the ARM cores for those who want only the risc-v cores (and maybe not pay ARM royalties on those chips).

jsheard 2y ago
See pg.1259 in the datasheet, the ARM_DISABLE fuse does exactly that. Putting that in was a smart move because it means they can always make a RISC-V-only variant of the chip without having to tape out new silicon - they just have to blow that fuse at the factory.
- londons_explore 2y ago
  I'm not sure "arm-core-disabled-by-fuse" is gonna be sufficient to persuade arms legal department to stop charging royalties...
  - axoltl 2y ago
    That's actually a super common way to handle licensing in hard IP. You don't want to spin different revisions of a chip with and without the IP, that's expensive. So you build in fuses to permanently disable the IP. This happens a lot with hardware video encoder/decoder blocks.
    - mschuster91 2y ago
      Or you do it the other way around in software - remember the Pi 1/2 era where you had to buy separate licenses for MPEG/h264 HW decoders, tied to the Pi's serial number?
      - sylware 2y ago
        That's why the fuse, with proper sales accounting, is so much important.
- sylware 2y ago

kristianp 2y ago

In case anyone missed the blog post about the Raspberry Pi Pico 2, here it is: https://www.raspberrypi.com/news/raspberry-pi-pico-2-our-new-5-microcontroller-board-on-sale-now/ https://www.raspberrypi.com/news/raspberry-pi-pico-2-our-new... It has quite a list of products already created by their partners.

peterburkimsher 2y ago

If anyone wants a PicoDVI sock or 4, I had 5 PCBs manufactured and am only using one. I wanted to put the HDMI connector onto the PCB itself, so I could avoid soldering the very small pin pitch. So I took some measurements and did my first KiCad project! https://forum.kicad.info/t/hdmi-pcb-edge-connector/48358/9 https://forum.kicad.info/t/hdmi-pcb-edge-connector/48358/9 Just send me an email if you're interested in having one, and I'd be happy to post it to you. The resistors are already populated, too, you just need to solder on the Pico using the through-hole connections.

amelius 2y ago

Is there any way to select between two HDMI signals? I have an nVidia Jetson but its boot sequence turns off the HDMI output at some point. It would be nice if I could show a placeholder screen while the Jetson is booting. Would something like that be possible by using this RP2350 board?

ale42 2y ago
Not directly because it can't handle retransmitting the existing HDMI signal. You probably need to add a HDMI/DVI switch IC like this one: https://www.analog.com/en/products/ad8196.html https://www.analog.com/en/products/ad8196.html
- psd1 2y ago
  I've tried a few commercial hdmi switches, to share a monitor between machines. The switch time is long and the monitor doesn't reliably wake up. Perhaps it's related to eDID? Do you know how that chip is for real-world usability?
  - kees99 2y ago
    > switch time is long and the monitor doesn't reliably wake up In my personal experience, obtuse monitor's firmware is often to blame. Using a switch with one extra input port, where all data pins are connected to ground (via series resistors), and switching to that port for a brief moment, before switching again to the real intended input, usually helps some.
    - amelius 2y ago
      Isn't it possible to create a valid and stable HDMI signal based on two inputs? I can imagine that some memory is required to synchronize both signals. Is there an IC that can do that?
dezgeg 2y ago
I'd be surprised if that was not fixable in the Jetson with pure software changes.

deleted 2y ago

[deleted]

phkahler 2y ago

It says 60 percent of CPU cycles on one core with the other core free. What does the available 40 percent of the one core look like? I'm assuming its got some high frequency interrupts going that can't be disturbed? What restrictions are on using the remaining capacity of the one core?