Wednesday, March 03


Daily News Stuff 3 March 2021

300% Overclock Edition

Tech News

  • Reading through the details of the PicoDVI project from yesterday I'm struck by how simple (relatively) it is to encode low-resolution graphics into a TMDS signal - the encoding used by DVI and HDMI.

    The project itself uses RGB pixels rather than a palette, which makes the code more complicated and slower.

    The hard part is sending out three synchronised bitstreams at 251.75MHz, but the Pi Pico can do that, if you first overclock it to 251.75MHz, which it can also do.  I don't know of any other microcontrollers that can do that - the clock-accurate encoding part, I mean - previously you used an external FPGA if you needed something like that.

    I want 480x360 rather than 640x480, and I don't know how well that will work, though there is no defined minimum resolution for DVI or HDMI.  It would still require a small overclock for a 60Hz refresh, but 6% rather than nearly 100%.

    The great thing though is I don't need to build anything to try it, just wait until one of these projects ships, then plug it into a cheap monitor and start fiddling with the timing parameters.

    For a real project I'd make a small board with two RP2040s - the Pi Pico chip - connected over SPI, and just a couple of SPI flash chips, a voltage regulator, and other minor things.  Simple enough when you know someone who does this for  living.

    VGA is even easier, of course - you don't have to do all that nasty encoding, which leaves headroom to actually draw stuff on screen.

  • At the other end of the scale the Threadripper Pro is now shipping to retail customers.  (AnandTech)

    Prices range from $1150 for 16 cores to $5490 for 64 cores.

    The Asus Threadripper Pro WS WRX80E-SAGE SE WIFI motherboard will set you back an additional $1000 or so, which is not a bad price given its feature set, which ranges from dual 10G Ethernet and remote management to seven PCIe 4.0 x16 slots, seven M.2 slots, and audio with a claimed 120dB SNR.  About the only thing it lacks is Thunderbolt 4.

    This is all still Zen 2, though; there are no Zen 3 Threadrippers or Epycs available at retail yet.

  • The Asus ProART B550-Creator does have Thunderbolt 4.  (Tom's Hardware)

    The only AMD motherboard I know of that does, though there are a handful with Thunderbolt 3.

    It maxes out at 16 cores and 128GB of RAM, only has one PCIe 4.0 x16 slot and two M.2 slots (and the second would be PCIe 3.0 since it's a B550 board), and the dual Ethernet ports are only 2.5G, but that's still a solid little platform.

  • The Radeon 6700 XT will be launching in four hours.  (WCCFTech)

    That's 3AM here.  Not staying up.

    The one thing I'm interested in is how much Infinity Cache it has - if any.  The 6800 and 6900 models have 128MB; leaks indicate this might have 96MB.  That should be enough for 1440p gaming, and could also hint at where AMD is going next with its APUs.

  • The US Navy is a bunch of pirates.  (TorrentFreak)

    Specifically, they have been found liable in a case before the Federal Circuit.  Given that they did in fact install the software in question on half a million computers without paying for it, this seems like a reasonable ruling.

    Of course, they won't pay for the (up to) $500 million in damages either.

Disclaimer: And neither will I.  At least, not directly.

Posted by: Pixy Misa at 10:16 PM | Comments (6) | Add Comment | Trackbacks (Suck)
Post contains 597 words, total size 5 kb.

1 The Pi Pico's just a Cortex M0+, albeit the fastest one I've seen (of the limited number of Cortex M MCU's I've looked at.)  M7s run up to 480MHz (actually I think there's a few by NXP that run at 600MHz) so it's probably possible for any of them to do this.
I think I spent $20 on an STM32H743VIT6 board:  480MHz max speed, a meg of RAM, 2MB of flash, and it comes with an SPI and a QSPI flash and a micro SD card slot (which apparently supports the full 4-wire interface, not just the normal 1-wire one you see on lower-end chips).

Posted by: Rick C at Thursday, March 04 2021 04:13 AM (eqaFC)

2 Huh.  Poking around, I see that the STM32F469 supports 720p30 displays via MIPI-DSI (the page 1 summary on the datasheet says "XGA resolution" which is normally 1024x768.) "The LCD-TFT display controller provides a 24-bit parallel digital RGB (Red, Green, Blue) and delivers all signals to interface directly to a broad range of LCD and TFT panels up to XGA (1024x76cool resolution".  It runs up to 180MHz.  Not quite the same as DVI but even higher resolutions than they got out of the Pi Pico. smile

Posted by: Rick C at Thursday, March 04 2021 04:22 AM (eqaFC)

3 Yep, there are a number of STM controllers that do video out over a parallel bus, and you can convert that into VGA up to about 15 bits of colour with just a handful of resistors.  That's what I was looking at doing previously.

But if you want to get DVI or HDMI you need a separate encoder chip - either a specific chip or a small FPGA - and a custom board.  The Pico is the first chip I've seen that has the necessary cycle-accurate control over I/O.

Posted by: Pixy Misa at Thursday, March 04 2021 10:40 AM (PiXy!)

4 Not sure if we're talking about the same thing or not.  Here's an example of a 180MHz STM32F4 driving a 320x240 LCD directly (not using the display's built-in ILI9341 controller, apparently.  The article--and the H742's reference manual) are full of charts talking about dot clocks and porches.)
I'm kind of talking out of my hat here, and I'll admit it, but I wouldn't be surprised if a typical Cortex-M with a fast enough clock couldn't also do the DVI signal.  I haven't heard anything about the Pi Pico being so much better than existing Cortex-M MCUs.  (And the 500-6000MHz ones seem like they should be able to even drive a higher resolution.)
Either way, it's really cool that anything that small can drive that level of signal.

Posted by: Rick C at Thursday, March 04 2021 12:47 PM (eqaFC)

5 LOL, now you got me curious.  The STM32MP1, a dual-core microprocessor (650MHz A7 and an M4 coprocessor) can output HDMI.  The bare chip is $16 QTY 1, which is admittedly getting pricey.

Posted by: Rick C at Thursday, March 04 2021 12:50 PM (eqaFC)

6 The thing about DVI (and HDMI, for basic stuff the signals are exactly the same) is that they are serial interfaces, rather than parallel, and require 10 bits per channel per pixel.

So for a standard 640x480 VGA display, which has a pixel clock of 25MHz, you three serial outputs clocked at 250MHz.

The LTDC on STM chips is a parallel interface, fine for VGA and directly connecting to LCD panels, but you need an external chip for DVI or HDMI.  Or...  Do something really clever with the pixel encoding.  I think that includes the STM32MP1.

Now that I've seen how it's done on the PICO, I can see how to generate DVI signals with a CPLD rather than an FPGA - smaller, cheaper, simpler chips that you don't need to reprogram on power-up.

If you have, say, a 32 colour palette, each entry is a 30-bit value, which is the TMDS-encoded data rather than the RGB values.  I think it would need two sets of values for odd and even pixels because of the TMDS sequencing.  The CPLD would just have shift registers sending out the colour and clock signals....  At several hundred MHz.

Posted by: Pixy Misa at Thursday, March 04 2021 02:42 PM (PiXy!)

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Apple pies are delicious. But never mind apple pies. What colour is a green orange?

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