5 ms·

At a 500kHz clock rate, with reasonable rise/fall times, the electrical effect of some 0.1" headers is minuscule. "Microstrip transmission lines" are enormous o

At a 500kHz clock rate, with reasonable rise/fall times, the electrical effect of some 0.1" headers is minuscule. "Microstrip transmission lines" are enormous overkill.

segfaultbuserr 3y ago
Unless you want to get a beautiful spectrum on a EMI/EMC test receiver, but it's certainly not relevant in this project... Building a discrete CPU at some MHz and passing modern EMI/EMC tests sound like a fun (and somewhat expensive [1]) nerd-snipping project to do. [1] The expense of EMI/EMC tests at an actual lab is well-known. Doing it in a home lab at a much lower expense is possible with pre-compliance tools like a TEM cell, a broadband antenna and a spectrum analyzer, but these equipment still costs a few thousand dollars.
- mips_r4300i 3y ago
  If he uses HC logic and is careful to keep traces pulled back from the edge of the pcbs, and also carefully deals with the interconnects (those might be the killer) then he could probably pass EMC. Maybe some series termination on the clock tree. At the speed it's running, the edge rates of the IO drivers and the input/output of the entire thing are the problem.
  - segfaultbuserr 3y ago
    A simple improvement of the interconnect is using two rows of pin headers, one rows for signals and another rows for dedicated grounds. This should significantly reduce the loop area. I once did a quick simulation and found that even controlled impedance is somewhat possible with 1.27 mm headers, although I haven't done any experiment with it (yet)... Another possible modification is converting all I/Os to differential signaling before they leave the board, a classic use is eliminating the radiation from long ribbon cables. Ribbon cables with ground planes also do exist if one's willing to pay...
dragontamer 3y ago
"Can do" vs "must do" is the key here. On a PCB, you "can" or you "may" perform advanced transmission line analysis upon your PCB traces. I don't think that a project like this needs much more than a 2-layer board (so microstrips are definitely overkill), but that doesn't change the fact that PCB parasitics analysis is basically a solved problem today and available on a lot of software packages. Anyway, my overall point is that large contiguous blocks of PCB are easier than thinking about connector issues (or for the matter: being forced to solder and manage all those connectors or headers). Not only is it easier to go header-free (and use a larger PCB), its better engineering due to tighter specs and analysis available. ------------ Now a bit of connector doodads and having fun is probably fun and all. But a stackup of 9x PCBs is reaching the point of absurdity. I can't think of any good engineering reason to have so many connectors. That's 9-separate PCBs on this design, meaning at a minimum, 9-separate ground planes on this design. Is that... good engineering? I don't think so. Its horribly complex but for seemingly no appreciable reason. Maybe sound / analog circuits go on a separate board, but I'm not seeing any sound here... ------------ That being said: I agree that at 500kHz maybe this level of analysis is not needed. But... on the other hand... these 74xxxx chips all have 30ns rise/fall times, so its not outside the realm of reality to be running this computer at 5MHz or 10MHz or so, 10x to 20x faster. At those higher speeds, you'll possibly need to think about these issues a bit more (or perhaps... more appropriately I should say... the author of this project "could have" aimed at a higher-speed target if they so chose)
- amelius 3y ago
  What open source software would you recommend for PCB parasitics analysis?
  - dragontamer 3y ago
    I dunno if anything is open source here... but the most important parameters are available on Javascript applications all over the place. https://saturnpcb.com/saturn-pcb-toolkit/ https://saturnpcb.com/saturn-pcb-toolkit/ I don't think this tool is open source, but via inductance, various heat-equations, trace-width calculations and the like are available on this tool. Its my understanding that higher-end PCB software integrates tools like these directly into the PCB-designers. --------- KiCAD has simple calculators like the SaturnPCB.com one (though with fewer features). I'm not sure if more advanced calculators exist though.

At a 500kHz clock rate, with reasonable rise/fall times, the electrical effect of some 0.1" headers is minuscule. "Microstrip transmission lines" are enormous overkill.

segfaultbuserr 3y ago

Unless you want to get a beautiful spectrum on a EMI/EMC test receiver, but it's certainly not relevant in this project... Building a discrete CPU at some MHz and passing modern EMI/EMC tests sound like a fun (and somewhat expensive [1]) nerd-snipping project to do. [1] The expense of EMI/EMC tests at an actual lab is well-known. Doing it in a home lab at a much lower expense is possible with pre-compliance tools like a TEM cell, a broadband antenna and a spectrum analyzer, but these equipment still costs a few thousand dollars.

mips_r4300i 3y ago
If he uses HC logic and is careful to keep traces pulled back from the edge of the pcbs, and also carefully deals with the interconnects (those might be the killer) then he could probably pass EMC. Maybe some series termination on the clock tree. At the speed it's running, the edge rates of the IO drivers and the input/output of the entire thing are the problem.
- segfaultbuserr 3y ago
  A simple improvement of the interconnect is using two rows of pin headers, one rows for signals and another rows for dedicated grounds. This should significantly reduce the loop area. I once did a quick simulation and found that even controlled impedance is somewhat possible with 1.27 mm headers, although I haven't done any experiment with it (yet)... Another possible modification is converting all I/Os to differential signaling before they leave the board, a classic use is eliminating the radiation from long ribbon cables. Ribbon cables with ground planes also do exist if one's willing to pay...

dragontamer 3y ago

"Can do" vs "must do" is the key here. On a PCB, you "can" or you "may" perform advanced transmission line analysis upon your PCB traces. I don't think that a project like this needs much more than a 2-layer board (so microstrips are definitely overkill), but that doesn't change the fact that PCB parasitics analysis is basically a solved problem today and available on a lot of software packages. Anyway, my overall point is that large contiguous blocks of PCB are easier than thinking about connector issues (or for the matter: being forced to solder and manage all those connectors or headers). Not only is it easier to go header-free (and use a larger PCB), its better engineering due to tighter specs and analysis available. ------------ Now a bit of connector doodads and having fun is probably fun and all. But a stackup of 9x PCBs is reaching the point of absurdity. I can't think of any good engineering reason to have so many connectors. That's 9-separate PCBs on this design, meaning at a minimum, 9-separate ground planes on this design. Is that... good engineering? I don't think so. Its horribly complex but for seemingly no appreciable reason. Maybe sound / analog circuits go on a separate board, but I'm not seeing any sound here... ------------ That being said: I agree that at 500kHz maybe this level of analysis is not needed. But... on the other hand... these 74xxxx chips all have 30ns rise/fall times, so its not outside the realm of reality to be running this computer at 5MHz or 10MHz or so, 10x to 20x faster. At those higher speeds, you'll possibly need to think about these issues a bit more (or perhaps... more appropriately I should say... the author of this project "could have" aimed at a higher-speed target if they so chose)

amelius 3y ago
What open source software would you recommend for PCB parasitics analysis?
- dragontamer 3y ago
  I dunno if anything is open source here... but the most important parameters are available on Javascript applications all over the place. https://saturnpcb.com/saturn-pcb-toolkit/ https://saturnpcb.com/saturn-pcb-toolkit/ I don't think this tool is open source, but via inductance, various heat-equations, trace-width calculations and the like are available on this tool. Its my understanding that higher-end PCB software integrates tools like these directly into the PCB-designers. --------- KiCAD has simple calculators like the SaturnPCB.com one (though with fewer features). I'm not sure if more advanced calculators exist though.