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| Complete newcomer to PCB layout - where do I start? |
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| tautech:
--- Quote from: nctnico on November 26, 2021, 01:55:53 am --- --- Quote from: tautech on November 25, 2021, 11:44:00 pm ---The trick to having the CAD program do all the work for you is to specify the correct power Net for IC supply pins and although some CAD programs hide IC supply pins when schematics are transferred to the PCB all the Net connections will be correct it you've set it all up correctly. --- End quote --- Back in the days when logic was powered from 5V, -5V and -2V power nets and hidden power pins where a good thing because you didn't need to draw all the power pins. Nowadays hidden power pins are horrible. Even a simple digital circuit with 3 chips (each 48 pins or less) can have a 4 or 5 power nets. --- End quote --- No worries if you assign them all to the correct IC pins. Who on earth needs to see power rail connectivity in a schematic ? :-// Assign rail nets/flags to the appropriate pins and supply rails and anyone can follow it. |
| bson:
--- Quote from: eti on November 26, 2021, 01:22:41 am ---but I’m still at a loss as to the specifics of what these “power flags” do, and when it’s apt to use them. Any help would be useful, as YouTube is hit and miss as regards finding a specific guide. Thanks --- End quote --- If you open the symbol editor (ctrl-e or cmd-e while hovering over the symbol) and then look at the pins (with the E key) you'll see pins say things like "input", "output", "power input", "bidirectional", "no connection" etc. Most ICs have power inputs. You'll connect these to say the GND and VCC nets, which then connect to a header someplace where you intend to hook up power. But the header pins are probably all defined as "passive" meaning they have no function associated with them. So you end up with a lot "power inputs" on a power net, with no "power output" to drive it. The power flag silences this particular rule violation (a bunch of power inputs with no output anywhere on the net). A fancier solution is to clone the header symbol and create your own, change pin functions on the header to reflect your actual use, and then use this symbol instead of the stock header. This also has the added advantage that you won't accidentally say connect the GND header pin to your VCC net or vice versa, which is easily done if you just drop down power flags willy nilly. |
| PlainName:
--- Quote ---Who on earth needs to see power rail connectivity in a schematic ? --- End quote --- How else do you know which is the power pin without either seeing it there or looking it up in the datasheet? When debugging some hardware, an unambiguous connection to a specific signal/rail is what I need. Further, with battery and low-power stuff the power pin may not actually be on a power rail, but if it's hidden it's not simple to see that. One solution is to have a table of pins vs rail off to one side, but that suffers from the same thing as comments in code: you gotta keep remembering to do maintenance on it. And you still have to actually say "This pin connects to that rail" when you really meant 'the other rail' but since no-once can see it after that you won't find out until the hardware doesn't work. |
| tautech:
--- Quote from: dunkemhigh on November 26, 2021, 12:28:29 pm --- --- Quote ---Who on earth needs to see power rail connectivity in a schematic ? --- End quote --- How else do you know which is the power pin without either seeing it there or looking it up in the datasheet? When debugging some hardware, an unambiguous connection to a specific signal/rail is what I need. Further, with battery and low-power stuff the power pin may not actually be on a power rail, but if it's hidden it's not simple to see that. One solution is to have a table of pins vs rail off to one side, but that suffers from the same thing as comments in code: you gotta keep remembering to do maintenance on it. And you still have to actually say "This pin connects to that rail" when you really meant 'the other rail' but since no-once can see it after that you won't find out until the hardware doesn't work. --- End quote --- Again I ask, please take the trouble to incorporate the poster into your Copy/Paste quotes. Isn't the PSU section also part of the set of schematics ? Well it certainly should be and in doing so every power rail will be labelled and if the designer intended the device to be repairable power rails will be labelled too. There are choices on how to do this if the designer wants to take the trouble to help with a properly serviceable design and a overlay is the most common used in good designs but marked test points or even symbols in the copper are used if there is sufficient space on the layout. All these features are in CAD tools if we take the trouble to use them...... Once you've done a few designs you know what things you need to include into both the schematic and layout to qualify as a good serviceable and easy to follow design. However as PCB designs get more compact for whatever reason much of what makes a good design is omitted to fit miniaturization requirements or lower the call on the worlds resources. However for commercially made products too much info on the PCB gives away some of the manufacturers IP........ |
| nctnico:
There are two things that need to be seperated here: 1) Hidden power pins: in modern designs these are a nuisance. Don't do this because it makes a circuit harder to debug due to the many different supply voltages and package variations in existence nowadays. 2) Draw all lines for a power net: don't do this, use power net symbols for this purpose or net labels if you want to save even more space. |
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