Author Topic: How to even start routing this?  (Read 6848 times)

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Offline Unixon

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Re: How to even start routing this?
« Reply #25 on: August 02, 2019, 09:41:18 am »
Consider enlarging the board, or using TSSOP devices (which I guess at this speed, should be 74HC for some, and AC or LVC when needed?).
SOIC packages have an underestimated and very useful feature: the ability to let traces go between pads on the same precision class board.
While TSSOP packages occupy less space, they potentially make routing harder.
 
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Offline spudboy488

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Re: How to even start routing this?
« Reply #26 on: August 02, 2019, 11:19:29 am »
Oh and I forgot, for dual, triple, quad and hex packages don't forget you can reassign gates to make routing easier.

The same holds for signals going through bus devices. For example, you don't have to keep D0 through D8 aligned through an 8-bit buffer/latch/etc. as long as the same order is applied on the other side.
 

Offline dmills

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Re: How to even start routing this?
« Reply #27 on: August 02, 2019, 11:56:45 am »
Also, (Static) RAM can usually freely swap address lines and data pins which can be VERY helpful, and is often forgotten.... 

4 Layers for me (It is pretty much my default position these days).

Regards, Dan.
 
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Offline ejeffrey

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Re: How to even start routing this?
« Reply #28 on: August 02, 2019, 12:35:12 pm »

If you do, make one layer a solid ground plane and another a solid Vcc plane. If they are inner planes then it is easier to probe and modify signals. If they are outer planes then EMI problems will be minimised. Choices, choices :)
)

Hardly a choice.  The EMI advantage of outer planes is hardly worth the ease of debug and tracing for what I assume is a one off. Plus for a project that is basically intended to show off old school 74 series logic design, it hard to give up the appearance of a nicely routed board for solid planes.
 

Online voltsandjolts

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Re: How to even start routing this?
« Reply #29 on: August 02, 2019, 01:02:19 pm »
4 layers will save your sanity.

pcbshopper.com is your friend. (no affiliation).
 

Offline tggzzz

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Re: How to even start routing this?
« Reply #30 on: August 02, 2019, 01:22:27 pm »

If you do, make one layer a solid ground plane and another a solid Vcc plane. If they are inner planes then it is easier to probe and modify signals. If they are outer planes then EMI problems will be minimised. Choices, choices :)
)

Hardly a choice.  The EMI advantage of outer planes is hardly worth the ease of debug and tracing for what I assume is a one off. Plus for a project that is basically intended to show off old school 74 series logic design, it hard to give up the appearance of a nicely routed board for solid planes.

Agreed. But the OP hadn't bothered to divulge that information when I wrote that post.
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Online SiliconWizard

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Re: How to even start routing this?
« Reply #31 on: August 02, 2019, 01:35:07 pm »
Just a tip, didn't really look closely how well your autorouter did, but if using autorouting, I would first hand-route all bypass caps and power supplies, then let the autorouter handle the rest. Autorouters can do a pretty atrocious job at routing supplies, and often like to create horrible loops, especially on a 2-layer board.

Of course to be half successful at routing this, placement will be key.
 

Offline Joanna_HTopic starter

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Re: How to even start routing this?
« Reply #32 on: August 02, 2019, 01:43:37 pm »

If you do, make one layer a solid ground plane and another a solid Vcc plane. If they are inner planes then it is easier to probe and modify signals. If they are outer planes then EMI problems will be minimised. Choices, choices :)
)

Hardly a choice.  The EMI advantage of outer planes is hardly worth the ease of debug and tracing for what I assume is a one off. Plus for a project that is basically intended to show off old school 74 series logic design, it hard to give up the appearance of a nicely routed board for solid planes.

Yeah, it's one off, showing off old 74 series, easier for someone looking at it to comprehend (well, the ic's at least.)   
 

Offline Joanna_HTopic starter

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Re: How to even start routing this?
« Reply #33 on: August 02, 2019, 01:48:49 pm »
4 layers will save your sanity.

pcbshopper.com is your friend. (no affiliation).

Thanks for the link..  cheapest for a 2 layer with delivery is about £20, cheapest for a 4 layer with delivery is about £40

So ... hmmm...
 

Online mikeselectricstuff

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Re: How to even start routing this?
« Reply #34 on: August 02, 2019, 03:03:00 pm »
Placement
Placement
Placement
Gate and pin-swapping for multi-gate packages, swapping gates between packages where possible.

Do you really need to keep it to two layers ?
How many of these are you making? - unless it's thousands then your time getting to 2 layers is probably worth mor than the additonal cost of 4 layers , bearing in mind that it can probably be smaller on a 4L PCB as well.
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Online mikeselectricstuff

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Re: How to even start routing this?
« Reply #35 on: August 02, 2019, 03:04:37 pm »
And as there is RAM, bear in mind that you can swap around any address and data pins to help layout.
 
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Offline Joanna_HTopic starter

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Re: How to even start routing this?
« Reply #36 on: August 02, 2019, 03:23:28 pm »
Placement
Placement
Placement
Gate and pin-swapping for multi-gate packages, swapping gates between packages where possible.

Do you really need to keep it to two layers ?
How many of these are you making? - unless it's thousands then your time getting to 2 layers is probably worth mor than the additonal cost of 4 layers , bearing in mind that it can probably be smaller on a 4L PCB as well.

I'm just having fun with a little project, doesn't have to be 2 layers, etc..   I'm just trying to keep costs down as it'll probably take a few revisions to debug it all.  It's also why I restricted each module size to 100x100mm, to keep costs down.   Yeah, I could have made it a lot cheaper by using a FPGA but that defeats the point of trying to create it out of 74 series.. :D

It's really just a mini (well not so mini any more) project.
 

Offline Joanna_HTopic starter

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Re: How to even start routing this?
« Reply #37 on: August 02, 2019, 03:48:32 pm »
It's all part of a bigger modular project as described earlier, so it's just lots of 100x100mm boards connected together.   Been going through doing each part and then reliased a graphical output would be nice.  So put that thing together, probably will get changed again, but not sure.  Other stuff has been done, but again needs to be manually routed, it's mostly auto atm just to get an idea of what is going on.

 

Offline KE5FX

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Re: How to even start routing this?
« Reply #38 on: August 02, 2019, 05:04:01 pm »
Don't bother with a power plane.

But... Why?? Do you hate power integrity? Do you appreciate high frequency noise? What? This will easily fit on 4-layers with GND and PWR planes.. Why not do it 'right'?

Power for components in this class that are mounted on boards of that size comes from capacitors, not planes.  She's got plenty of bypassing, but not enough routing area.

One nice thing about power planes, however, is that they can be used to maintain copper symmetry.  The board will less prone to warpage if large planes are balanced from top to bottom.   If the power plane is omitted, it's good to maintain a lot of ground islands for that reason.  Don't remove copper unnecessarily, in other words.... but don't ascribe any magical electrical properties to it, either. 
 

Offline jmelson

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Re: How to even start routing this?
« Reply #39 on: August 02, 2019, 06:45:23 pm »
Forgot to say, it's a two layer board.
2 LAYERS?  The power and ground nets, if made with even slightly thicker traces, will fill almost the entire area, leaving no space for signal routing.  I really don't think you can do it.
If this is all digital stuff, why not use an FPGA?  You might be able to do it in one chip.

Jon
 

Offline T3sl4co1l

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Re: How to even start routing this?
« Reply #40 on: August 02, 2019, 10:41:33 pm »
Yikes! EMI-wise, you're probably slightly better off building this on solderless breadboard with a metal backplane.  It absolutely will ruin your design: I've breadboarded a mere Z80 + RAM + couple bus latches system on one solderless breadboard panel, and had it freeze up at random times due to poor signal quality.  (The randomness was due to implementing an LFSR in the main loop; with the usual loop (an LED display routine), it was fine.)

The resistor placements look like they were determined based on spacial balancing (i.e., a row of this here, a row of that there..), with no regard for where they connect?

Tim
« Last Edit: August 02, 2019, 10:46:30 pm by T3sl4co1l »
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Offline IconicPCB

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Re: How to even start routing this?
« Reply #41 on: August 02, 2019, 11:20:37 pm »
I would like to let Topor chew on this and see what it spits out.

If You are Ok with this please send pcb file.
 

Offline Joanna_HTopic starter

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Re: How to even start routing this?
« Reply #42 on: August 03, 2019, 03:01:38 pm »
I'm seriously considering abandoning the TTL approach, it's fun, it's challenging, but I just went throught the BOM of what I've developed so far and lets just say that it's starting to get pricey....   lots of different boards, lots of B2B connectors, lots and lots of TTL chips.   Adds up quickly.

I'm starting to think I could achieve the same goal using a number of FGPA's (cheaper ones) that would 1) lead to lower cost, 2) require less PCB's and 3) a hell of a lot less routing!
 

Offline rstofer

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Re: How to even start routing this?
« Reply #43 on: August 03, 2019, 06:30:04 pm »
Without a BOM, it's pretty hard to see how much logic is involved with the project.  If most of those chips are MSI, I think the entire project will fit in a small FPGA.  Perhaps we're not seeing the entire project, just one sub-assembly.

A smallish FPGA will have millions of gate equivalents.  Think of an FPGA as a dumpster full of logic with a bunch of RAM on the side.

The bigger problem with the FPGA is getting all the external signals to fall in the range acceptable to the chip.  These days that is usually 3.3V signals.

A separate issue is packaging.  Most of the FPGAs use BGA packaging and I'm not up for that.  But I can buy 'stamp format' boards and just solder them in to a daughter card.

I might consider something like the A7-35T variant if I wanted a 'stamp format' board:
https://store.digilentinc.com/cmod-a7-breadboardable-artix-7-fpga-module/

All the hard work is done, all I have to do is handle the inputs and outputs and write a little code.
 

Online mikeselectricstuff

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Re: How to even start routing this?
« Reply #44 on: August 03, 2019, 07:08:00 pm »
A separate issue is packaging.  Most of the FPGAs use BGA packaging and I'm not up for that.  But I can buy 'stamp format' boards and just solder them in to a daughter card.

There are plenty of FPGAs in QFP, which isn't a big deal to hand solder
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Online SiliconWizard

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Re: How to even start routing this?
« Reply #45 on: August 03, 2019, 08:23:10 pm »
Sure. We don't exactly know what the complete circuit is, but it may perfectly fit in a MachXO2. If something a bit beefier is required, you can go for a Spartan 6 LX4/LX9 - all available in QFP.
 

Offline amyk

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Re: How to even start routing this?
« Reply #46 on: August 03, 2019, 08:46:04 pm »
I didn't read each of the replies in detail to see if this was already posted: Think about how your signals flow and use that as a guide to placement. It may sound obvious but the more net crossings you eliminate, the easier it gets to route.
 

Offline Joanna_HTopic starter

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Re: How to even start routing this?
« Reply #47 on: August 03, 2019, 09:34:18 pm »
Sure. We don't exactly know what the complete circuit is, but it may perfectly fit in a MachXO2. If something a bit beefier is required, you can go for a Spartan 6 LX4/LX9 - all available in QFP.

I was looking at the MachX02 ( I think these are closer to CPLD than FPGA) , nice cheap little chip.  Could fit a hell of a lot of logic in one of those :D

I could still keep the modular design but massively reduce the chip count.   Would mean switching to 3v3 though.   But no biggy.  Should be fun, haven't touched verilog since the 90's lol
« Last Edit: August 03, 2019, 09:38:38 pm by Joanna_H »
 

Online mikeselectricstuff

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Re: How to even start routing this?
« Reply #48 on: August 03, 2019, 10:07:22 pm »
Nice thing about the XO2 is it has onboard config memory, core voltage regulator and also an oscilllator, so minimal extra parts. A few package options from QFN32 to QFP144.
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Offline rstofer

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Re: How to even start routing this?
« Reply #49 on: August 03, 2019, 10:09:28 pm »
Start by selecting a small chip and write the code.  If it won't fit, move up a notch.  Unless you want to do a lot of extra work on the constraints file (pinout definition), just do synthesis and check for fit.  You can use simulation without defining the pinout, I think.

I tend to go the other way:  I buy a big chip and leave a lot of empty logic laying around.  I only have experience with Xilinx devices from Spartan II up into Artix 7.
 


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