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Thread: Localtalk bridge


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Permlink Replies: 144 - Pages: 10 [ Previous | 1 ... 4 5 6 7 8 9 10 | Next ] - Last Post: Apr 16, 2018 12:39 AM Last Post By: fraveydank
bigmessowires


Posts: 217
Registered: 10/29/13
Re: Localtalk bridge
Posted: Dec 14, 2014 11:36 AM   in response to: dougg3 in response to: dougg3
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Looks pretty good to me. It's surprising how few of the pins you're using on each chip.

What software do you use for designing PCBs? I've been using Eagle for a long time, but tried KiCad for my most recent board. They're both OK, I guess, but I wouldn't say I actually like either of them.
dougg3

Posts: 190
Registered: 8/13/12
Re: Localtalk bridge
Posted: Dec 14, 2014 2:33 PM   in response to: bigmessowires in response to: bigmessowires
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Attachment FinalCape.png (126.3 K)
Yeah...the SCC has two channels and I'm only using one, so there are a lot of unused pins. Oh, and the PLCC version of the SCC has several NC pins. The LPC only has to talk to the SCC, generate an 8 MHz clock, and use a couple of UART pins. There is actually a smaller 48-pin variant of the LPC chip, but nobody stocks it so I went with the 64-pin version.

I'm using Eagle. I paid for the cheap hobbyist version. The main reason I use it is because it's what they use at work so I'm comfortable in it. I don't design PCBs at work but I sometimes have to look at the schematics to figure something out. I really want to learn KiCad, but I totally agree. PCB software in general sucks...

I added an LED, moved a few things around for better routing, and finished. Just placed an order with OSHPark! OSHPark claims to allow non-rectangular board sizes, but I don't think their previewer works correctly with them. Hopefully it gets cut out as requested. I've never tried a non-rectangular board from them before. Their previewer also doesn't show the board title, which I did as an image so I wouldn't be stuck with the ugly Eagle vector font. Hopefully that prints out OK.

MacTjaap


Posts: 14
Registered: 11/22/14
Re: Localtalk bridge
Posted: Dec 14, 2014 6:08 PM   in response to: dougg3 in response to: dougg3
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Looks very professional! I will read carefully how you build your first one!
techknight

Posts: 110
Registered: 10/13/12
Re: Localtalk bridge
Posted: Dec 14, 2014 8:49 PM   in response to: MacTjaap in response to: MacTjaap
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Looks like you might have a couple pins off on your ADB connector, drooping?
dougg3

Posts: 190
Registered: 8/13/12
Re: Localtalk bridge
Posted: Dec 14, 2014 8:56 PM   in response to: techknight in response to: techknight
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It's not an ADB connector, it's a serial port. Do you mean how some of the pins are out of alignment from the rest? That's the standard footprint of that connector, already tested on my breakout board. I was surprised when I first saw it too :-)
techknight

Posts: 110
Registered: 10/13/12
Re: Localtalk bridge
Posted: Dec 14, 2014 9:03 PM   in response to: dougg3 in response to: dougg3
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ahh.. ok, Yea I was wondering. that is kinda odd ;-)
uniserver


Posts: 53
Registered: 12/11/14
Re: Localtalk bridge
Posted: Dec 15, 2014 2:48 PM   in response to: techknight in response to: techknight
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I'm thinking, I should should buy the base board, to use these bad ass break out's you keep making!!!
dougg3

Posts: 190
Registered: 8/13/12
Re: Localtalk bridge
Posted: Dec 16, 2014 12:30 AM   in response to: uniserver in response to: uniserver
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Go for it! Who knows what other crazy BeagleBone Black capes we might think of in the future? :-)
techknight

Posts: 110
Registered: 10/13/12
Re: Localtalk bridge
Posted: Dec 16, 2014 10:29 PM   in response to: dougg3 in response to: dougg3
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Well, it was a beaglebone black cape that dumped the MFM macbottom drive.
dougg3

Posts: 190
Registered: 8/13/12
Re: Localtalk bridge
Posted: Dec 26, 2014 6:39 PM   in response to: techknight in response to: techknight
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Attachment LocalTalkBoard.jpg (104.3 K)
Look at what arrived in the mail today :-)



BBB, which I already had, is there for reference. So far, so good! The PCB appears to be the right side and I think the holes are in the right place.
dougg3

Posts: 190
Registered: 8/13/12
Re: Localtalk bridge
Posted: Dec 27, 2014 2:01 AM   in response to: dougg3 in response to: dougg3
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Got the LPC11U68 portion of the circuit put together. It works! I'm blinking the LED.



Unfortunately the LPC11U68 is intermittently not responding to me when I tell it to go into its on-chip ISP command handler by pulling P0.1 down before powering it up. Sometimes it doesn't respond to me after entering the command handler. There's another pin (P0.3) it samples to decide to go into the UART ISP handler or a USB bootloader when P0.1 is low on boot -- I have that pin pulled low with a 100k pulldown to tell it to go into the UART handler. I suspect it's going into the USB handler when it fails, but I don't know for sure.

Before I panic, I'm going to test it on the actual BBB which will control the LPC11U68's !RESET pin instead of leaving it floating (which is OK to do on this chip, BTW). I suspect that once I'm actually in control of all the pins, it might behave nicer. It's also possible I have an intermittent connection somewhere.

Anyway, yay! Now onto assembling the rest of the circuit and porting the software over to the LPC11U68 (which I don't anticipate will take much effort)
bigmessowires


Posts: 217
Registered: 10/29/13
Re: Localtalk bridge
Posted: Dec 27, 2014 12:15 PM   in response to: dougg3 in response to: dougg3
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dougg3 wrote:
Before I panic, I'm going to test it on the actual BBB which will control the LPC11U68's !RESET pin instead of leaving it floating (which is OK to do on this chip, BTW). I suspect that once I'm actually in control of all the pins, it might behave nicer. It's also possible I have an intermittent connection somewhere.

Looks great!

If there are floating inputs (other than reset), I could definitely see that causing weird problems. A 100K pulldown might also be too large of a resistance, so the voltage climbs above the V input low threshold. Have you checked the voltage at the input pin, when that resistor is in place? Maybe try a 10K resistor instead, or just a direct connection to GND.
dougg3

Posts: 190
Registered: 8/13/12
Re: Localtalk bridge
Posted: Dec 28, 2014 12:05 AM   in response to: bigmessowires in response to: bigmessowires
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Thanks for the advice BMOW! Actually, I didn't even bother checking the voltage...what you suggested made a lot of sense. The 100K pulldown was too weak. I found that shorting across it temporarily was making it work every time without problems. I changed it to a 10K, and now it's working fine every time. I've never really understood how people choose pull-up/pull-down resistor values, but it seems like 100K is too weak for what I'm doing.

I think I should add a pull-down resistor to the reset pin of the LPC11U68 just to be safe as well -- keep it in reset until I explicitly tell it to go out of reset. I notice that the main program runs for a second or two at power-up before something happens that puts it back into reset, which kind of bothers me.

The rest of the board is assembled (in the process, I remembered that I suck at soldering PLCC chips).



Anyway, now I need to get LPC11U68 flashing working from the BBB, and then port my old LPC1769 software over! Shouldn't be too hard -- there are already some open-source tools for LPC flashing from Linux, and I've written one from scratch at work before.
dougg3

Posts: 190
Registered: 8/13/12
Re: Localtalk bridge
Posted: Dec 28, 2014 1:15 PM   in response to: dougg3 in response to: dougg3
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Just for completeness, after changing the resistor I went ahead and measured. The voltage across the 10k pull-down is now 0.586 volts, so the pin is being pulled down to 0.586 volts. This is well within the acceptable range of 0 to 0.3*VDD for a low value (VDD=3.3V, so 0.3*VDD = 0.99V). It was probably in limbo before with the 100k resistor.

I'm looking at the LPC11U68 datasheet to determine how I can avoid this mistake in the future, and thought I'd share here:

  • The pin I'm pulling down has an internal pull-up enabled by default.
  • Pull-up voltage for internal pull-ups is 3.3V (had to check, because the LPC1769 only pulls pins up to 2.8V with its internal pull-ups)
  • Pull-up current on 0V input (Ipu) for internal pull-ups is -10 to -85 uA, typically -50 uA (I think the negative sign is just indicating direction of current in this case)

I don't know if it's totally valid to do the calculation like this, but it probably gives me a ballpark figure at least. If I use ohm's law it looks like it's essentially the effect of a 66k resistor (3.3V/0.000050A), but it can vary between 39k and 330k. In series with my 100k resistor as a voltage divider, I can see why it wasn't working. :-) Even at 39k, the 10k resistor is more than enough to make it well below the 0.99V threshold.
bigmessowires


Posts: 217
Registered: 10/29/13
Re: Localtalk bridge
Posted: Dec 28, 2014 4:15 PM   in response to: dougg3 in response to: dougg3
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Your calculation looks right to me. Can you turn off the internal pull-up?

Even without an internal pull-up, you still need to worry about the input pin current. This should be listed in the datasheet. Then use v=ir with your pull-down resistance to calculate the voltage across the resistor. With old chips and 74LS stuff I think the input current can be in the tens of milliamps, which can easily cause problems with larger resistance values. But for CMOS chips I think the input pin current is normally a few microamps at most.

In general, I think pull-downs are more problematic than pull-ups, but I don't completely understand why. I've rarely encountered a pull-down, if ever, in the circuits I've worked with.

Anyway, glad you got it working!

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