How delicate are internal pull-up resistors?

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Hi,

I have a SOIC packaged ATmega1284 which I have hand soldered to a custom PCB, in fact, for various reasons I've taken it off and resoldered it 3 or 4 times.

I don't have a proper hot air rework station yet, so I used a paint-stripper heat gun.

The chip works ok except for some of the input pins I'm using with buttons. They seem to be stuck pulled to ground. I can't find any errors in the button circuit, so I'm wondering if my excessive heating has killed the internal pull-up resistors in the chip?

-Mike

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Not so likely the pull-up resistors.

 

More likely, the static discharge protection diodes on every input. Each pin has a diode to ground and one to Vcc, back biased during normal operation. A bit less likely is that the pull-down FET in the totem-pole output is gone/fried/otherwise shorted.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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You can use your multimeter to determine if the pin is intact by using the diode feature on your multimeter. A short will read as 0 or thereabouts, whereas an ok pin will read as a diode. If you read a short it is usually on the pcb. A port pin has to be seriously damaged to read other than a diode. Measure between 0V and the port pin. Swap the probes - ie measure one way, then the other.

 

I've used this technique many times - especially when soldering QFN packaged parts.

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MalphasWats wrote:
I don't have a proper hot air rework station yet

Here you go: https://www.ebay.com/itm/2in1-86...

 

Jim

 

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@Jim,

 

I keep meaning to ask you about these stations.

e.g. 852D+, 862D+, Lodestar 502878, KS8586, ..., 858D, 936, 998

 

They all seem to have similar handsets.    So differences probably come down to the firmware.

I need a reliable soldering iron + hot air.

 

Your experience is more important than Ebay sales patter.

 

I am a hobbyist.

 

David.

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david.prentice wrote:
I am a hobbyist.

Me too! Ok, I do "work" as an engineer some days!

I have the 852D+ at home, while we have an 862+ at my day job.

They all seem to work fine, I looked for whichever one was on sale when I needed one....

Takes a little while to get the hang of hot air but its fun to watch, use plenty of liquid flux,

I like the water soluble flux, but the board must be rinsed and dried as it is conductive, but the board comes out clean!

The no-clean flux is ok for repair work, but does not look as nice.

 

Jim

 

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MalphasWats wrote:
so I'm wondering if my excessive heating has killed the internal pull-up resistors in the chip?

 

My replacement chip arrived this morning and is now *VERY* carefully soldered onto a fresh board. SAME PROBLEM.

 

So I've spent about an hour checking and rechecking all of my connections, circuit diagrams *EVERYTHING*. Then I start to doubt that I've even got the buttons connected to the pins I think I have, so I check the data sheet:

 

 

I have 7 push-buttons connected to PC0-7. 3 of my buttons were working as expected. 4 buttons weren't. GUESS WHICH ONES WEREN'T WORKING!

 

I recalculated my fuse bits to disable JTAG and re-flashed. Now it works.

 

Thank you all for your efforts, it's hard to provide an answer when the asker is an idiot but I learnt a lot of things to check next time. Also the SOIC ATMega1284P can take a LOT of punishment, just FYI.

 

As for SMD work, I'm saving up wife points to justify purchasing one of these http://reflowr.com sometime soon :)

 

-Mike

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Next time specify what pins you are having problems with - the response would've been swift - DISABLE JTAG! You can also do it in code just in case you accidentally forget to set the jtag fuse.

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Sorry to derail the thread, but Jim, is the 852D+ the sort of tool that I might be able to use to replace a QFN48 6mm x 6mm package, for example?

Four legs good, two legs bad, three legs stable.

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Yep! just wave the hot air around the device and in a few seconds you should be able to pick the device off. I did a 64QFN with heatsink pad recently - took a bit of heating to remove that sucker. Then I reflowed a new device on. And it worked!

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If you are curious as to how (de) soldering with hot air looks like, and how to do it watch some video's / tutorials on youtube.

Plenty of soldering over there:

 

https://www.youtube.com/results?search_query=hot+air+soldering

 

De-soldering is mainly applying heat evenly and gently tapping it untill the component moves, then you can remove it with some tweezers.

Be very carefull to not lift your part too soon.

If one of the solder joints has not melted yet it is very easy to rip a pad of because the epoxy is also getting soft at soldering temperatures.

 

For soldering there are some different techniques. They always start with thoroughly cleaning the area (no old solder on the pads, everything should be flat) and they always use plenty of flux.

 

In the repair corners the low temperature (with bismuth) solder seems to be popular. It makes soldering easier with less disturbance of the rest of the circuit, but I have never used it myself.

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

Last Edited: Mon. Dec 3, 2018 - 05:46 PM
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John_A_Brown wrote:
Sorry to derail the thread, but Jim, is the 852D+

Sorry I did not reply sooner, but Kartman is correct, I wave the heat around until I see the solder melt and use an exacto knife to flip the chip off, or use tweezers to lift it.

Once the chip is off, flux the area and reheat to have clean pads ready for the next chip.

 

Jim

 

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Paulvdh wrote:

They always start with thoroughly cleaning the area (no old solder on the pads,

everything should be flat) and they always use plenty of flux.

 

I do quite a lot of PCB rework, all vintage (70's, 80's) through-hole boards

such as Rockman gear and lately Asteroids arcade machine boards.  I've

discovered the magic of flux, though I usually just add a bit of new solder

to a joint I want to remove (which introduces fresh flux) before desoldering.

 

The warnings on flux are much scarier than on solder, not sure why, but

this is part of the reason I prefer my current method.

 

So I'm curious, how do you get ALL of the solder off the pads?  Doesn't seem

possible, or do you just mean the pads should be flat?  I'm hoping to venture

into SMT parts some day, but so far the vintage stuff has held my interest.

 

--Mike

 

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avr-mike wrote:
So I'm curious, how do you get ALL of the solder off the pads?

Generally you don't, in fact if the pads don't have solder, I add it with a solding iron.

Exception, with two terminal parts, resistors, diodes, Caps, I put solder on one pad only with iron, then place part using tweezers.

Then solder the other end, followed by reflow using flux and hot air.  

For multi leg parts, chips, avr's, I use the iron to wet all the pads, then tac one or two opposing leads to hold the part in place, then reflow using flux and hot air.

It's fun watching the parts float into position.  Easier than through hole, no flipping of the pcb and trimming leads, etc. 

Leadless parts are similar, wet the pads with solder using iron, heat pads until solder melts, remove hot air, carefully drop part into place using tweezers.  Reflow if needed.

Takes practice, but doable.  The above works for parts that are 0402 or larger, start with 0805 parts, and work down to smaller parts.

I've not tried the above with 0204 or smaller! 

 

Jim

 

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"and lately Asteroids arcade machine boards "

I'm sorry, but nobody could ever repair Asteroids PCBs at the time(or Battlezone) by any means other than swapping chips until it works. I used to repair Atari game PCBs, and most of the raster(rather than rasta) based games, I could pretty much figure out which chip from the descibed symptoms, but those vector graphics games.... scary stuff.

 

Aside from that, maybe I'll get me one o' them fancy hot-air rework engines and have a go...

Thanks for the helpful replies.

Four legs good, two legs bad, three legs stable.

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John_A_Brown wrote:

I'm sorry, but nobody could ever repair Asteroids PCBs at the time...

Fortunately my board was "working" already, though the picture was

starting to go.  I bought a spare "for parts" board to practice on, and

stripped every part other than the IC's and replaced them.  Then did

this to my game board.  The result was amazing, picture as good as

if new!  Glad I don't need to source one of those vector monitors.

 

--Mike