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Topic: Atmega2561 (Read 7387 times) previous topic - next topic

DrAzzy

#15
Oct 22, 2015, 07:07 am Last Edit: Oct 22, 2015, 08:51 am by DrAzzy
How many Vcc pins do you make available?

When working with MCUs on breakouts like that, I feel like I never have enough Vcc and Gnd pins; if you've looked at my designs , you'll note how I give 4-5 Vcc pins on each board, so someone can attach some basic components with jumper cables without having to bodge up some way to get more points to connect Vcc.


Have you considered an outline that could use a common HC/49 crystal? For most people, a strange surface mount 4-pin crystal is likely something they won't have on hand (and they're more expensive). Take a look at how I do pads for crystals - that will take through-hole HC/49, SMD HC/49, and those cute mini HC/49 ones (the .28"x.16" ones), as well as many 2-pin SMD crystals, or through-hole cans style. Of course, if you're going to be selling it assembled only, this doesn't matter so much.
ATTinyCore for x4/x5/x61/x7/x8/x41/1634/828/x313 megaTinyCore for the megaavr ATtinies - Board Manager:
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ATtiny breakouts, mosfets, awesome prototyping board in my store http://tindie.com/stores/DrAzzy

larryd

#16
Oct 22, 2015, 07:33 am Last Edit: Oct 22, 2015, 07:41 am by LarryD
I was suggesting a diode between +5 and GND.
i.e. cathode to +5V and anode to GND.
The diode in this configuration does not conduct, however, if some old man accidentally hooks power backwards, the diode conducts fully therefore taking out the power supply but protecting the 2561.

Looks very nice.
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CrossRoads

VCC is available near IC pin 20, 49, 64, the FTDI header, the ICSP header. I could make the 5V select jumper shorted by default (jumper pad with trace) then 2 more pads would be available full time. I could add another by the ICSP header, and one more in the upper right corner.

Diode from Gnd (anode) to Vcc (cathode). How hefty of a diode? 2A capable?
Something like that would be in SMB type package
http://www.digikey.com/product-detail/en/S2J-TP/S2J-TPMSCT-ND/773715
and have to go on the top of the board so the bottom could stay low  height.
I'll see if I can put it in the top right hand corner, give up the board name and move the mounting hole towards the edge more.  Will have to beef up the power traces, can't have them acting like fuses if power should be backwards.

Crystal - I wasn't planning on offering this as a kit, so having place for  HC49 type parts (or even the smaller XC1776
http://www.digikey.com/product-detail/en/ECS-160-20-3X-TR/XC1776CT-ND/2676640
which I have on the Bobweeny

would need to go on the top, requiring the board footprint to grow to make room for it.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

larryd

#18
Oct 22, 2015, 07:20 pm Last Edit: Oct 22, 2015, 07:35 pm by LarryD
This board will be used with limited output power supplies.
It would be great to have the diode survive an accident but, not necessary.
The diode could be considered sacrificial, replace it, if it shorts.
I am sure your experience is as mine, silicon diodes go short in a situation like I am suggesting.
This diode hopefully will never be used, only in an accidental situation, which may happen during experimental or inexperienced user applications.
The diode's function is to protect the hard to replace/unsolder and expensive 2561.
If nothing else, pads to accomidate it would be nice.
The diode you linked looks great, but a 1/2 or 1 amp package would work, may have to be replaced on a fault.

Edit:
I like beefy power traces. :)


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larryd

My guess is Bob will supply a finished product, the crystal selection is his choice not the end user.
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CrossRoads

#20
Oct 22, 2015, 07:49 pm Last Edit: Oct 22, 2015, 07:52 pm by CrossRoads
Okay, I'll back off on the current capability for the diode. At the same time, 27 cents is a good price.

Crystal, yes, it has to fit the pad selected, in this case 6 x 3.5mm:
http://www.digikey.com/product-detail/en/406I35D16M00000/CTX1206CT-ND/4308783
Altho this one looks good too, but may not have been around when I did the original 2560 design, or I didn't look hard enough for an appropriate symbol:
3.2 x 2.5mm, much lower ppm #s, and also lower cost. I like it!
http://www.digikey.com/product-detail/en/TSX-3225%2016.0000MF09Z-AC0/SER4069CT-ND/5413938
Or this 2-pin device.
http://www.digikey.com/product-detail/en/ABM7-16.000MHZ-D2Y-T/535-9840-1-ND/2001463
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

CrossRoads

#21
Oct 23, 2015, 05:58 am Last Edit: Oct 23, 2015, 05:59 am by CrossRoads
Think this is what I am finishing with, unless I change the crystal to  this
http://www.digikey.com/product-detail/en/TSX-3225%2016.0000MF09Z-AC0/SER4069CT-ND/5413938
I've asked Atmel what caps would go with it.
Any last thoughts from anyone?
I've gone with a 1206 size protection diode, can be 150ma, 500mA, 1A, 2A. If we're just looking for something to fail and short out if power is connected backwards, I don't what's best to go with. Or go with the higher current and let it survive shorting out a 1A switching regulator wallwart that can autorecover once the short is removed.

I've got a I2C header in one corner with SDA-SCL-GBD, and SCL-SDA-GND available along 3 header pins.
I've also got Tx1-Rx1-Gnd available along 3 header pins.
I couldn't arrange a clean way to get Rx0-Tx0-Gnd along another header, but they're on the FTDI header in a regular FTDI arrangement anyway.
I've got 5 sets of +5/Gnd pairs available, along +5/Gnd available on the ICSP and FTDI header.

+5 on FTDI can be separated from +5 on the rest of the board. A solder jumper/trace on the bottom of the board allows that to separated.
DTR can be setarated from the FTDI header so Reset can be disabled with USB connection. A solder jumper/trace on the bottom of the board allows that to be separated. Male pins can also be installed if one wants to jump back & forth a lot during development/test.

Power LED has been added.
"L" LED has been added, connected to SCK, which is on pin 9.  Unless the SPI pins are re-arranged in pins_arduino.h, currently they are here as discussed yesterday:
Code: [Select]


static const uint8_t SS   = 8;
static const uint8_t SCK  = 9;
static const uint8_t MOSI = 10;
static const uint8_t MISO = 11;


PB , // PB 0 ** 8 ** SS
PB , // PB 1 ** 9 ** SCK
PB , // PB 2 ** 10 ** MOSI
PB , // PB 3 ** 11 ** MISO
PB , // PB 4 ** 12 ** PWM3
PB , // PB 5 ** 13 ** PWM4
PB , // PB 6 ** 14 ** PWM5
PB , // PB 7 ** 15 ** PWM6


2mm mounting holes are at:
100, 350 mil lower left
1450, 200 mil lower right
200, 1250 upper left
1450, 1250 upper right
so not quite symmetrical, I tried, while keep traces away pretty well.
Board size remains 1600 x 1400 mil, or 40.6 x 35.6 mm

So, the only thing that may change still is the size of the crystal and the 2 caps to go with it, hope to find out tomorrow. I think I'll order up 20 boards worth of stuff and see how it prices out with solder stencil and assembly, I don't think we can reliably hand place the uC and not have a lot of solder bridges to clean up.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

larryd

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larryd

#23
Oct 23, 2015, 07:17 am Last Edit: Oct 23, 2015, 09:28 am by LarryD
Side note, some designs include a 22pf cap from reset to ground for noise consideration.

Edit:
This may not work in this case but, I place a small drop of glue, center bottom of multi lead SMDs.
Place the component, dry over night.
Flux, solder, clean.

.
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larryd

#24
Oct 23, 2015, 09:27 am Last Edit: Oct 23, 2015, 09:30 am by LarryD
Is there any advantage of having a 1K pull down resistor on the DTR side (left side) of C1.
As they have in Rev. 3 UNO?
May aid in power up reset.
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CrossRoads

22pF cap, I think any marginal benefit is dwarfed by the 0.1uF DTR cap. User can add one across the ICSP Reset/Gnd pins if it is shown to be needed. I have not seen it to be needed on any of the 328P or 1284P boards I have delivered.

DTR cap,1K pulldown on one side, 10K pullup on the other. If the user's choice of USB/serial adapter shows that a 1K pulldown is needed, one can be added across the DTR/Gnd pins on the FTDI header.  I have not seen it to be needed on any of the 328P or 1284P boards I have delivered that I use an FTDI module to interface with.

I am not satisfied with the 9pF load needed by the smaller crystal, the only info I can find in the datasheet seems to indicate that the uC presents 10pF load, so it is not possible to match that up. I think I'll stick with the slightly larger device with its easier to work with 18pF load.  I just need to connect pads 2 & 4 to Gnd per its datasheet.
"3. Terminations #2, #4 and metal lid are connected internally and may be connected to ground for EMI suppression."
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

CrossRoads

What kind of glue do you use?
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

larryd

#27
Oct 23, 2015, 06:43 pm Last Edit: Oct 23, 2015, 07:06 pm by LarryD
I have used Gorilla Super Glue gel, and high temperature Loctite.
The Gorilla glue is what I use the most.
I place some glue on a disposable surface.
Using a dental tool, transfer very a small amount to the center bottom of the chip.
Position, let dry, liquid flux, solder, clean.

http://www.homedepot.ca/product/20g-gorilla-super-glue/992228#

https://www.google.com/search?site=&tbm=isch&source=hp&biw=981&bih=625&q=loctite+high+temp+thread+sealant&oq=loctite+high&gs_l=img.1.4.0i8i30j0i24l9.6110.11987.0.19834.12.12.0.0.0.0.82.855.12.12.0.ekpsrh...0...1.1.64.img..0.12.851.2m7AIMA2CtI#imgrc=_


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CrossRoads

I tried Gorilla Glue to attach a wire into an epee. Seemed like it foamed up/expanded as it dried (cured?). I wouldn't want to use that under a chip I don't think.

I wonder what the board houses use, altho not that hard, as I don't plan on reflow soldering any dual sided boards, will let the board house deal with that.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

larryd

The trick is: use a small amount.
This causes tight surface tension adhesion, no floating while drying.
The gel (shown in the link) does not foam.

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