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nuhamind2
PostPosted: Apr 16, 2012 - 12:42 AM
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Location: Yogyakarta, Indonesia

Hi.
I need to do prototyping for my project. I need to connect CKOUT pin from uC to an external shift register (74LS166). The uC itself is in a dev board and well running. The shift register is in breadboard.
So will it work well, because breadboard have stray capacitance and the uC will be clocked at 20Mhz.

Thanks
 
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valusoft
PostPosted: Apr 16, 2012 - 02:08 AM
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Keep the lead(s) short.

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DocJC
PostPosted: Apr 16, 2012 - 02:38 AM
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And be sure to connect the grounds from the development board and the breadboard together.

Welcome to the Forum.

JC
 
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nuhamind2
PostPosted: Apr 16, 2012 - 05:44 AM
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So, it means it can work.
Thanks
I will sure connect the ground, i have experience in forgetting the ground on my very first avr project.
 
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LDEVRIES
PostPosted: Apr 16, 2012 - 09:30 AM
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And
-run it at the same Vcc as the development board.
-bypass the IC's supply (Vcc & Gnd) with a 0.1uF monolithic capacitor, with the shortest possible leads(place diagonally over the top of the IC).

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Plons
PostPosted: Apr 16, 2012 - 10:42 PM
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All good tips so far.
But you also need to prevent ringing. Do you have an oscilloscope, 100MHz bandwidth, and probes 10x ?
Beacause AVR's switch very fast, the problem is with the rise- and fall-times. Slewrate.
You need to add a resistor in the clock-line, as close as possible to dev board. 330 Ohm is a good starter value. Some pictures:
Without 330 Ohm: http://www.avrfreaks.net/index.php?name ... 176#946176
With 330 Ohm: http://www.avrfreaks.net/index.php?name ... 236#946236

The correct value for your setup: you need to hookup a scope.

It's unlikely that stray capacitance of the breadboard will cause a problem.

Why do you bring the 20MHz to the breadboard, rather than Sck ?
Btw, on Sck you need that resistor as well.

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ka7ehk
PostPosted: Apr 17, 2012 - 12:31 AM
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It depends on the kind of "breadboard" you use. If it is one of those with spring contacts in parallel rows and columns, you will have to contend with a LOT of extra pin-pin capacitance. 20MHz could be a challenge. With such boards, it is also very difficult, if not impossible, to keep leads short and, especially to have good grounds. As a result, even using a scope is hard because everything rings like a bell.

Jim

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Someguy22
PostPosted: Apr 17, 2012 - 01:35 AM
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ka7ehk wrote:
It depends on the kind of "breadboard" you use. If it is one of those with spring contacts in parallel rows and columns, ...
Bob Pease referred to those breadboards as "slabs of trouble". I have to agree.
 
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smileymicros
PostPosted: Apr 17, 2012 - 02:14 AM
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Shift registers on a breadboard with no problems:



16Mhz Crystal with AVR on a breadboard with no problems:



YMMV

Smiley

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DocJC
PostPosted: Apr 17, 2012 - 03:30 AM
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I do lots of projects and tests on breadboards.

That said, I've never routed the 16 MHz (?) clock signal across boards. SPI, I2C, USART, etc., but these are all at a significantly lower freq than the micro's clock.

This was a GPS project, (ZBasic, (AVR), micro), GPS, Bluetooth, 3-Axis Accel. This version has a string of external memory chips, but the real project units ended up with an MMC card.

JC
 
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nuhamind2
PostPosted: Apr 17, 2012 - 03:47 AM
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Well, it seem problematic. I can use lab's oscilospoce, but it is limited to a few Mhz. It seem i have to make it on PCB.
Thanks everyone.

edit:thanks smilemicros and DoJc for the pic. Probably it won't hurt trying on breadboard
 
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bobgardner
PostPosted: Apr 17, 2012 - 04:00 AM
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Yikes. Remind me to not uploaad any of my ugly breadboard monstrosities.

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gchapman
PostPosted: Apr 17, 2012 - 04:26 AM
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nuhamind2 wrote:
It seem i have to make it on PCB.
Not necessarily. Consider:
BusBoard - double sided = PadBoard, SMTBoard, SMT Pads.
Vectorbord - used a 2-sided version of this with wire wrap for one prototype (MPU, external clock oscillator, external static RAM, external UART).
Vero Technologies - Square Pad Board with Colander Ground Plane
Progressive Wiring Techniques by ChaN.
i.o.w., likely will need a ground plane.
 
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Plons
PostPosted: Apr 17, 2012 - 12:30 PM
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Breadboards often get the blame. But actual measuring the capacitance between two adjacent contact-strips shows it's not that bad: 1.6pF. The powerstrips are much longer: 5x and give 10.5pF
Measurements done with an aluminium plate under the breadboard.
Without the plate, values are 2.5pF and 10pF respectively.

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Lennart
PostPosted: Apr 17, 2012 - 01:38 PM
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I do almost all early prototyping with breadboards.
Very rare that the breadboard is to blame for anything not working.
Far more often due to my sloppiness with wires.
Sometimes a wire that seem to be properly connected is not.
You can detect this by measuring the voltage drop.
When I want real stable connections I solder wires to gold plated male header pins.

The highest speed I've had running across the breadboard is SPI clock at 8MHz for an LCD display.
Appx 5cm wire voltage divided to 3V3 crossing and parallel to other signal wires.
Never seen any issues.
But 20MHz is more than twice that speed.
My guess is that you will have no problem (if the wires are properly connected and short of course).
 
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Plons
PostPosted: Apr 17, 2012 - 01:39 PM
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The proof of the pudding:
ATtiny2313 with 20 MHz crystal, CKOUT enabled.

2x 200mm breadboardwire.
No resistor in series: overshoot to 6.8V, undershoot -1.6V. Not liked by the 74LS166 shiftregister.
Adding 150 Ohm: a nice 20MHz signal, no overshoots.

With the 330 Ohm I mentioned before: a bit too much damping, but the 74LS166 would accept it as a clock.

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ka7ehk
PostPosted: Apr 17, 2012 - 08:52 PM
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One difference is that Plons has taken appropriately extreme care in both what is on the board and the measurement. He has shown what is possible. BUT, you have to do stuff RIGHT in order to achieve that (as he shows in the second picture, above).

Jim

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