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Author Topic: Atmega 328 and TLC5940 routing  (Read 5873 times)
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and added the bypass to the tlc5940

The processor also needs them.


Where? i haven't seen one any ware?
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http://www.atmel.com/images/doc2521.pdf

Page 2: 2.1 Digital supply section.

Page 14: Figure 6-1...
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Decoupling on all VCC/GND pairs

Put a 0.1uF capacitor across all (A)VCC / GND pairs as close as possible to the processor.
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?? not on here?

http://arduino.cc/en/Main/Standalone

Stupid additional question but I plan on running this of a 3.6v PSP battery (always have 2 spare in camera bag + 1 in each of my psp1001 consoles)

Am I supposed to change some component values?

Stupid question and you have permission to roll your eyes.
« Last Edit: April 01, 2013, 12:54:19 pm by AJB2K3 » Logged

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The bypass caps are there (2 x 10uF caps): one is in the output of the 7805 voltage regulator. In all the pictures where the breadboard appears with the 7805, the bypass cap for the Atmega328 is the upper rightmost cap. Note that the 7805 also requires an input capacitor and it is also there, to the left of the other capacitor, also 10uF.

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?? not on here?

Unfortunately, it is a common mistake to exclude them.

With those two digital chips on your board there is a high probability you will have problems without the bypass capacitors. If you do not include them and have problems you will be pulling your hair out trying to figure out what is wrong.
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The bypass caps are there (2 x 10uF caps): one is in the output of the 7805 voltage regulator.

Those capacitors serve a different purpose than the VCC/GND capacitors.  Both are important.  The 0.1uF capacitors across VCC/GND are not present in that circuit but should be.

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Stupid additional question but I plan on running this of a 3.6v PSP battery (always have 2 spare in camera bag + 1 in each of my psp1001 consoles)

The voltage is in range for the TLC5940.

The voltage is slightly out of range for the AVR processor at 16 MHz.  Many many people run the processor slightly out of the voltage range.  I have never read about any problems.  You will have to use your own judgement.  Obviously, if you were building a pacemaker, you should choose a different path.  If you were building a light display, running the processor a bit out of specifications is not a problem.
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The bypass caps are there (2 x 10uF caps): one is in the output of the 7805 voltage regulator.

Those capacitors serve a different purpose than the VCC/GND capacitors.  Both are important.  The 0.1uF capacitors across VCC/GND are not present in that circuit but should be.

You are correct.

After I read your answer I wrote some gibberish here and then I deleted it after I studied some more.

Although the cap I mentioned helps a bit, each and every chip should have its own decoupling (aka "bypass") capacitor, to prevent it from suffering voltage drops caused by the rest of the circuit. I am glad I have always done that in my designs.
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My pcb is a varient of the trihole pad design but has two tracks made to run under the chips as supply lines. Would it be ok to put it across these lines at one end of the chips? or should I hide them under the ic sockets as close as possible to the chips power pins.
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I can't help with that one.  If you don't get an answer in a few days, let me know (personal message) and I'll use my moderater-fu to try and get you an answer.
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Even more confused. now with bypass caps.

In one diagram I see them placed between the + supply and the VCC pins, and another diagram put across the + & - supply lines.
Which one is correct?
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In one diagram I see them placed between the + supply and the VCC pins

Wrong.

Quote
and another diagram put across the + & - supply lines.

Correct.

The capacitor is connected from (A)VCC to GND as close as possible to the chip.
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In one diagram I see them placed between the + supply and the VCC pins

Wrong.

Quote
and another diagram put across the + & - supply lines.

Correct.

The capacitor is connected from (A)VCC to GND as close as possible to the chip.


Thats what I thought, thanks.
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