Atmega 328 and TLC5940 routing

Crystal is rather far from the processor. I believe Atmel recommends as close as possible with a ground plane.

No bypass capacitors?

I believe the TLC5940 datasheet calls for a capacitor of several microfarads across VCC and GND.

Ah yes, bypass caps.. I can't believe I missed those..

Now that I missed because I didn't see a referance for it on the sheet (still trying to understand it tbh) but the arduino prototype doesn't have one.

Codelink - still building the schematic for this :astonished: =(

AJB2K3:
Codelink - still building the schematic for this :astonished: =(

Eagle my friend, Eagle.

I prefer DipTrace, and I've been using it for the last year for my personal projects. I know Eagle might be more powerful and has a gigantic user base, but I like DipTrace better. The free version is great and should be enough even for more advanced hobbists.

Back on topic: ditch that layout, since there are several problems with it:

  • the crystal is too far from the 328
  • there are sharp corners in the traces. Never, NEVER route traces with 90º angles.
  • there is no bypass capacitors for either chip
  • it is good practice to use every space left in the board (in its signal side) for a copper pour linked to the ground net.

If you decide to go ahead and use that circuit anyway, that one is easier done with a perfboard, rather than etched. Make use you use DIP sockets, instead of soldering the chips directly to the board.

AlxDroidDev:
I prefer DipTrace, and I've been using it for the last year for my personal projects. I know Eagle might be more powerful and has a gigantic user base, but I like DipTrace better. The free version is great and should be enough even for more advanced hobbists.

Back on topic: ditch that layout, since there are several problems with it:

  • the crystal is too far from the 328
  • there are sharp corners in the traces. Never, NEVER route traces with 90º angles.
  • there is no bypass capacitors for either chip
  • it is good practice to use every space left in the board (in its signal side) for a copper pour linked to the ground net.

If you decide to go ahead and use that circuit anyway, that one is easier done with a perfboard, rather than etched. Make use you use DIP sockets, instead of soldering the chips directly to the board.

I have moved the crystal under the board and added the bypass to the tlc5940
Still playing with layout designs.
Need to order some electronics to check out part size's

and added the bypass to the tlc5940

The processor also needs them.

Where? i haven't seen one any ware?

Page 2: 2.1 Digital supply section.

Page 14: Figure 6-1...

Decoupling on all VCC/GND pairs

Put a 0.1uF capacitor across all (A)VCC / GND pairs as close as possible to the processor.

?? not on here?

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.

AJB2K3:
?? not on here?

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

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.

AJB2K3:
?? 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.

AlxDroidDev:
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.

AJB2K3:
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.

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.

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.

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.

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?

AJB2K3:
In one diagram I see them placed between the + supply and the VCC pins

Wrong.

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.