USB JTAG Programmer - PCB Layout Advice

[embedded]

What are peoples opinions on the routing shown in the attached picture? I know there are right angles where the smaller ceramic caps are connected but I think this is acceptable?

Right angles become important as one looks for RF performance of the decoupling.  The rule of thumb is up to 30Mhz this is not terribly important.  But I suspect that you will have 200Mhz + clocking.  So you are looking for the impedance at that frequency.  For sake of argument 200Mhz is about 1.5m wavelength then feature size directly relates to the impedance.  (And the feature size is very relevant to that wavelength, be well under 1/1000'th of the wavelength since the wavelength is ~300 ohms at wavelength!  So you are getting impedance of 300ohms x fraction of wavelength.)

Certainly you can decrease impedance by smoothing corners and I would definitely do that for 1Ghz.  But is it necessary?  Really it's a judgment call.  I have not given you an opinion I hope I have given you a tool to look at the issue.

Just remember more parallel traces decrease impedance as a square of traces ie faster than width or anything else.

That's why Dave is so fanatical about stitching grounds.  He is creating parallel traces decreasing impedance by a square rule.

[TheDirty]

You're feeding your lower voltage regulators from the higher ones.  I'm assuming the low voltage ones take very little current.

[David]

You're feeding your lower voltage regulators from the higher ones.  I'm assuming the low voltage ones take very little current.

The lower voltage regulators will pull quite a bit of current... I did think about using two 5V regulators in parralel to provide more?

This is currently how the regulators are daisy chained:



Dave

[jahonen]

Sharp corners matter only if one aims for constant specific impedance. You can imagine that track gets little wider at corners, so impedance drops there. For power supply rails, you don't target to any specific impedance, but lowest possible one. So having a right angles is not a problem in that sense. A real culprit here is that T-branches increase the impedance of the small capacitors, making them considerably less effective. It is just like adding an series inductor (and very small resistor) with the capacitor.

I'd remove those T-branches and route the tracks via the small capacitors (think the capacitor as 4-terminal device, separate input and output tracks), as it should not be very difficult to do so.

Regards,
Janne

[David]

I'd remove those T-branches and route the tracks via the small capacitors (think the capacitor as 4-terminal device, separate input and output tracks), as it should not be very difficult to do so.

Regards,
Janne

I'm not quite sure I understand what you mean by this? Any chance of a very quick diagram?

Cheers,
Dave

[jahonen]

I'd remove those T-branches and route the tracks via the small capacitors (think the capacitor as 4-terminal device, separate input and output tracks), as it should not be very difficult to do so.

Regards,
Janne

I'm not quite sure I understand what you mean by this? Any chance of a very quick diagram?

Cheers,
Dave

Here's a picture what I mean, it is difficult to explain it in words, but as you can see, the concept is quite simple, minimize the loop area and treat capacitors as 4-terminal devices.

Regards,
Janne

[TheDirty]

He's saying what I said in post 50, you are routing power around your caps instead of through them.  Your traces need to go through the caps rather than the caps are just attached to the traces somewhere.

[David]

FINALLY!!!! 

[EEwannabe]

Congrats,very nice looking board.

-Patrick

[DavidDLC]

Excellent work.

What I have done is to create a generic board with inputs/outputs, so I can use this generic I/O board with any other development board for microcontroller, instead of having everything together.

I will post a picture of this board

I ordered one pcb and the manufacturer sent me two, if any of you is interested to get the board you may need to deal with:

- The board has my name on it
- Pay shipment and I will send it to you.
- Buy the components and populate, it's a mix between smd and through hole components.

[DavidDLC]

Here is the picture of the board I was talking about:



It has:

- 7 x 24 LED matrix, this is to be driven by 4 eight bits ports.
- 2 x 16 LCD for 8 bits connection and 3 control lines, but It could be managed in 4 bits and 3 control lines with an additional adapter (not included). It has contrast control

The rest from here is to be managed by 1 eight bit port:

- Two 8 SMD LEDs ports
- 4 x 7 LED segment
- 4 x 4 keypad matrix, you need to have a port with internal pull-ups
- 8 push buttons, they can be read high or low
- 8 position dip switch, they can be read high or low
- 2 hex switch, they can be read high or low
- 2 H-Bridge for motor control
- 1 port with 4 NPN optocoupler and 4 SPDT relays

As I mentioned before I use this for prototyping.

[DavidDLC]

Attach is the original picture file

[Mechatrommer]

nice! how much it cost to get a sample PCB? including the shipping.

[DavidDLC]

I'm giving away an unpopulated PCB, as I explained before, I ordered one and the manufacturer sent me to.

Whoever wants it should buy the components and pay the shipping.

I'm located in San Diego, Ca. USA you can calculate the shipping cost and make the decision