First time doing big layout

[jdhar]

Hi everyone,

This is my first time on this forum, and I thought I would give it a try since there seems to be lots of knowledgable people here.

I have completed a design for what is essentially a single-board computer for my company. I'm at the point where I need to start layout/board design, but my problem is that I have never done anything of this magnitude. Right now, I am constrained to a 6.5x5" 6-layer board, with a fair amount of Analog circuitry. I was hoping that some people could give me good tips or point me to resources that talk about general strategies for laying out and routing. I am using Protel DXP 2004, and my general strategy so far has been to arrange components in individual (mostly modular) blocks off the board, and then put each block on the board (rearranging if necessary), and manually routing within each block. I then plan on letting the auto-router complete the connections between the blocks and to/from the DSP.

I hope this in the right place - forgive me if it is not. Thank you kindly,

Jai

 

[Frosty]

You have not really given enough information for precise answers

I have tried DXP2004 on an old PC104 design, which was done before in 99SE/Specctra with little trouble.

The Situs router hardly looks at it, seems there are issues with rounding errors during rules setup and so you need to allow additional tolerances within the channel rules to get it done.

Using rooms (blocks) is good, if you keep all of the system level blocks on one SCH sheet, the rooms will be generated for you.

If you expect the autorouter to do the job for you you will be waiting a long time.......... i recommend working a while on your placement and getting that right.

If you geta good placement the interactive routing tools should be enough to get the job done for you.

Autorouters are REALLY fussy about how they are set up and instructed, if you expect ANY router to just 'go do it' you will always be dissapointed, but Protels is one of the worst routers i have sen, so not a good starting point.

:R

 

[jdhar]

Thanks for your reply - I am aware that it's almost impossible to give precise answers given the nature of my question. However, I will provide more information on the board, and maybe that may help.

The core is a TI TMS320F2812 DSP, with an External bus interface. This is one of the biggest routing challenges I see, since the bus can't exactly be confined to one section of the board, since both analog and digital devices use it. For the analog devices, there is a 24-bit Sigma-delta ADC, 1 MSPS ADC, and 16-bit DAC, all with many opamps attached. On the digital side, there is Ethernet, USB, Modem, 4 UART's, PS/2 and LCD.

At this point, my biggest concern isn't really achieving ultimate precision from all the analog components. Due to my inexperience with a board of this density, I am more worried about proper placement of components, and things such as via sizes/placement and general good routing practice. Ground/Power plane layout is also proving to be quite difficult since we have Digital devices requiring 1.9,3.3,5 and 12V, and Analog devices requiring 5 and 12 (maybe). While I can confine most of the analog section to one side of the board, I can't seem to separate all of the Analog 5 and Analog 12V devices.

These are just worries regarding the PCB.. who knows if my design will even work

 

[rauol]

8) Seperate out Analog and digital grounds and use Ground planes.
Also note that the track resistance of the signal track going to the AD converter can influence the accuracy of your AD converter.