optimizing line routing, transmission lines, reflections

Jon Fick wrote:

All,

 

A search of the subject lines of the last several thousand posts in this

newsgroup didn't reveal an answer to my question, nor did searchs in

Google...

 

I commonly use the Eagle Autorouter to avoid routing by hand.  (I know

there are a variety of opinions on this!)  I'm a middle of the road

type...where in certain situations I'll either do the layout of certain

transmission lines prior to the autorouter, or modify routing after the

autorouter is done.  I'm wondering if some of the Eagle "power users"

might post a few references (online, printed, etc.) that would

illustrate how to optimize the layout of transmission lines, for

instance for feeding multiple I2C devices.  Daisy chain?  Star layout?

Everything I've found online is essentially yet another "transmission

line calculator" but there's nothing about actual routing design.

 

Thanks for any references or pointers.

 

Jon Fick

Transmission lines should be routed in a daisy chain fashion.  Stubs can

cause problems, depending on the data rates and the signal's rise and

fall times.  Terminators must be mounted at the end of the transmission

line.  I've just looked up the signaling rate for I2C and found that

it's between 100Kbps and 400Kbps.  So layouts 1 and 2 should work just

  fine.

1) GOOD - daisy chain:

      [chip]//[chip]--[term]

    Note that for really high speeds (Gbps), the terminating resistor is

often built into the chip.

2) Not so Good / Not so bad - termination resistors should be after the

connection to the end chip.  However, if the terminator is very close to

the chip's pin, this will probably work.  For I2C speeds, this layout

should work.

     +[chip]//+--[chip]

3) BAD - this layout has stubs.  If the stubs are long compared to the

rise/fall time of the signal you will get reflections, and this will

most likely cause grief.

            +//--

      long trace==> |              |

      (AKA "stub")  |              | <== stub

                     |

HTH

-Dave Pollum

Jon Fick wrote:

All,

 

A search of the subject lines of the last several thousand posts in this

newsgroup didn't reveal an answer to my question, nor did searchs in

Google...

 

I commonly use the Eagle Autorouter to avoid routing by hand.  (I know

there are a variety of opinions on this!)  I'm a middle of the road

type...where in certain situations I'll either do the layout of certain

transmission lines prior to the autorouter, or modify routing after the

autorouter is done.  I'm wondering if some of the Eagle "power users"

might post a few references (online, printed, etc.) that would

illustrate how to optimize the layout of transmission lines, for

instance for feeding multiple I2C devices.  Daisy chain?  Star layout?

Everything I've found online is essentially yet another "transmission

line calculator" but there's nothing about actual routing design.

 

Thanks for any references or pointers.

 

Jon Fick

Transmission lines should be routed in a daisy chain fashion.  Stubs can

cause problems, depending on the data rates and the signal's rise and

fall times.  Terminators must be mounted at the end of the transmission

line.  I've just looked up the signaling rate for I2C and found that

it's between 100Kbps and 400Kbps.  So layouts 1 and 2 should work just

  fine.

1) GOOD - daisy chain:

      [chip]//[chip]--[term]

    Note that for really high speeds (Gbps), the terminating resistor is

often built into the chip.

2) Not so Good / Not so bad - termination resistors should be after the

connection to the end chip.  However, if the terminator is very close to

the chip's pin, this will probably work.  For I2C speeds, this layout

should work.

     +[chip]//+--[chip]

3) BAD - this layout has stubs.  If the stubs are long compared to the

rise/fall time of the signal you will get reflections, and this will

most likely cause grief.

            +//--

      long trace==> |              |

      (AKA "stub")  |              | <== stub

                     |

HTH

-Dave Pollum