Characteristic Impedance of 60x bus of MPC8270 for PCB Stackup and Routing

[muhammadahmed47]

Hi,  i am routing 60x bus of MPC8270 at 100Mhz for SDRAM interfacing and PCI bus also.

In data sheet of MPC8270 output buffer impedances are given as follows: 

Are these also characteristic impedance of PCB Track or these are just output impedances and i need to change my PCB Track impedance. Like when we route USB Signals we must make sure that the differential impedance of PCB Track must be 90 ohm. So what is the characteristic impedance of 60x / local and PCI bus.  

Thanks

[alexander_yakov · Accepted solution]

Our documentation does not specify PCB track impedance for 60x bus and PCI bus.

For 60x bus you have to perform signal integrity simulations using device IBIS model and your actual trace impedance.

For PCI bus - please refer to PCI standard.

Have a great day,
Alexander
TIC

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[muhammadahmed47]

PCI Standard says PCI Impedance is 60-100 Ohms so i do not know why the output buffer impedance of PCI is 27 Ohm. Because for matching 60 Ohm i need to add 33 Ohm of series resistance which will take a lot of space in my board.

[alexander_yakov]

As far as I remember, PCI standard says PCI interface is designed to work without impedance matching - PCI buffers have strong protection diodes, so overshoot/undershoot voltages caused by signal reflection do not damage output buffers.

Moreover, PCI uses signal reflections - output driver initially drives output line only partially (1/2 of desired voltage), than wavefront arrives another end of line, it reflects on unterminated end and doubles.

Please search in Google for "reflected wave switching" keyword for more information.

For example, this link:

http://www.sigcon.com/Pubs/news/2_28.htm

Also, I recommend looking Section "PCI Method: Reflected-Wave Switching" in "PCI System Architecture (4th Edition)" book from MindShare.