Re: [SI-LIST] : 66 MHz AGP Clock Signal
Tue, 14 Apr 1998 09:45:58 +0800

At 11:58 AM 4/13/98 -0800, Diaco Davari wrote:
>I've been working on a Intel based PC design, where I think
>ringing/reflection on the
>66 MHz clock to AGP (Video port) is going to be an EMC & Signal Integrity
>issue due to long track lengths.
>We have used a 6 layer standard design layout of
>Signal/GND/Signal/Signal/VCC/Signal, with all clock lines buried
>interlayer. The fab house designs the geometry to control the stackup to
>make the 65 Ohm on these critical lines. As you may already know, this 66
>MHz clock line splits into two line which one loops back to the internal
>PLL on the Intel chip and the other feeds directly to AGP port. Each
>branch has its own damping resistor. According to design specification
>both lines have equal length (Approx. 8 inch) but the loop back line is
>cerpentined. The cerpentined feed back segment Placed interlayer next to
>GND plane and placed right under the Intel chip. The clock line that
>feeds AGP connector only switches once from inter layer plane A to B.
>According to the Emission data gathered with AGP video card in place, we
>are still marginally above limit. We have already tried different values
>of damping resistor at source and load side. Also, tried small value of
>Caps on both ends to match the impedance of the line with source and
>load, but no cleaner signal or better emission performance observed.
>Also, enabling the Spread Spectrum on clock generator has no effect on
>the 66 MHz clock that comes out of the PLL section of the Intel chip.
>Does anyone know how to best resolve this Signal Integrity and EMC issue?
>What is the best termination configuration?

If you have split lines, normally the termination values are 2x65 Ohm at
each end.
Although the split lines are of the same width. (assuming).

>What is the best layout design to cure this problem?

It seems that you have 2 signals between the gnd and Vcc, this is not good for
high speed circuits > 100MHz, I should think it will still work for 66
Mhz. We normally
recommend to have gnd and Vcc coupled together, S,S,GND,VCC,S,S
1,2, 3 , 4 , 5 , 6

The most critical clocks and lines are routed in 2 and 5. We have some
with signal integrity simulation that show the integrity will degrade if
you put the signals
in between the Vcc and Gnd.

Good Luck.
>Best Regards,
Cheah Soo Lan