On Apr 6, 2009, at 2:51 PM, [1]carzr...@optonline.net wrote: tune down your edge rate in the FPGA to make the effective
bandwidth of your edges lower. square waves are an infinite series of sine waves, trapezoidal are finite. ( roughly speaking ) 3 inches is a 1/8th wave antenna for 500MHz, again roughly ( 1GHz ~= 12 inches ~=1 nS at the speed of light. ) if you worried about signal integrity, ignore the logic analyzer Yeah but on a pcb, flight-time on FR-4 is around 170pS / inch (off the top of my head, anyway). By way of 'rule of thumb' the trace looks like a transmission line if the flight time is greater than 1/4 wavelength. If 1ns edge rate, then it's 1/4 of 4nS, which translates into about 1" (I think I did that right). So anything longer than 1" will have transmission line affects, and you should consider termination. my old timer gave me a different way.... use 1/8th wave antenna on PCB, and the speed of light..... 1/4 wave antenna open air.... interestingly enough they are about the same, it's interesting to see the difference in rules of thumb. a 1ns edge rate if perfectly toggled looks like a sine wave (RC effects) that has just over 2nS period or just under 500MHz 500MHz on FR4 using your numbers of 170ps/inch, about 0.5C :-). From Mathematica, to keep track of my units. <<Units` Take 170 ps per inch and multiply it by 500 Mega Hertz (170. Pico Second)/(1 Inch) 500 Mega Hertz (85000. Hertz Mega Pico Second)/Inch What is per inches, invert..... 1/% (0.0000117647 Inch)/(Hertz Mega Pico Second) clean it up to inches, nice that Mathematica leaves all of the units in there :-) Convert[%, Inch] 11.7647 Inch Make it a quarter wave antenna %/4 2.94118 Inch How long you need to care after .... about 3 inches. So slow up your edge rate, with some source resistors or through your drive strength and other settings in the FPGA. now for the 1/8th wave rule of thumb <<PhysicalConstants` Lets look at the 1/8 th rule and speed of light SpeedOfLight /( 500 Mega Hertz) (149896229 Meter)/(250 Hertz Mega Second) Take it to inches N[Convert[%,Inch]] 23.6057 Inch 1/8th wave antenna %/8 2.95071 Inch All rules of Thumb..... Xilinx fpga's are very capable of producing 1 nS edges or maybe better. That translates to around 500 MHz. My experience with the slew rate limiting on the spartan-2 is that it doesn't do very much. Dropping the drive strength will definitely slow the edges since the output impedance of the gate goes up and you get an RC rise time affect on the line. As long as the degraded rise-time is ok in your design, this is a good method. Check your ram's spec sheets, also look at the additional delay that the traces on the far side will be exposed. longest clock Vs. shortest data and vice versa. You want your data eye to be nice and big, the sampling clock right in the middle of it. The cleaner the eye the slower your edge rates can go. gene _______________________________________________ geda-user mailing list [2]geda-u...@moria.seul.org http://www.seul.org/cgi-bin/mailman/listinfo/geda-user References 1. mailto:carzr...@optonline.net 2. mailto:geda-user@moria.seul.org
_______________________________________________ geda-user mailing list geda-user@moria.seul.org http://www.seul.org/cgi-bin/mailman/listinfo/geda-user