electrical level conversion

[John Larkin]

Unfortunately the MGTCLK_IN inputs have a built-in termination, so I see a
problem in terminating #2 clock pair and not terminating clock #1 !? Has
anyone made experiences with the RocketIO clock inputs, yet?

Oh. If both clock inputs are internally terminated, the simplest thing
might to be to use an active buffer, an lvds-lvds part with two
differential outputs. It could be a dual buffer or one of the fanout
things with one input and multiple outputs. Four resistors at the pecl
output would pull down the pecl and shift the dc level down to make
the lvds buffer inputs all nice and legal. Then just route pairs from
each lvds output to the Vertex clock inputs.

Check TI and National for apropriate lvds buffers. You'll need some
software to calculate the differential trace impedances, Txline maybe.

John

 

[Richard Henry]

The Xilinx people are probably on comp.arch.fpga

Hoping they will chime in, I have added caf.

 

[Austin Lesea]

All,

I have previously posted on the differential input circuit that we
(Xilinx) use.

I will repeat what I have said before: the differential input circuit
is a full CMOS differential comparator. It will operate (function) from
rail to rail on its inputs. Its performance has only been characterized
for LVDS, and low voltage LVPECL common mode voltages and swings.

Now for the new part: there are no configuration bits to select
anything. The comparator is the comparator, and it is the same circuit
regardless of standard selected. If it is differential, it is the same
circuit.

Hope this helps,

Austin

 

[PeteS]

Oh. If both clock inputs are internally terminated, the simplest thing
might to be to use an active buffer, an lvds-lvds part with two
differential outputs. It could be a dual buffer or one of the fanout
things with one input and multiple outputs. Four resistors at the pecl
output would pull down the pecl and shift the dc level down to make
the lvds buffer inputs all nice and legal. Then just route pairs from
each lvds output to the Vertex clock inputs.

Check TI and National for apropriate lvds buffers. You'll need some
software to calculate the differential trace impedances, Txline maybe.

John

I have a perl script I posted some time ago that calculates
differential impedance (there's a bug I have never fixed in the
propagation velocity, though).

I've used this a number of times and empirically it agrees with the
board manufacturer's data when they calculate the requirements for me.

Once I get home, I'll post it again.

Cheers

PeteS

 

[PeteS]

I have a perl script I posted some time ago that calculates
differential impedance (there's a bug I have never fixed in the
propagation velocity, though).

I've used this a number of times and empirically it agrees with the
board manufacturer's data when they calculate the requirements for me.

Once I get home, I'll post it again.

Cheers

PeteS

# top-level options

# Globals

$Pi = 3.1415926535;

# main
print "\n\n";
print "Impedance calculator\n\n";
print "Source material:\n Johnson & Graham, ";
print "\"High-Speed Digital Design.\" 1993. Appendix C.\n";
print "Differential impedance calculations based on empirical data\n";
{
my ($choice);
do {
print "\nEnter S for stripline, M for microstrip, X to exit
<$choice>: ";
$entry = <>;
chomp $entry;
$choice = uc $entry unless $entry eq "";
print "\n";

if ($choice eq "S") {
STRIPLINE();
}

if ($choice eq "M") {
MICROSTRIP();
}
} until ($choice eq "X")
}

# Subroutines
BEGIN {

sub E_skny($$$) {
local ($h,$w,$Er) = @_;
return ($Er + 1) / 2 + ($Er - 1) / 2 * ((1 + 12 * $h / $w) ** -0.5 +
0.04*(1-$w/$h)**2);
}

sub E_wide($$$) {
local ($h,$w,$Er) = @_;
return ($Er + 1) / 2 + ($Er - 1) / 2 * (1 + 12 * $h / $w) ** -0.5;
}

sub E_temp($$$) {
local ($h, $w, $Er) = @_;
return ( $w > $h ? E_wide($h, $w, $Er) : E_skny($h, $w, $Er));
}

sub EEFF($$$$) {
local ($h, $w, $t, $Er) = @_;
return E_temp($h, $w, $Er) - (($Er - 1) * $t / $h) / 4.6 * ($w / $h) **
0.5;
}

sub WE_skny($$$) {
local ($h, $w, $t) = @_;
return $w + (1.25 * $t)/$Pi * (1 + log(4 * $Pi * $w / $t));
}

sub WE_wide($$$) {
local ($h, $w, $t) = @_;
return $w + (1.25 * $t)/$Pi * (1 + log(2 * $h / $t));
}

sub WE($$$) {
local ($h, $w, $t) = @_;
return ($w > $h / (2 * $Pi) ? WE_wide($h, $w, $t) : WE_skny($h, $w, $t));
}

sub ZMS_skny($$$) {
local ($h, $w, $t) = @_;
return 60 * log((8 * $h / WE($h, $w, $t)) + (WE($h, $w, $t) / (4 * $h)));
}

sub ZMS_wide($$$) {
local ($h, $w, $t) = @_;
return 120 * $Pi / (WE($h, $w, $t) / $h + 1.393 + 0.667 *
log(WE($h, $w, $t) / $h + 1.444));
}

sub ZMSTRIP($$$$) {
local ($h, $w, $t, $Er) = @_;
return ($w > $h ? ZMS_wide($h, $w, $t) : ZMS_skny($h, $w, $t)) /
EEFF($h, $w, $t, $Er) ** 0.5;
}

sub PMSTRIP($$$$) {
local ($h, $w, $t, $Er) = @_;
return 84.72e-12 * EEFF($h, $w, $t, $Er) ** 0.5;
}

sub LMSTRIP($$$$) {
local ($h, $w, $t, $x) = @_;
return PMSTRIP($h, $w, $t, 1) * ZMSTRIP($h, $w, $t, 1) * $x;
}

sub CMSTRIP($$$$$) {
local ($h, $w, $t, $Er, $x) = @_;
return PMSTRIP($h, $w, $t, $Er) / ZMSTRIP($h, $w, $t, $Er) * $x;
}

{ # this block makes the "my" below permanent
my ($h, $w, $t, $s);
sub MICROSTRIP {
print "\nMicrostrip configuration\n";
$h *= 1000;
$w *= 1000;
$t *= 1000;
$s *= 1000;
print "Enter height to reference plane (mil) <$h>: ";
$input = <>;
$h = $input unless $input eq "\n";
print "Enter width of track (mil) <$w>: ";
$input = <>;
$w = $input unless $input eq "\n";
print "Enter separation of diff traces <$s>: ";
$input = <>;
$s = $input unless $input eq "\n";
print "Enter thickness of trace (mil) (1oz = 1.37) <$t>: ";
$input = <>;
$t = $input unless $input eq "\n";
print "Enter dielectric constant <$Er>: ";
$input = <>;
$Er = $input unless $input eq "\n";

$h /= 1000;
$w /= 1000;
$t /= 1000;
$s /= 1000;
chomp $Er;

$hi_accuracy = 0;
if (($t / $h > 0) && ($t / $h < 0.2)) {
if (($w / $h > 0.1) && ($w / $h < 20)) {
if (($Er > 0) && ($Er < 16)) {
$hi_accuracy = 1;
}
}
}

print "\n";
$Zo = ZMSTRIP($h, $w, $t, $Er);
printf "Calculated impedance (ohm): %6.2f ", $Zo;
print ($hi_accuracy ? "(accuracy > 98%)" : "(accuracy undefined)");
print "\n";
printf "Differential impedance (ohm): %6.2f (+/-10%)\n",
ZMDIFF ($Zo, $s, $h);
printf "Calculated prop delay (in/ns): %6.2f\n", 1e-9 /
PMSTRIP($h, $w, $t, $Er);
printf "Calculated inductance (nH/in): %6.2f\n", 1e9 *
LMSTRIP($h, $w, $t, 1);
printf "Calculated capacitance (pF/in): %6.2f\n", 1e12 *
CMSTRIP($h, $w, $t, $Er, 1);
}
}

sub ZSTR_K1 ($$) {
local ($w, $t) = @_;
local ($tmp1, $tmp2, $tmp3) = 0;
$tmp1 = 1 + log(4 * $Pi * $w / $t);
$tmp2 = 1 + $t * $tmp1 / ($Pi * $w);
$tmp3 = $tmp2 + 0.255 * ($t / $w) ** 2;
return $w * $tmp3 / 2;
}

sub ZSTR_skny ($$$$) {
local ($b, $w, $t, $Er) = @_;
return 60 / ($Er ** 0.5) * log (4 * $b / ($Pi * ZSTR_K1($w, $t)));
}

sub ZSTR_K2 ($$) {
local ($b, $t) = @_;
local ($tmp1) = 0;
$tmp1 = 1 - $t / $b;
return 2 / $tmp1 * log(1 / $tmp1 + 1) - (1 / $tmp1 - 1) *
log(1 / $tmp1 ** 2 - 1);
}

sub ZSTR_wide ($$$$) {
local ($b, $w, $t, $Er) = @_;
return 94.15 / $Er ** 0.5 / ( ($w / $b) / (1 - $t / $b) +
ZSTR_K2 ($b, $t) / $Pi);
}

sub ZSTRIP ($$$$) {
local ($b, $w, $t, $Er) = @_;
return ($w > 0.35 * $b ? ZSTR_wide($b, $w, $t, $Er) :
ZSTR_skny ($b, $w, $t, $Er));
}

sub ZOFFSET ($$$$$) {
local ($h1, $h2, $w, $t, $Er) = @_;
return (2 * ZSTRIP(2 * $h1 + $t, $w, $t, $Er) *
ZSTRIP(2 * $h2 + $t, $w, $t, $Er)) /
(ZSTRIP(2 * $h1 + $t, $w, $t, $Er) +
ZSTRIP(2 * $h2 + $t, $w, $t, $Er));
}

sub PSTRIP($) {
local $Er = @_;
return 84.72e-12 * $Er ** 0.5;
}

sub LSTRIP($$$$) {
local ($b, $w, $t, $x) = @_;
return PSTRIP(1) * ZSTRIP($b, $w, $t, 1) * $x;
}

sub LOSTRIP($$$$$) {
local ($h1, $h2, $w, $t, $x) = @_;
return PSTRIP(1) * ZOFFSET($h1, $h2, $w, $t, 1) * $x;
}

sub CSTRIP($$$$$) {
local ($b, $w, $t, $Er, $x) = @_;
return PSTRIP($Er) / ZSTRIP($b, $w, $t, $Er);
}

sub COSTRIP($$$$$$) {
local ($h1, $h2, $w, $t, $Er, $x) = @_;
return PSTRIP($Er) / ZOFFSET($h1, $h2, $w, $t, $Er);
}

{ # this line makes the "my" below permanent
my ($h1, $h2, $w, $t, $s);
sub STRIPLINE {
print "\nStripline configuration\n";
$h1 *= 1000;
$h2 *= 1000;
$w *= 1000;
$t *= 1000;
$s *= 1000;
print "Enter distance to reference plane above (mil) <$h1>: ";
$input = <>;
$h1 = $input unless $input eq "\n";
print "Enter distance to reference plane below (mil) <$h2>: ";
$input = <>;
$h2 = $input unless $input eq "\n";
print "Enter width of track (mil) <$w>: ";
$input = <>;
$w = $input unless $input eq "\n";
print "Enter separation of diff traces <$s>: ";
$input = <>;
$s = $input unless $input eq "\n";
print "Enter thickness of trace (mil) (1oz = 1.37) <$t>: ";
$input = <>;
$t = $input unless $input eq "\n";
print "Enter dielectric constant <$Er>: ";
$input = <>;
$Er = $input unless $input eq "\n";

$h1 /= 1000;
$h2 /= 1000;
$w /= 1000;
$t /= 1000;
$s /= 1000;
chomp $Er;

$b = $h1 + $h2 + $t;
$hi_accuracy = 0;

if ($h1 == $h2) {
print "Symmetric stripline configuration\n\n";
if (($t / $b < 0.25) && ($t / $w < 0.11)) { $hi_accuracy = 1 }
$symmetric = 1;
}
else {
print "Asymmetric stripline configuration\n\n";
$symmetric = 0;
}

$Zo = ($symmetric ? ZSTRIP($b,$w,$t,$Er) : ZOFFSET($h1, $h2, $w,
$t, $Er));
printf "Calculated impedance (ohm): %6.2f ", $Zo;
print ($hi_accuracy ? "(accuracy > 98.7%)" : "(accuracy undefined)");
print "\n";
printf "Differential impedance (ohm): %6.2f (+/-10%)\n",
ZSDIFF($Zo, $s, $h1, $h2);
printf "Calculated prop delay (in/ns): %6.2f\n", 1e-9 /
PSTRIP($Er);
printf "Calculated inductance (nH/in): %6.2f\n", 1e9 *
($symmetric ? LSTRIP($b, $w, $t, 1) : LOSTRIP($h1, $h2, $w, $t, 1));
printf "Calculated capacitance (pF/in): %6.2f\n", 1e12 *
($symmetric ? CSTRIP($b, $w, $t, $Er, 1) :
COSTRIP($h1, $h2, $w, $t, $Er, 1));
}
}
sub ZMDIFF ($$$) {
my ($Zo, $s, $h) = @_;
return 2*$Zo*(1-0.48*exp(-.96*$s/$h));
}
sub ZSDIFF ($$$$) {
my ($Zo, $s, $h1, $h2) = @_;
return 2*$Zo*(1-0.374*exp(-2.9*$s/($h1+$h2)));
}
}

 

[John Larkin]

All,

I have previously posted on the differential input circuit that we
(Xilinx) use.

I will repeat what I have said before: the differential input circuit
is a full CMOS differential comparator. It will operate (function) from
rail to rail on its inputs. Its performance has only been characterized
for LVDS, and low voltage LVPECL common mode voltages and swings.

Now for the new part: there are no configuration bits to select
anything. The comparator is the comparator, and it is the same circuit
regardless of standard selected. If it is differential, it is the same
circuit.

Hope this helps,

Austin

It is, incidentally, a *very good* comparator!

John