FPGA Central

The lane signals should be AC coupled:
a. on the motherboard between the controller and PCIe socket
b. on the PCIe card between the FPGA GTP and PCIe PCB connector
c. on both

?
thx,
Vasile

The spec says all _transmitters_ shall be AC coupled. You just lead the
RX line directly to your component.

In terms of your a) b) c) options, the answer is therefore c) but only
the TX line(s).

By the way as far as I remember, there is some Intel doc recommending
placing the cap at 1/3 of the distance from plug to component (i.e.
closer to the plug than to the component).

On Aug 29, 2:55 pm, "Charles, NG" <site_blackh...@trellisys.ie> wrote:
> The spec says all _transmitters_ shall be AC coupled. You just lead the
> RX line directly to your component.
>
> In terms of your a) b) c) options, the answer is therefore c) but only
> the TX line(s).
>
> By the way as far as I remember, there is some Intel doc recommending
> placing the cap at 1/3 of the distance from plug to component (i.e.
> closer to the plug than to the component).


Also if you're designing a PCIe plug-in card, remember that the signal
names on the connector are based on the motherboard. So you need to
connect your transmitter to the receive signals (PERP0/PERN0 ...) and
your receiver to the transmit signals (PETP0/PETN0 ...). Since the Rx
and Tx signals are on the opposite board surface, it is very hard to
correct swapped signals using wires! I got bit by this on my first
PCIe design.

On Aug 29, 12:43 pm, Gabor <ga...@alacron.com> wrote:
> On Aug 29, 2:55 pm, "Charles, NG" <site_blackh...@trellisys.ie> wrote:
>
> > The spec says all _transmitters_ shall be AC coupled. You just lead the
> > RX line directly to your component.
>
> > In terms of your a) b) c) options, the answer is therefore c) but only
> > the TX line(s).
>
> > By the way as far as I remember, there is some Intel doc recommending
> > placing the cap at 1/3 of the distance from plug to component (i.e.
> > closer to the plug than to the component).
>
> Also if you're designing a PCIe plug-in card, remember that the signal
> names on the connector are based on the motherboard. So you need to
> connect your transmitter to the receive signals (PERP0/PERN0 ...) and
> your receiver to the transmit signals (PETP0/PETN0 ...). Since the Rx
> and Tx signals are on the opposite board surface, it is very hard to
> correct swapped signals using wires! I got bit by this on my first
> PCIe design.

Thx for attentioning but my intention is to route differential signals
on stripline
(between ground planes) and I guess will be no wire.
I'm designing both the PCIe card and the motherboard and...

Vasile

On Aug 29, 5:50 pm, PeteS <axk...@dsl.pipex.com> wrote:
> Gabor wrote:
> > On Aug 29, 2:55 pm, "Charles, NG" <site_blackh...@trellisys.ie> wrote:
> >> The spec says all _transmitters_ shall be AC coupled. You just lead the
> >> RX line directly to your component.
>
> >> In terms of your a) b) c) options, the answer is therefore c) but only
> >> the TX line(s).
>
> >> By the way as far as I remember, there is some Intel doc recommending
> >> placing the cap at 1/3 of the distance from plug to component (i.e.
> >> closer to the plug than to the component).
>
> > Also if you're designing a PCIe plug-in card, remember that the signal
> > names on the connector are based on the motherboard. So you need to
> > connect your transmitter to the receive signals (PERP0/PERN0 ...) and
> > your receiver to the transmit signals (PETP0/PETN0 ...). Since the Rx
> > and Tx signals are on the opposite board surface, it is very hard to
> > correct swapped signals using wires! I got bit by this on my first
> > PCIe design.
>
> It is very important to make sure your coupling capacitor is large
> enough to handle the 30kHz beacons, so *if* (as I do on occasion) you ac
> couple at _each end of the link_, make sure you used double the
> recommended value of capacitor if it's a short (in the transmission line
> sense) link.

This is new for me. What do you mean with 30KHz beacon at 3.1Gbps ?
The 0.1uF impedance is large enough for a Ghz signal.

Gabor wrote:
> On Aug 29, 2:55 pm, "Charles, NG" <site_blackh...@trellisys.ie> wrote:
>> The spec says all _transmitters_ shall be AC coupled. You just lead the
>> RX line directly to your component.
>>
>> In terms of your a) b) c) options, the answer is therefore c) but only
>> the TX line(s).
>>
>> By the way as far as I remember, there is some Intel doc recommending
>> placing the cap at 1/3 of the distance from plug to component (i.e.
>> closer to the plug than to the component).
>
>
> Also if you're designing a PCIe plug-in card, remember that the signal
> names on the connector are based on the motherboard. So you need to
> connect your transmitter to the receive signals (PERP0/PERN0 ...) and
> your receiver to the transmit signals (PETP0/PETN0 ...). Since the Rx
> and Tx signals are on the opposite board surface, it is very hard to
> correct swapped signals using wires! I got bit by this on my first
> PCIe design.
>

It is very important to make sure your coupling capacitor is large
enough to handle the 30kHz beacons, so *if* (as I do on occasion) you ac
couple at _each end of the link_, make sure you used double the
recommended value of capacitor if it's a short (in the transmission line
sense) link.

Cheers

PeteS

On Aug 31, 8:52 pm, PeteS <axk...@dsl.pipex.com> wrote:
> vasile wrote:
> > On Aug 29, 5:50 pm, PeteS <axk...@dsl.pipex.com> wrote:
> >> Gabor wrote:
> >>> On Aug 29, 2:55 pm, "Charles, NG" <site_blackh...@trellisys.ie> wrote:
> >>>> The spec says all _transmitters_ shall be AC coupled. You just lead the
> >>>> RX line directly to your component.
> >>>> In terms of your a) b) c) options, the answer is therefore c) but only
> >>>> the TX line(s).
> >>>> By the way as far as I remember, there is some Intel doc recommending
> >>>> placing the cap at 1/3 of the distance from plug to component (i.e.
> >>>> closer to the plug than to the component).
> >>> Also if you're designing a PCIe plug-in card, remember that the signal
> >>> names on the connector are based on the motherboard. So you need to
> >>> connect your transmitter to the receive signals (PERP0/PERN0 ...) and
> >>> your receiver to the transmit signals (PETP0/PETN0 ...). Since the Rx
> >>> and Tx signals are on the opposite board surface, it is very hard to
> >>> correct swapped signals using wires! I got bit by this on my first
> >>> PCIe design.
> >> It is very important to make sure your coupling capacitor is large
> >> enough to handle the 30kHz beacons, so *if* (as I do on occasion) you ac
> >> couple at _each end of the link_, make sure you used double the
> >> recommended value of capacitor if it's a short (in the transmission line
> >> sense) link.
>
> > This is new for me. What do you mean with 30KHz beacon at 3.1Gbps ?
> > The 0.1uF impedance is large enough for a Ghz signal.
>
> PCIe (as with most high speed links) uses beacon signals to determine if
> the 'other end' of the link is active. That beacon is a relatively low
> frequency pulse to minimise EMC/EMI. 0.1uF is easily sufficient, but at
> 2.5Gb/s (1.25GHz fundamental) a 100pF cap would be sufficient - the cap
> recommended in the PCIe spec (you do have a copy right?) is sufficient
> for the beaconing. (I don't have a copy in front of me, but I seem to
> remember it's 0.075uF).
>
> Cheers
>
> PeteS


..075uF (75nF) is the minimum spec. There is also a maximum of 0.2uF
(200nF)
so theoretically 0.1uF -25/+100 % is good. Z5U or Y5U bypass quality
caps
won't give you this tolerance. X7R or X5R will. The 1.1
specification
talks about 0603 components, but I would think 0402 would work better.

vasile wrote:
> On Aug 29, 5:50 pm, PeteS <axk...@dsl.pipex.com> wrote:
>> Gabor wrote:
>>> On Aug 29, 2:55 pm, "Charles, NG" <site_blackh...@trellisys.ie> wrote:
>>>> The spec says all _transmitters_ shall be AC coupled. You just lead the
>>>> RX line directly to your component.
>>>> In terms of your a) b) c) options, the answer is therefore c) but only
>>>> the TX line(s).
>>>> By the way as far as I remember, there is some Intel doc recommending
>>>> placing the cap at 1/3 of the distance from plug to component (i.e.
>>>> closer to the plug than to the component).
>>> Also if you're designing a PCIe plug-in card, remember that the signal
>>> names on the connector are based on the motherboard. So you need to
>>> connect your transmitter to the receive signals (PERP0/PERN0 ...) and
>>> your receiver to the transmit signals (PETP0/PETN0 ...). Since the Rx
>>> and Tx signals are on the opposite board surface, it is very hard to
>>> correct swapped signals using wires! I got bit by this on my first
>>> PCIe design.
>> It is very important to make sure your coupling capacitor is large
>> enough to handle the 30kHz beacons, so *if* (as I do on occasion) you ac
>> couple at _each end of the link_, make sure you used double the
>> recommended value of capacitor if it's a short (in the transmission line
>> sense) link.
>
> This is new for me. What do you mean with 30KHz beacon at 3.1Gbps ?
> The 0.1uF impedance is large enough for a Ghz signal.
>
>

PCIe (as with most high speed links) uses beacon signals to determine if
the 'other end' of the link is active. That beacon is a relatively low
frequency pulse to minimise EMC/EMI. 0.1uF is easily sufficient, but at
2.5Gb/s (1.25GHz fundamental) a 100pF cap would be sufficient - the cap
recommended in the PCIe spec (you do have a copy right?) is sufficient
for the beaconing. (I don't have a copy in front of me, but I seem to
remember it's 0.075uF).

Cheers

PeteS

Gabor wrote:
> On Aug 31, 8:52 pm, PeteS <axk...@dsl.pipex.com> wrote:
>> vasile wrote:
>>> On Aug 29, 5:50 pm, PeteS <axk...@dsl.pipex.com> wrote:
>>>> Gabor wrote:
>>>>> On Aug 29, 2:55 pm, "Charles, NG" <site_blackh...@trellisys.ie> wrote:
>>>>>> The spec says all _transmitters_ shall be AC coupled. You just lead the
>>>>>> RX line directly to your component.
>>>>>> In terms of your a) b) c) options, the answer is therefore c) but only
>>>>>> the TX line(s).
>>>>>> By the way as far as I remember, there is some Intel doc recommending
>>>>>> placing the cap at 1/3 of the distance from plug to component (i.e.
>>>>>> closer to the plug than to the component).
>>>>> Also if you're designing a PCIe plug-in card, remember that the signal
>>>>> names on the connector are based on the motherboard. So you need to
>>>>> connect your transmitter to the receive signals (PERP0/PERN0 ...) and
>>>>> your receiver to the transmit signals (PETP0/PETN0 ...). Since the Rx
>>>>> and Tx signals are on the opposite board surface, it is very hard to
>>>>> correct swapped signals using wires! I got bit by this on my first
>>>>> PCIe design.
>>>> It is very important to make sure your coupling capacitor is large
>>>> enough to handle the 30kHz beacons, so *if* (as I do on occasion) you ac
>>>> couple at _each end of the link_, make sure you used double the
>>>> recommended value of capacitor if it's a short (in the transmission line
>>>> sense) link.
>>> This is new for me. What do you mean with 30KHz beacon at 3.1Gbps ?
>>> The 0.1uF impedance is large enough for a Ghz signal.
>> PCIe (as with most high speed links) uses beacon signals to determine if
>> the 'other end' of the link is active. That beacon is a relatively low
>> frequency pulse to minimise EMC/EMI. 0.1uF is easily sufficient, but at
>> 2.5Gb/s (1.25GHz fundamental) a 100pF cap would be sufficient - the cap
>> recommended in the PCIe spec (you do have a copy right?) is sufficient
>> for the beaconing. (I don't have a copy in front of me, but I seem to
>> remember it's 0.075uF).
>>
>> Cheers
>>
>> PeteS
>
>
> .075uF (75nF) is the minimum spec. There is also a maximum of 0.2uF
> (200nF)
> so theoretically 0.1uF -25/+100 % is good. Z5U or Y5U bypass quality
> caps
> won't give you this tolerance. X7R or X5R will. The 1.1
> specification
> talks about 0603 components, but I would think 0402 would work better.
>

A major issue is parasitics. 0402 is ok, 0603 is marginal.

As to type, I wouldn't use anything less than X5R anyway for a signal path.

Cheers

PeteS

The Beacon is one way a wakeup from L2 LTSSM state (deep power save).
Depends on whether your Phy implementation supports this method or
uses the Wake# or even whether the deep power save feature is
supported.

John Adair
Enterpoint Ltd.
www.enterpoint.co.uk

On 30 Aug, 01:49, vasile <picli...@gmail.com> wrote:
> On Aug 29, 5:50 pm, PeteS <axk...@dsl.pipex.com> wrote:
>
>
>
>
>
> > Gabor wrote:
> > > On Aug 29, 2:55 pm, "Charles, NG" <site_blackh...@trellisys.ie> wrote:
> > >> The spec says all _transmitters_ shall be AC coupled. You just lead the
> > >> RX line directly to your component.
>
> > >> In terms of your a) b) c) options, the answer is therefore c) but only
> > >> the TX line(s).
>
> > >> By the way as far as I remember, there is some Intel doc recommending
> > >> placing the cap at 1/3 of the distance from plug to component (i.e.
> > >> closer to the plug than to the component).
>
> > > Also if you're designing a PCIe plug-in card, remember that the signal
> > > names on the connector are based on the motherboard. So you need to
> > > connect your transmitter to the receive signals (PERP0/PERN0 ...) and
> > > your receiver to the transmit signals (PETP0/PETN0 ...). Since the Rx
> > > and Tx signals are on the opposite board surface, it is very hard to
> > > correct swapped signals using wires! I got bit by this on my first
> > > PCIe design.
>
> > It is very important to make sure your coupling capacitor is large
> > enough to handle the 30kHz beacons, so *if* (as I do on occasion) you ac
> > couple at _each end of the link_, make sure you used double the
> > recommended value of capacitor if it's a short (in the transmission line
> > sense) link.
>
> This is new for me. What do you mean with 30KHz beacon at 3.1Gbps ?
> The 0.1uF impedance is large enough for a Ghz signal.- Hide quoted text -
>
> - Show quoted text -