As I understand it, the VIIPro devices have on-chip differential termination resistors which can be brought into play by selecting the XXX_LVDS_25_DT buffers. I've looked at several designs using VIIPro devices from different manufacturers but they all seem to use external 100R resistors to achieve LVDS termination. Can anyone tell me why this would be the case please? I thought the reduction in component count would be very welcome. Is it because of heat dissipation? TIA, Rog.
VIIPro on-chip LVDS termination
Started by ●May 5, 2005
Reply by ●May 5, 20052005-05-05
Roger, This is likely a case of old habits die hard. Also since this wasn't officially supported when Virtex-II Pro was first released the early the board was created the more likely the external resistor would be present. Since the "_DT" is true differential termination, there is no significant heat dissipation caused by using it. Unlike, the previous thevenin equivalent split termination version with a two 100 ohm loads between 2.5V and GND. Ed Roger wrote:> As I understand it, the VIIPro devices have on-chip differential termination > resistors which can be brought into play by selecting the XXX_LVDS_25_DT > buffers. I've looked at several designs using VIIPro devices from different > manufacturers but they all seem to use external 100R resistors to achieve > LVDS termination. Can anyone tell me why this would be the case please? I > thought the reduction in component count would be very welcome. Is it > because of heat dissipation? > > TIA, > > Rog. > >
Reply by ●May 5, 20052005-05-05
Ed, Thanks again. So do the later Xilinx boards utilise the on-chip termination? Rog. "Ed McGettigan" <ed.mcgettigan@xilinx.com> wrote in message news:427A69B1.9060709@xilinx.com...> Roger, > > This is likely a case of old habits die hard. Also since this > wasn't officially supported when Virtex-II Pro was first released > the early the board was created the more likely the external > resistor would be present. > > Since the "_DT" is true differential termination, there is no > significant heat dissipation caused by using it. Unlike, the > previous thevenin equivalent split termination version with a > two 100 ohm loads between 2.5V and GND. > > Ed > > Roger wrote: >> As I understand it, the VIIPro devices have on-chip differential >> termination resistors which can be brought into play by selecting the >> XXX_LVDS_25_DT buffers. I've looked at several designs using VIIPro >> devices from different manufacturers but they all seem to use external >> 100R resistors to achieve LVDS termination. Can anyone tell me why this >> would be the case please? I thought the reduction in component count >> would be very welcome. Is it because of heat dissipation? >> >> TIA, >> >> Rog.
Reply by ●May 5, 20052005-05-05
They should, but sometimes a circuit cut-and-paste from a previous schematic may have left in place or we may have left the external pads in place for flexibility. You can always desolder the external resistor and use the internal one instead. I'm fairly certain I had all of the external differential resistors removed for the Virtex-4 boards (ML40x, ML41x, ML42x) during the schematic reviews, but we may have simply missed one or two. Ed Roger wrote:> Ed, > > Thanks again. So do the later Xilinx boards utilise the on-chip termination? > > Rog. > > "Ed McGettigan" <ed.mcgettigan@xilinx.com> wrote in message > news:427A69B1.9060709@xilinx.com... > >>Roger, >> >>This is likely a case of old habits die hard. Also since this >>wasn't officially supported when Virtex-II Pro was first released >>the early the board was created the more likely the external >>resistor would be present. >> >>Since the "_DT" is true differential termination, there is no >>significant heat dissipation caused by using it. Unlike, the >>previous thevenin equivalent split termination version with a >>two 100 ohm loads between 2.5V and GND. >> >>Ed >> >>Roger wrote: >> >>>As I understand it, the VIIPro devices have on-chip differential >>>termination resistors which can be brought into play by selecting the >>>XXX_LVDS_25_DT buffers. I've looked at several designs using VIIPro >>>devices from different manufacturers but they all seem to use external >>>100R resistors to achieve LVDS termination. Can anyone tell me why this >>>would be the case please? I thought the reduction in component count >>>would be very welcome. Is it because of heat dissipation? >>> >>>TIA, >>> >>>Rog. > > >
