Hub-Module FEX MGT Data Fanout ----------------------------------- Original Rev. 20-Mar-2015 Current Rev. 16-Feb-2016 This note describes the 74 channel MGT Fanout on the Hub Module. This fanout receives FEX readout data from the Hub's ATCA Zone 2 connectors and sends this data to MGT transceiver inputs on both the ROD mezzanine card and to the Hub's own UltraScale Virtex FPGA. This fanout is based on the NB7VQ14M chip from On-Semi. AC Coupling: ------------ The outputs from the NB7VQ14M fanout chips are AC coupled. 100 nFd 0201 size DC Blocking capacitors are used for this AC coupling. These capacitors are located very close the the output pins on the NB7VQ14M fanout chips. Input Common Mode Reference: ---------------------------- The signals send to the Hub's MGT Fanout are all AC coupled at the sending end. Thus the input common mode voltage to NB7VQ14M fanout chips can and must be set with circuits on the Hub Module. The fanout chips internal VRef generator is used to set their common mode input voltage. When powered with a Vcc of 1.800 Volts this reference generator provides a 1.300 V +- 150 mV common mode input voltage to the fanout chips. This makes sense as the input circuit of these fanout chips is the bases of a long-tail NPN pair the emitters of which are feed by a current source. Input Termination: ------------------ The NB7VQ14M fanout chips contain their own input termination resistors. From a center tap there is a 50 Ohm resistor to each input. This center tap is tied to the common mode reference supply and bypassed to ground with a 47 nFd capacitor. In this way the common mode input voltage to the receiver is set, the 100 Ohm differential termination is realized, and a common mode termination is implemented. Equalizer Enable: ----------------- The NB7VQ14M fanout chips contain an equalizer that can reduce inter symbol interference on long copper PCB traces. This equalizer is enabled or removed from the fanout circuit by means of a logic level control pin. Because it is not yet clear if this equalizer will improve the MGT readout signals in the ATCA backplane application we will make the Equalizer Enable pin accessible on the Hub Module on a per slot basis. The assumption is that for the backplane losses from a given slot that either all 6 of the MGT readout signals from that slot will be improved by the equalizer or else none of them will. Thus from the point of view of the Equalizer Enables, the MGT fanout it is divided into 13 section (12 FEXs and the Other Hub) with the equalizer enables individually programmable to each of these sections. The equalizer enable input pin has a 75k Ohm pull-down resistor. 6 of these in parallel is 12.5k Ohm. When this logic input is at its default Low the equalizer is bypassed. The equalizer enable logic levels are: Low is < 0.35 Vcc High is > 0.65 Vcc. With 1.8 Volt Vcc Low is < 0.630 and High is > 1.170 volts. Current is under +- 150 uAmp. The fanout equalizer enable pins are driven by a 1V8 HP Select I/O Bank on the Hub FPGA. In the Hi state, these control signals from the Hub FPGA are guaranteed to reach 1.35 Volts (in LVCMOS_1V8 mode) or 1.400 Volts (in HSTL mode). So we are guaranteed to be able to remain in control of these equalizer enable inputs with the fanout chips operating from a Vcc as high as 2.154 Volts. Reference Designators: ---------------------- The Hub Module uses an overall scheme to organize the reference designators of its many components. The fanout chips themselves run from U401 through U475 with an increment of 1 between channels. The various capacitors in these circuits (DC blocking, Vcc bypass, VRef bypass) run from C401 through C919 with an increment of 7 between channels. Fanout Vcc Supply: ------------------ The NB7VQ14M fanout chips can be powered with anything from 1.8 V to 3.3V. The lower supply voltages are attractive because of the reduced heat dissipation. From the data sheet I see no advantage in powering the fanout circuits from a 3.3 Volt supply. Ed's work indicates that 1V8 works as well or better than 2V5. There may be some advantage in using a fanout Vcc supply somewhat higher than the 1V8 minimum listed in the NB7VQ14M data sheet. In any case this supply must be quiet as any noise that it puts onto these high speed MGT signals is likely to interfere with reception of these signals from across the ATCA backplane. Thus the Hub Module provides a private supply just for powering these fanout chips. This supply, FAN_1V8, is nominally set for 1.8 Volts. Because it does not power anything besides the fanout chips it could be adjusted up to at least 2.150 Volts if that is necessary. The expected current draw for the 74 fanout chips is 13 Amps. At maximum the fanout Vcc current draw should be less than 15 Amps. The heat from the 74 fanout chips is expected to be about 23 Watts. The Vcc pins on each fanout chip are bypassed with a 4.7 nFd and 100 nFd 0402 size ceramic capacitors as close as possible to the Vcc pins. Routing Challenges: ------------------- All of the differential pair traces associated with the FEX MGT data fanout circuits must be able to cleanly support 10 Gb/sec data rates. Interference and cross-talk must be kept to a minimum. This is especially true on the traces that run from the Zone 2 connectors to the inputs of the fanout chips. These MGT signals may be in poor condition after their travel across the backplane and through two ADFplus connectors. The 74 outputs running to the ROD must route out and separate from the 74 pairs that run to the Hub's FPGA. There is little space available on the Hub Module to hold the MGT fanout. Fanout chips are located on both sides of the card and the design of the side 2 cover needs to take the height and heat dissipation from these parts into consideration. Fanout Name: ------------ In a few places the documentation for the Hub Module may still call its FEX MGT data fanout by the name "GTH Fanout". This old name came from the original design of the Hub Module that used a Virtix-7 FPGA with only GTH type MGT transceivers. The old name has been replaced in most of the Hub's documentation but I know that it is still used in the net names.