Termination Resistor Calibration Circuit
One resistor calibration circuit (RCAL) is shared between all GTM transceiver DUAL primitives in
a GTM transceiver Dual column. The MGTAVTTRCAL and MGTRREF pins connect the bias
circuit power and the external calibration resistor to the RCAL circuit. The RCAL circuit performs
the resistor calibration only during configuration of the Ult device. Prior to configuration,
all analog supply voltages must be present and within the proper tolerance as specified in the
Ult device data sheets (see
http://www.xilinx.com/documentation
). If an entire power
supply group (PSG) is not used by any Duals, MGTAVTTRCAL and MGTRREF should be tied to
ground. See
for more details regarding RCAL biasing recommendations
when there are unused Duals.
The RCAL circuit is associated with the GTM transceiver Dual that is the RCAL master. The RCAL
master performs the termination resistor calibration during configuration of the Ult
device and then distributes the calibrated values to all of the GTM transceiver Duals in the
column. The Dual in which the RCAL circuit is located must be powered on. For Stacked Silicon
Interconnect (SSI) technology devices, each slice to be used (that contains multiple Duals) must
be powered on.
Connect the MGTAVTTRCAL pin to the MGTAVTT supply and to a pin on the 100Ω precision
external resistor. The other pin of the resistor is connected to the MGTRREF pin. The resistor
calibration circuit provides a controlled current load to the resistor connected to the MGTRREF
pin. It then senses the voltage drop across the external calibration resistor and uses that value to
adjust the internal resistor calibration setting. The quality of the resistor calibration is dependent
on the accuracy of the voltage measurement at the MGTAVTTRCAL and MGTRREF pins. To
eliminate errors due to the voltage drop across the traces that lead from the resistor and to the
Ult device pins, the trace from the MGTAVTTRCAL pin to the resistor should have the
same length and geometry as the trace that connects the other pin of the resistor to the
MGTRREF pin. Also, the maximum DC resistance of the PCB trace must be limited to less than
0.5Ω. (See the suggested layout in the following figure.)
Figure 47:
PCB Layout for the RCAL Resistor
100Ω
Trace length from the resistor pins to the
FPGA pins MGTRREF and MGTAVTTRCAL
must be equal in length.
Connection
to AVTT
MGTAVTTRCAL
MGTRREF
X20930-053118
Chapter 5: Board Design Guidelines
UG581 (v1.0) January 4, 2019
Virtex Ult GTM Transceivers
119