Stratix GX to Mercury
Interoperability
November 2002, ver. 1.0
Introduction
Application Note 247
The introduction of the StratixTM GX device family enables a new level of
integration by combining 3.125-gigabits per second (Gbps) transceivers
with a high-performance FPGA core. The Stratix GX family is the
industry’s most capable FPGA/transceiver combination. However, to
properly interface with existing systems or new implementations that use
MercuryTM devices, Stratix GX devices must be able to interoperate with
Mercury devices.
This application note describes the recommended approach to connect
Mercury devices to Stratix GX devices. The Mercury device’s transceivers
support the LVDS, LVPECL, and 3.3-V PCML I/O standards, while the
Stratix GX transceivers support the 1.5-V PCML standard, resulting in a
different common-mode voltage (VCM). However, the two devices can
effectively interconnect via AC coupling. AC coupling of the transmission
line removes the VCM variable or offset voltages (VOS) that are not at the
same level.
1
Considerations
for Connecting
the Devices
Altera Corporation
AN-247-1.0
Recommendations within this application note do not differ in
any way from the original Mercury board design
recommendations. As a result, any existing Mercury board can
maintain its current configuration while interoperating with new
Stratix GX designs.
Mercury clock data recovery (CDR) supports LVDS, LVPECL, and 3.3-V
PCML. The techniques for using these I/O standards have not changed
since the release of Mercury devices. Stratix GX gigabit transceiver blocks
support 1.5-V PCML, which may have a different VCM than the Mercury
I/O standards. These two devices can still interconnect when they are AC
coupled. The AC coupling of the transmission line enables devices with
different VCM to interoperate.
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AN 247: Stratix GX to Mercury Interoperability
Table 1 shows the minimum and maximum ratings on the transceivers
and the recommended VCM. Mercury LVPECL maximum output values
exceed the Stratix GX maximum input values; therefore, you cannot use
Mercury LVPECL to connect to Stratix GX devices. Stratix GX devices
have a programmable output differential (VOD) level, so the higher
maximum level is not an issue. It is important to always consider the
minimum input value, since Mercury devices have a larger minimum
requirement for input. When designing with Stratix GX devices, use a
larger output voltage swing to ensure that the signal level at the end of the
line, including attenuation, still meets the minimum requirement.
Table 1. Minimum & Maximum Input & Output Signal Capability
Voltage
Mercury
LVPECL
Stratix GX
3.3-V PCML
LVDS
Unit
1.5-V PCML
VID (maximum)
1,200
800
600
800
mV
VOD (maximum)
1,200
800
600
800
mV
VID (minimum)
400
400
200
100
mV
VCM (input) typical
2.1
3.0
2.1
1.1
V
VOS (output) typical
2.1
3.0
1.25
0.75
V
Programmable VOD & Pre-Emphasis
Stratix GX devices have programmable VOD and pre-emphasis. You can
use pre-emphasis to improve the ability of the device, allowing you to
drive a long transmission line. The programmed output current driver
level controls VOD. Changing the level of VOD can improve the signal
quality at the end of a long trace length or back plane. Tables 2 and 3 show
the choices for Stratix GX VOD and pre-emphasis programming. You
should consider these programming choices for the best signal integrity at
the end of the transmission line at the Mercury device’s receiver.
Table 2. Programmable VOD Settings
Termination (Ω)
2
mA
50
60
75
4
200
240
300
8
400
480
600
10
500
600
750
12
600
720
-
14
700
-
-
16
800
-
-
Altera Corporation
AN 247: Stratix GX to Mercury Interoperability
Table 3. Pre-Emphasis Settings
VOD (mV)
5%
10%
15%
20%
25%
200.0
210.0
220.0
230.0
240.0
250.0
240.0
252.0
264.0
276.0
288.0
300.0
300.0
315.0
330.0
345.0
360.0
375.0
400.0
420.0
440.0
460.0
480.0
500.0
480.0
504.0
528.0
552.0
576.0
600.0
500.0
525.0
550.0
575.0
600.0
625.0
600.0
630.0
660.0
690.0
720.0
750.0
600.0
630.0
660.0
690.0
720.0
750.0
600.0
630.0
660.0
690.0
720.0
750.0
700.0
735.0
770.0
-
-
-
720.0
756.0
792.0
-
-
-
750.0
787.5
-
-
-
-
800.0
-
-
-
-
-
Table 4 shows the minimum and maximum values for Stratix GX 1.5-V
PCML.
Table 4. Stratix GX 1.5-V PCML
Symbol
Parameter
Minimum
Typical
Maximum
Units
VOD
Differential
output voltage
200
-
800
mV
VOS (input)
Output offset
voltage
1.025
1.100
1.175
V
VOS (output)
Output offset
voltage
-
0.750
-
V
Altera Corporation
3
AN 247: Stratix GX to Mercury Interoperability
Stratix GX
Devices Driving
Mercury LVDS
& LVPECL
A Stratix GX device can drive a Mercury device with the Mercury device’s
inputs receiving LVDS or LVPECL. Table 5 shows the LVDS specifications
for Mercury devices. The VOS or VCM is 1.2 V for Mercury devices;
therefore, the connection to the Stratix GX device must be AC coupled to
accommodate different voltages. The minimum input differential (VID)
for Mercury LVDS is 200 mV; therefore, you can program the VOD for
Stratix GX devices to a value above 200 mV factoring in any attenuation
losses.
Table 5. Mercury 3.3-V LVDS I/O Specifications
Minimum
Typical
Maximum
Unit
VOD
Symbol
Differential
RL = 100 Ω
output voltage
Parameter
Conditions
250
510
600
mV
VID
Differential
Input Voltage
RL = 100 Ω
200
400
1,200
mV
VOS
Output offset
voltage
RL = 100 Ω
1.125
1.250
1.375
V
VTH
Differential
input
threshold
VCM = 2.1 V
−100
-
100
mV
The minimum VID for Mercury LVPECL is 400 mV. You should program
the Stratix GX device so that the VOD is a value above 400 mV, factoring
in any attenuation losses. Table 6 shows the LVPECL specification for
Mercury devices.
Table 6. Mercury LVPECL I/O Specifications
Symbol
Parameter
Conditions
Min
Typ
Max
Unit
VID
Differential
input voltage
RL = 100 Ω
400
600
1,200
mV
VOD
Differential
RL = 100 Ω
output voltage
525
1,050
1,200
mV
Since the transmission line is AC coupled, the Mercury device requires
VCM regeneration before going to the input. You can execute this
regeneration by incorporating two sets of pull-up and pull-down
resistors. See Figure 1.
4
Altera Corporation
AN 247: Stratix GX to Mercury Interoperability
Figure 1. Stratix GX Transceiver to Mercury LVDS & LVPECL Receiver Connection
Stratix GX 1.5-V PCML
Mercury LVDS
8 mA
10 - 100 nF
Z = 50 Ω
50 Ω
50 Ω
R1
R2
100 Ω
R1
Z = 50 Ω
10 - 100 nF
R2
8 mA
If the design is the same as the schematic in Figure 1, you do not have to
change the Mercury device’s connections from the original design. Use
the following guidelines to choose the resistor values:
■
■
■
■
Mercury LVDS/
LVPECL Driving
a Stratix GX
Device
Altera Corporation
When using the internal differential termination, ensure that the
resistors R1 and R2 are in the kΩ range.
R1  R2  50 Ω ≈ 50Ω. In this equation, the 50-Ω value on the left of
the equal sign comes from the fact that 100-Ω differential = 50 Ω to
ground on a positive signal arm and +50 Ω to ground on a negative
signal arm.
R2/(R1 + R2) × 3.3 V = VCM. Calculate the R1 and R2 using a value for
the common-mode voltage (VCM) in the middle of the Mercury
receiver tolerance. Although VCM can range from 0 to 2.4 V, Altera
recommends that VCM be between 0 to 0.7 V or 1.8 to 2.4 V for speeds
greater than 1 Gbps.
R1 = 3.3 kΩ and R2 = 5.6 kΩ are standard resistor values that meet the
requirement for VCM = 2.0 V. However, other values are possible.
The Stratix GX receiver connection does not need VCM reconstruction
because it can accept AC-coupled signals. The Stratix GX device’s VID
minimum is 100 mV (see Table 4), making the receiver very sensitive to
the signal range being generated by the Mercury device’s transmitter. For
signals that have a higher signal-to-noise ratio (SNR), you can program
the receiver sensitivity such that it is increased in the equalizer module
(for more information, see AN 237: Using High-Speed Transceiver Blocks in
Stratix GX Devices). The schematic for a Mercury device driving a
Stratix GX device via LVDS (connection is the same for LVPECL) is shown
in Figure 2. Since Stratix GX devices are 1.5-V devices, the maximum
input voltage for the receiver is 1.7 V. For an LVPECL input with a voltage
swing of ± 600 mV, set the Stratix GX VCM input to 1.1 V or lower.
5
AN 247: Stratix GX to Mercury Interoperability
Figure 2. Mercury LVDS Transceiver Output to Stratix GX Receiver
Mercury LVDS
Stratix GX 1.5-V PCML
10 - 100 nF
Z = 50 Ω
50 Ω
50 Ω
Z = 50 Ω
10 - 100 nF
Stratix GX
Device Driving
Mercury 3.3-V
PCML Inputs
Mercury 3.3-V PCML is terminated at 3.3 V (see Table 7). This I/O
standard has a different resistor network than the previous LVDS setups.
Table 7. Mercury 3.3-V PCML Specifications
Symbol
Parameter
Min
Typ
Max
Unit
VID
Differential input
voltage
400
-
800
mV
VOD
Differential
output voltage
400
700
800
mV
If the design is the same as the schematic in Figure 3, the Mercury
connections do not have to change from the original design. The
minimum VID for Mercury 3.3-V PCML is 400 mV; therefore, you can
program the VOD for Stratix GX devices to a value above 400 mV factoring
in any attenuation losses.
6
Altera Corporation
AN 247: Stratix GX to Mercury Interoperability
Figure 3. Stratix GX Transceiver to Mercury 3.3-V PCML Receiver Connection
Stratix GX 1.5-V PCML
Mercury PCML
VTT
8 mA
10 - 100 nF
50 Ω
Z = 50 Ω
50 Ω
VTT
50 Ω
50 Ω
Z = 50 Ω
R1
10 - 100 nF
8 mA
Mercury 3.3-V
PCML
Transmitter
Driving a
Stratix GX
Device
The 400 mV output of the Mercury 3.3-V PCML driver is well above the
100 mV minimum of the Stratix GX receiver. The Mercury connection
does not have to change, but the line going to the Stratix GX device must
be AC coupled. For signals that have a higher SNR, you can program the
receiver sensitivity to increase. See Figure 4.
Figure 4. Mercury 3.3-V PCML Transmitter to Stratix GX CML
Mercury PCML
Stratix GX CML
VTT
50 Ω
10 - 100 nF
Z = 50 Ω
50 Ω
VTT
50 Ω
50 Ω
Z = 50 Ω
10 - 100 nF
Conclusion
Altera Corporation
Using the programmable VOD feature of Stratix GX devices and the AC
coupling capability for input VCM reconstruction, systems using Mercury
devices can easily interconnect with Stratix GX devices. No modifications
are required to existing systems or designs, allowing these two device
families to effectively communicate, and allowing new Stratix GX boards
to be added to existing Mercury-based systems.
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AN 247: Stratix GX to Mercury Interoperability
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