DS031 Datasheet, PDF (16/318 Pages) Xilinx, Inc – Summary of Features

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Virtex-II Platform FPGAs: Functional Description
Digitally Controlled Impedance (DCI)
Today’s chip output signals with fast edge rates require ter-
mination to prevent reflections and maintain signal integrity.
High pin count packages (especially ball grid arrays) can
not accommodate external termination resistors.
Virtex-II XCITE DCI provides controlled impedance drivers
and on-chip termination for single-ended and differential
I/Os. This eliminates the need for external resistors, and
improves signal integrity. The DCI feature can be used on
any IOB by selecting one of the DCI I/O standards.
When applied to inputs, DCI provides input parallel termina-
tion. When applied to outputs, DCI provides controlled
impedance drivers (series termination) or output parallel
termination.
DCI operates independently on each I/O bank. When a DCI
I/O standard is used in a particular I/O bank, external refer-
ence resistors must be connected to two dual-function pins
on the bank. These resistors, voltage reference of N transis-
tor (VRN) and the voltage reference of P transistor (VRP)
are shown in Figure 9.
1 Bank
DCI
DCI
DCI
DCI
VCCO
VRN
VRP
R_REF (1%)
R_REF (1%)
GND
DS031_50_101200
Figure 9: DCI in a Virtex-II Bank
When used with a terminated I/O standard, the value of
resistors are specified by the standard (typically 50Ω).
When used with a controlled impedance driver, the resistors
set the output impedance of the driver within the specified
range (25Ω to 100Ω). For all series and parallel termina-
tions listed in Table 6 and Table 7, the reference resistors
must have the same value for any given bank. One percent
resistors are recommended.
The DCI system adjusts the I/O impedance to match the two
external reference resistors, or half of the reference resis-
tors, and compensates for impedance changes due to volt-
age and/or temperature fluctuations. The adjustment is
done by turning parallel transistors in the IOB on or off.
Controlled Impedance Drivers (Series Term.)
DCI can be used to provide a buffer with a controlled output
impedance. It is desirable for this output impedance to
match the transmission line impedance (Z₀). Virtex-II input
buffers also support LVDCI and LVDCI_DV2 I/O standards.
IOB
Z
Z
Virtex-II DCI
V_CCO = 3.3 V, 2.5 V, 1.8 V or 1.5 V _{DS031_51_110600}
Figure 10: Internal Series Termination
Table 6: SelectI/O-Ultra Controlled Impedance Buffers
V_CCO
3.3 V
DCI
LVDCI_33
DCI Half Impedance
LVDCI_DV2_33
2.5 V
LVDCI_25
LVDCI_DV2_25
1.8 V
LVDCI_18
LVDCI_DV2_18
1.5 V
LVDCI_15
LVDCI_DV2_15
Controlled Impedance Drivers (Parallel)
DCI also provides on-chip termination for SSTL3, SSTL2,
HSTL (Class I, II, III, or IV), and GTL/GTLP receivers or
transmitters on bidirectional lines.
Table 7 and Table 8 list the on-chip parallel terminations avail-
able in Virtex-II devices. V_CCO must be set according to
Table 3. Note that there is a V_CCO requirement for GTL_DCI
and GTLP_DCI, due to the on-chip termination resistor.
Table 7: SelectI/O-Ultra Buffers With On-Chip Parallel
Termination
IOSTANDARD Attribute
I/O Standard
Description
SSTL3 Class I
SSTL3 Class II
SSTL2 Class I
SSTL2 Class II
External
Termination
SSTL3_I
SSTL3_II
SSTL2_I
SSTL2_II
On-Chip
Termination
SSTL3_I_DCI ⁽¹⁾
SSTL3_II_DCI ⁽¹⁾
SSTL2_I_DCI ⁽¹⁾
SSTL2_II_DCI ⁽¹⁾
HSTL Class I
HSTL_I
HSTL_I_DCI
HSTL Class II
HSTL_II
HSTL_II_DCI
HSTL Class III
HSTL_III
HSTL_III_DCI
HSTL Class IV
HSTL_IV
HSTL_IV_DCI
GTL
GTL
GTL_DCI
GTLP
GTLP
GTLP_DCI
Notes:
1. SSTL-compatible
DS031-2 (v3.5) November 5, 2007
Product Specification
www.xilinx.com
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