IBUF_<I>selectIO</I>

Libraries GuideChapter 6: Design Elements (GCLK to KEEPER)

IBUF_selectIO

Single Input Buffer with Selectable I/O Interface

XC3000	XC4000E	XC4000X	XC5200	XC9000	Spartan	SpartanXL	Spartan2	Virtex
N/A	N/A	N/A	N/A	N/A	N/A	N/A	Primitive	Primitive

For Virtex and Spartan2, IBUF and its variants (listed below) are single input buffers whose I/O interface corresponds to a specific I/O standard. The name extensions (LVCMOS2, PCI33_3, PCI33_5, etc.) specify the standard. For example, IBUF_SSTL3_II is a single input buffer that uses the SSTL3_II I/O-signaling standard.

An IBUF isolates the internal circuit from the signals coming into a chip. For Virtex and Spartan2, the dedicated GCLKIOB pad is input only. IBUF inputs (I) are connected to an IPAD or IOPAD. IBUF outputs (O) are connected to the internal circuit.

The hardware implementation of the I/O standards requires that you follow a set of usage rules for the SelectI/O buffer components. Refer to the “SelectI/O Usage Rules” section below for information on using these components.

Component	I/O Standard	VREF
IBUF	LVTTL	N/A
IBUF_LVCMOS2	LVCMOS2	N/A
IBUF_PCI33_3	PCI33_3	N/A
IBUF_PCI33_5	PCI33_5	N/A
IBUF_PCI66_3	PCI66_3	N/A
IBUF_GTL	GTL	0.80
IBUF_GTLP	GTL+	1.00
IBUF_HSTL_I	HSTL_I	0.75
IBUF_HSTL_III	HSTL_III	0.90
IBUF_HSTL_IV	HSTL_IV	0.75
IBUF_SSTL2_I	SSTL2_I	1.10
IBUF_SSTL2_II	SSTL2_II	1.10
IBUF_SSTL3_I	SSTL3_I	0.90
IBUF_SSTL3_II	SSTL3_II	1.50
IBUF_CTT	CTT	1.50
IBUF_AGP	AGP	1.32

SelectI/O Usage Rules

The Virtex and Spartan2 architectures include a versatile SelectI/O interface to multiple voltage and drive standards. To select an I/O standard, you must choose the appropriate component from the Virtex or Spartan2 library. Each standard has a full set of I/O buffer components (input, in/out, output, 3-state output). For example, for an input buffer of the GTL standard, you would choose IBUF_GTL. Refer to the “IBUF_selectIO”, “IBUFG_selectIO”, “IOBUF_selectIO”, “OBUF_selectIO”, and “OBUFT_selectIO” sections for information on the various input/output buffer components available to implement the desired standard.

The hardware implementation of the various I/O standards requires that certain usage rules be followed. As shown in the following table, each I/O standard has voltage source requirements for input reference (VREF), output drive (VCCO), or both. Each Virtex and Spartan2 device has eight banks (two on each edge). Each bank has voltage sources shared by all I/O in the bank. Therefore, in a particular bank, the voltage source (for either input or output) must be of the same type. The Input Banking (VREF) Rules section and the Output Banking (VCCO) Rules section below summarize the SelectI/O component usage rules based on the hardware implementation.

I/O Standard	VCCO	VREF
LVTTL	3.3	N/A
LVCMOS2	2.5	N/A
PCI33_3 (PCI 33MHz 3.3V)	3.3	N/A
PCI33_5 (PCI 33MHZ 5.0V)	3.3	N/A
PCI66_3 (PCI 66MHz 3.3V)	3.3	N/A
GTL	N/A	0.80
GTL+	N/A	1.00
HSTL_I	1.5	0.75
HSTL_III	1.5	0.90
HSTL_IV	1.5	0.75
SSTL2_I	2.5	1.10
SSTL2_II	2.5	1.10
SSTL3_I	3.3	0.90
SSTL3_II	3.3	1.50
CTT	3.3	1.50
AGP	3.3	1.32

Input Banking (VREF) Rules

The low-voltage I/O standards that have a differential amplifier input require a voltage reference input (VREF). The VREF voltage source is provided as an external signal to the chip that is banked internal to the chip.

Any input buffer component that does not require a VREF source (LVTTL, LVCMOS2, PCI*) can be placed in any bank.
All input buffer components that require a VREF source (GTL*, HSTL*, SSTL*, CTT, AGP) must be of the same I/O standard in a particular bank. For example, IBUF_SSTL2_I and IBUFG_SSTL2_I are compatible since they are the same I/O standard (SSTL2_I).
If the bank contains any input buffer component that requires a VREF source, the following conditions apply.
- One or more VREF sources must be connected to the bank via an IOB.
- The number of VREF sources is dependent on the device and package.
- The locations of the VREF sources are fixed for each device/package.
- All VREF sources must be used in that bank.
If the bank contains no input buffer component that requires a VREF source, the IOBs for VREF sources can be used for general I/O.
Output buffer components of any type can be placed in the bank.

Output Banking (VCCO) Rules

Because Virtex and Spartan2 have multiple low-voltage standards and also needs to be 5V tolerant, some control is required over the distribution of VCCO, the drive source voltage for output pins. To provide for maximum flexibility, the output pins are banked. In comparison to the VREF sources described above, the VCCO voltage sources are dedicated pins on the device and do not consume valuable IOBs.

Any output buffer component that does not require a VCCO source (GTL, GTL+) can be placed in any bank.
To be placed in a particular bank, all output buffer components that require VCCO must have the same supply voltage (VCCO). For example, OBUF_SSTL3_I and OBUF_PCI33_3 are compatible in the same output bank since VCCO=3.3 for both.
Input buffer components of any type can be placed in the bank.
The configuration pins on a Virtex and Spartan2 device are on the right side of the chip. When configuring the device through a serial prom, the user is required to use a VCCO of 3.3V in the two banks on the right hand side of the chip. If the user is not configuring the device through a serial prom, the VCCO requirement is dependent upon the configuration source.

Banking Rules for OBUFT_selectIO with KEEPER

If a KEEPER symbol is attached to an OBUFT_selectIO component (3-state output buffer) for an I/O standard that requires a VREF (for example, OBUFT_GTL, OBUFT_SSTL3_I), then the OBUFT_selectIO component follows the same rules as an IOBUF_selectIO component for the same standard. It must follow both the input banking and output banking rules. The KEEPER element requires that the VREF be properly driven.