BUFGP

Primary Global Buffer for Driving Clocks or Longlines (Four per PLD Device)

BUFGP, a primary global buffer, is used to distribute high fan-out clock or control signals throughout PLD devices. In Virtex, BUFGP is equivalent to an IBUFG driving a BUFG. In CPLD designs, BUFGP is treated like BUFG. A BUFGP provides direct access to Configurable Logic Block (CLB) and Input Output Block (IOB) clock pins and limited access to other CLB inputs. Four BUFGPs are available on each XC4000E and Spartan device, one in each corner. The input to a BUFGP comes only from a dedicated IOB.

Alongside each column of CLBs in an XC4000E or Spartan device are four global vertical lines, which are in addition to the standard vertical longlines. Each one of the four global vertical lines can drive the CLB clock (K) pin directly. In addition, one of the four lines can drive the F3 pin, a second line can drive the G1 pin, a third can drive the C3 pin, and a fourth can drive the C1 pin. Each of the four BUFGPs drives one of these global vertical lines. These same vertical lines are also used for the secondary global buffers (refer to the “BUFGS” section for more information).

Because of its structure, a BUFGP can always access a clock pin directly. However, it can access only one of the F3, G1, C3, or C1 pins, depending on the corner in which the BUFGP is placed. When the required pin cannot be accessed directly from the vertical line, PAR feeds the signal through another CLB and uses general purpose routing to access the load pin.

To use a BUFGP in a schematic, connect the input of the BUFGP element directly to the PAD symbol. Do not use any IBUFs, because the signal comes directly from a dedicated IOB. The output of the BUFGP is then used throughout the schematic. For a negative-edge clock, insert an INV (inverter) element between the output of the BUFGP and the clock input. This inversion is performed inside each CLB or IOB.

A Virtex BUFGP must be sourced by an external signal. Other BUFGPs can be sourced by an internal signal, but PAR must use the dedicated IOB to drive the BUFGP, which means that the IOB is not available for use by other signals. If possible, use a BUFGS instead, because they can be sourced internally without using an IOB.

The dedicated inputs for BUFGPs are identified by the names PGCK1 through PGCK4 in pinouts in XC4000E and Spartan. The package pin that drives the BUFGP depends on which corner the BUFGP is placed by PAR.

Figure 3.38 BUFGP Implementation XC5200