BUFFER in Xilinx FPGA

Jul 14, 2025#FPGA2

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The document discusses the use of buffers in Xilinx FPGAs, emphasizing the importance of synchronous timing circuits in large FPGA designs. It highlights the need for precise clock management, including the use of global clock resources and dedicated clock buffers. ### Key Points: 1. **Buffers and Drivers**: - **Input/Output Buffers**: Essential for handling external clock signals and differential inputs. Signals must pass through IBUFGDS or IBUFDS buffers for processing. - **Drivers**: Adding BUFG can enhance signal stability when fan-out is high, introducing a delay of about 10ns, but this delay is negligible for outputs to internal units. 2. **Types of Buffers**: - **BUFG**: A high-fan-out buffer connecting signals to global routing resources, minimizing delay and jitter, commonly used for clock and reset signals. - **BUFGCE**: A global clock buffer with a single gate control input, outputting zero when the control signal is low. - **BUFH**: Allows access to unused parts of the global clock network for high-speed local routing. - **IBUFDS**: A buffer for low-voltage differential signal inputs with two ports for opposite-phase signals. - **IBUFDS_GTE2**: A buffer for Gbit transceiver inputs in 7-series devices, requiring a dedicated reference clock input. - **OBUFDS**: A differential signal output buffer. Overall, the document provides a comprehensive overview of the various buffer types and their roles in ensuring effective signal management within Xilinx FPGAs.

In large FPGA designs, synchronous sequential circuits are recommended. Synchronous sequential circuits are designed based on clock-triggered edges, which have higher requirements for clock period, duty cycle, delay, and jitter. To meet timing requirements, a global clock resource is generally used to drive the main clock. The main clock of the FPGA is usually implemented using a full copper layer process and is designed with dedicated clock buffering and driving structures.

1 Buffering and Driving#

1.1 Buffering#

Input and output buffering is mainly used for off-chip input clocks or off-chip differential input signals.

Differential signals and differential clocks cannot be processed directly after entering the chip without going through IBUFGDS or IBUFDS buffering.

1.2 Driving#

When the signal fan-out is too large, adding a BUFG can increase signal stability.

There is about a 10ns delay through a BUFG, but the delay to all on-chip units after passing through the BUFG can be ignored.

2 Types and Functions of Buffers#

2.1 BUFG#

BUFG is a high-fan-out buffer that connects signals to global routing resources, minimizing signal delay and jitter.

It is typically used in clock networks and other high-fan-out networks, such as reset and enable signals.

2.2 BUFGCE#

BUFGCE is a global clock buffer with a single gate-controlled input, where CE is active high.

When CE is low, the O port outputs 0.

2.3 BUFH#

The BUFH primitive allows direct access to the clock region entry of BUFG, enabling access to unused parts of the global clock network as high-speed, low-skew local routing resources (single clock region).

2.4 IBUFDS#

A buffer that supports low-voltage differential signal input, with two input ports, one for the main port and one for the secondary port, where the input signal phases are opposite.

2.5 IBUFDS_GTE2#

Input buffer for Gbit transceivers in 7 series devices, REFCLK should be connected to the dedicated reference clock input pin of the serial transceiver.

2.6 OBUFDS#

Differential signal output buffer.