I am looking at getting fpga to generate pwm and quadrature decoders for dc motor and then interface fpga to ROS.

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I'm not aware of an existing FPGA to ROS framework.

It shouldn't be terribly difficult to do, but an FPGA is probably overkill for most applications.

There are number of implementation details that you'll have to work out:

• You'll have to come up with a way to wire your FPGA to your ROS computer. Serial, USB and Ethernet may all be choices here.
• You'll need to find or write a protocol that defines how the FPGA communicates with the computer. This could be a simple serial protocol, or it could be UDP over Ethernet, or something else.
• You'll need to program the FPGA appropriately. This is usually done in Verilog or VHDL. This will include writing the appropriate hardware to generate your PWM signals, read your encoders, and communicate with your PC over the chosen protocol.
• You'll need to write a ROS node which communicates with your FPGA and publishes and subscribes to ROS topics.

Unless you really need the flexibility or speed of an FPGA, there are better, pre-built solutions for interfacing ROS to motors and encoders such as ros_arduino_bridge