How to set Speed: 100 Mbits full duplex instead oft Auto-negotiation on DE2-115 board

You'll need to write to the PHY's mdio registers. Register 0 is the control register, where you can disable autonegotiation and force 100Mbps speed.

IEEE 802.3 Clause 22 contains the full details of this register, but here's the register contents:

15 - reset

14 - loopback

13 - speed lsb 6,13 = 01 for 100mbps

12 - aneg enable

11 - power down

10 - isolate

9 - restart aneg

8 - duplex, 1 = full, 0 = half

7 - collision test

6 - speed, msb

5 - unidirectional enable

4:0 - rsvd (0)

For 100Mbps, full duplex:

1010 0001 0000 0000

or, 0xA100

Are you positive you want to disable autoneg? If you connect the board to a device that is configured for auto-negotiation, it will parallel detect and you'll get a duplex mismatch. You could also configure the device to advertise 100Mbps only, but still use autonegotiation. You can do this via register 4 (and don't forget to hit the soft reset bit in register 0 after making any changes in register 4). If you have a 10/100/1000 PHY and you want to disable advertising for 1000BaseT, you'll also have to configure register 9.