TX Pause Frame

Automatic transmission of pause frames is supported through the transmit pause frame bits of the network control register and from the external input signals tx_pause, tx_pause_zero, and tx_pfc_sel. If either bit [11] or bit [12] of the network control register is written with a logic 1, or if the input signal tx_pause is toggled when tx_pfc_sel is Low, an IEEE Std 802.3 pause frame is transmitted providing full duplex is selected in the network configuration register and the transmit unit is enabled in the network control register.

Pause frame transmission occurs immediately if transmit is inactive or if transmit is active between the current frame and the next frame due to be transmitted.

Transmitted pause frames comprise of the following:

• A destination address of 01-80-C2-00-00-01.
• A source address taken from specific address register 1.
• A type ID of 88-08 (MAC control frame).
• A pause opcode of 00-01.
• A pause quantum register.
• Fill of 00 to take the frame to the minimum frame length.
• A valid FCS.

The pause quantum used in the generated frame depends on the trigger source for the frame.

• If bit [11] is written with a one, the pause quantum is taken from the transmit pause quantum register. The transmit pause quantum register resets to a value of 0xFFFF giving maximum pause quantum as the default.
• If bit [12] is written with a one, the pause quantum is zero.
• If the tx_pause input is toggled, tx_pfc_sel is Low and the tx_pause_zero input is held Low until the next toggle, the pause quantum is taken from the transmit pause quantum register.
• If the tx_pause input is toggled, tx_pfc_sel is Low and the tx_pause_zero input is held High until the next toggle, the pause quantum is zero.

After transmission, a pause frame transmitted interrupt is generated (bit [14] of the interrupt status register) and the only statistics register incremented is the pause frames transmitted register. Pause frames can also be transmitted by the MAC using normal frame transmission methods.