31.6.16.2 802.3 Pause Frame Transmission
Automatic transmission of pause frames is supported through the transmit pause frame bits of the Network Control register. If either bit 11 (NCR.TXPF) or bit 12 (NCR.TXZQPF) of the Network Control register is written with logic 1, an 802.3 pause frame will be transmitted, providing full duplex is selected in the Network Configuration register and the transmit block is enabled in the Network Control register.
Pause frame transmission will happen 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 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 minimum frame length
- Valid FCS
The pause quantum used in the generated frame will depend on the trigger source for the frame as follows:
- If bit 11 (NCR.TXPF) is written with a '1', the pause quantum will be taken from the Transmit Pause Quantum register. The Transmit Pause Quantum register resets to a value of 0xFFFF giving maximum pause quantum as default.
- If bit 12 (NCR.TXZQPF) is written with a '1', the pause quantum will be zero.
After transmission, a pause frame transmitted interrupt will be generated (bit 14 (ISR.PFTR) of the Interrupt Status register) and the only statistics register that will be incremented will be the Pause Frames Transmitted register.
Pause frames can also be transmitted by the MAC using normal frame transmission methods.