RX Pause Frames

Versal ACAP Technical Reference Manual (AM011)

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1.4 English

Bit [13] of the network configuration register is the pause enable control for reception. If this bit is set and a non-zero pause quantum frame is received, transmission pauses.

If a valid pause frame is received, then the pause time register is updated with the new frame's pause time regardless of whether a previous pause frame is active. An interrupt (either bit [12] or bit [13] of the interrupt status register) is triggered when a pause frame is received, but only if the interrupt is enabled (bit [12] and bit [13] of the interrupt mask register). Pause frames received with non-zero quantum are indicated through the interrupt bit [12] of the interrupt status register. Pause frames received with zero quantum are indicated on bit [13] of the interrupt status register.

When the pause time register is loaded and the frame currently being transmitted is sent, no new frames are transmitted until the pause time reaches zero. The loading of a new pause time, and the pausing of transmission, only occurs when the controller is configured for full-duplex operation. If the controller is configured for half-duplex there is no frame is defined as having a destination address that matches either the address stored in specific address register 1 or if it matches the reserved address of 0x0180C2000001. It must also have the MAC control frame type ID of 0x8808 and have the pause opcode of 0x0001.

Pause frames that have FCS or other errors are treated as invalid and are discarded. IEEE Std 802.3 pause frames that are received after priority-based flow control (PFC) is negotiated are also discarded. Valid pause frames received increment the pause frames received statistic register. The pause time register decrements every 512-bit times once transmission has stopped. For test purposes, the retry test bit can be set (bit [12] in the network configuration register) which causes the pause time register to decrement every tx_clk cycle when transmission has stopped.

The interrupt (bit [13] in the interrupt status register) is asserted whenever the pause time register decrements to zero (assuming it was enabled by bit [13] in the interrupt mask register). This interrupt is also set when a zero quantum pause frame is received.