SPRUIV7B May 2022 – September 2023 AM620-Q1 , AM623 , AM625 , AM625-Q1 , AM625SIP
Incoming pause frames are acted upon, when enabled, to prevent the Ethernet port from transmitting any further frames. Incoming pause frames are only acted upon when the [0] FULLDUPLEX and [4] TX_FLOW_EN bits in the CPSW_PN_MAC_CONTROL_REG register are set. Pause frames are not acted upon in half-duplex mode. Pause frame action will be taken if enabled, but normally the frame will be filtered and not transferred to memory. MAC control frames will be transferred to memory if the [24] RX_CMF_EN (RX Copy MAC Control Frames Enable) bit in the CPSW_PN_MAC_CONTROL_REG register is set. The [4] TX_FLOW_EN and [0] FULLDUPLEX bits effect whether or not MAC control frames are acted upon, but they have no effect upon whether or not MAC control frames are transferred to memory or filtered.
Pause frames are a subset of MAC Control Frames with an opcode field = 0001h. Incoming pause frames will only be acted upon by the port if:
The pause time value from valid frames will be extracted from the two bytes following the opcode. The pause time will be loaded into the port's transmit pause timer and the transmit pause time period will begin.
If a valid pause frame is received during the transmit pause time period of a previous transmit pause frame then:
If [4] TX_FLOW_EN in CPSW_PN_MAC_CONTROL_REG register is cleared, then the pause-timer will immediately expire.
The port will not start the transmission of a new data frame any sooner than 512-bit times after a pause frame with a non-zero pause time has finished being received (MRXDV going inactive). No transmission will begin until the pause timer has expired (the port may transmit pause frames in order to initiate outgoing flow control). Any frame already in transmission when a pause frame is received will be completed and unaffected.
Incoming pause frames consist of the below:
All quantities above are hexadecimal and are transmitted most-significant byte first. The least-significant bit is transferred first in each byte.
The padding is required to make up the frame to a minimum of 64 Bytes. The standard allows pause frames longer than 64 Bytes to be discarded or interpreted as valid pause frames. The Ethernet port will recognize any pause frame between 64 Bytes and CPSW_PN_RX_MAXLEN_REG[13-0] RX_MAXLEN bytes in length.