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Packets transmitted or received on each Ethernet port can generate Ethernet Transmit Events or Ethernet Receive Events, respectively. The CPTS hardware will decode each packet to determine if it is a valid CPTS time sync event.
According to the IEEE 802.3 Ethernet standard, each Ethernet frame contains a 2-octet EtherType field to indicate which protocol is encapsulated in the PayLoad field, as shown in Figure 12-358. For standard time sync packets, this will contain the EtherType for the Precision Time Protocol (IEEE 1588), which is defined as 0x88F7. The CPTS hardware will compare this field to the TS_LTYPE1 field in the CPSW_PN_TS_SEQ_LTYPE_REG or the TS_LTYPE2 field in CPSW_PN_TS_CTL_LTYPE2_REG register (depending on which enable bit was set) , which should also be programmed to 88F7h.
When a virtual LAN is used, an additional 4-octet 802.1Q tag is inserted in the Ethernet frame before the EtherType field, as shown in Figure 12-358. To indicate to the CPTS hardware that a virtual LAN is in use, the TS_TX_VLAN_LTYPE1_EN (or TS_TX_VLAN_LTYPE2_EN) enable bit must be set in the CPSW_PN_TS_CTL_REG register. The EtherType for the 802.1Q tag is defined as 0x8100, and the CPTS hardware will compare this value to the TS_VLAN_LTYPE1 (or TS_VLAN_LTYPE2 depending on which enable bit was set) field in the CPSW_PN_TS_VLAN_LTYPE_REG register, which should also be programmed to 0x8100.
When two stacked VLANs are used, two additional 4-octet 801.Q tags are inserted in the Ethernet frame before the EtherType field, as shown in Figure 12-358. In this case, both TS_VLAN_LTYPE1 and TS_VLAN_LTYPE2 must be enabled. The outer tag must match the value of the TS_VLAN_LTYPE1 field, and the inner tag must match the value of the TS_VLAN_LTYPE2 field.