The pad field can vary from 0 to 46 bytes in length. This field is used to ensure that the frame length is at least 64 bytes in length (the preamble and SFD fields are not considered part of the frame for this calculation), which is required for successful CSMA/CD operation. The values in this field are used in the frame check sequence calculation but are not included in the length field value when it is used. The length of this field and the data field combined must be at least 46 bytes. If the data field contains 0 bytes, the pad field is 46 bytes. If the data field is 46 bytes or more, the pad field has 0 bytes.
For the transmission, this field can be inserted automatically by the Ethernet MAC or supplied by you. If the pad field is inserted by the Ethernet MAC, the FCS field is calculated and inserted by the Ethernet MAC. If the pad field is supplied by you, the FCS can either be inserted by the Ethernet MAC or provided by you, as indicated by a configuration register bit.
For the reception, if the length/type field has a length interpretation, any pad field in the incoming frame is not passed to you unless the Ethernet MAC is configured to pass the FCS field on to you.