When frame filtering is enabled, the MAC header of the packet is investigated by the radio CPU. The frame control field (FCF) is checked first. The frame type subfield is the first subfield of the FCF to be checked, and determines the further processing. The MSB of the frame type is processed according to frameFiltOpt.modifyFtFilter before the check is made. The result of this modification is only used when checking, not when storing the FCF in the RX queue entry. For each of the eight possible values of the frame type field (including four reserved values), the frame can be setup to be accepted or rejected. This is controlled by the bits of frameTypes. If the frame type is Acknowledgment (010b) and a CMD_RX_ACK operation is running in the foreground, the packet is processed further even if frameTypes.bAcceptFt2Ack is 0. Section 26.5.4.5 gives more details on the processing in that case.

Filtering is performed on the Frame Version and Reserved subfields. If the frame version is greater than frameFiltOpt.maxFrameVersion, the frame is rejected.

If the Reserved subfield ANDed with frameFiltOpt.fcfReservedMask is nonzero, the frame is rejected. The addressing fields are checked to see if the frame must be accepted or not. This filtering follows the rules for third-level filtering (refer to the IEEE 802.15.4 standard). When checking against the local address, the localExtAddr or localShortAddr field is used, and when checking against the local PAN ID, the localPanID field is used.

If frameFiltOpt.bStrictLenFilter is 1 and the frame type indicates that the frame is an acknowledgment frame, the frame is rejected if the length of the PHY payload is not 5, which is the length of a correctly-formulated ACK frame.

If frameFiltOpt.frameFiltStop is 1 and the frame filtering gives the conclusion that the frame is to be rejected, reception stops and the radio returns to sync search. Otherwise, the frame is received to the end.

The radio CPU checks the header to see if an acknowledgment is to be transmitted. This gives a preliminary result; the actual transmission of the ACK depends on the status at the end of the frame. The condition for transmitting an acknowledgment frame is given in Section 26.5.4.1.3.