Whenever the PHY detects the data-prefix state on the serial bus, it initiates a receive operation by asserting receive on the CTL terminals and a logic 1 on each of the D bus terminals (data-on indication). The PHY indicates the start of a packet by placing the speed code (encoded as shown in Table 12-11) on the D terminals, followed by packet data. The PHY holds the CTL terminals in the receive state until the last symbol of the packet has been transferred. The PHY indicates the end of packet data by asserting idle on the CTL terminals. All received packets are transferred to the LLC. Note that the speed code is part of the PHY-LLC protocol and is not included in the calculation of CRC or any other data protection mechanisms.

It is possible for the PHY to receive a null packet, which consists of the data-prefix state on the serial bus followed by the data-end state, without any packet data. A null packet is transmitted whenever the packet speed exceeds the capability of the receiving PHY, or whenever the LLC immediately releases the bus without transmitting any data. In this case, the PHY asserts receive on the CTL terminals with the data-on indication (all 1s) on the D bus terminals, followed by Idle on the CTL terminals, without any speed code or data being transferred. In all cases, in normal operation, the TSB41BA3F-EP sends at least one data-on indication before sending the speed code or terminating the receive operation.

The TSB41BA3F-EP also transfers its own self-ID packet, transmitted during the self-ID phase of bus initialization, to the LLC. This packet is transferred to the LLC just as any other received self-ID packet.

A. SPD = Speed code, see Table 12-11. d0–dn = Packet data

Figure 12-4 Normal Packet Reception Timing

The sequence of events for a normal packet reception is as follows:

1. Receive operation initiated. The PHY indicates a receive operation by asserting receive on the CTL lines. Normally, the interface is idle when receive is asserted. However, the receive operation can interrupt a status transfer operation that is in progress so that the CTL lines can change from status to receive without an intervening idle.
2. Data-on indication. The PHY can assert the data-on indication code on the D lines for one or more cycles preceding the speed code.
3. Speed code. The PHY indicates the speed of the received packet by asserting a speed code on the D lines for one cycle immediately preceding packet data. The link decodes the speed code on the first receive cycle for which the D lines are not the data-on code. If the speed code is invalid or indicates a speed higher that which the link is capable of handling, then the link must ignore the subsequent data.
4. Receive data. Following the data-on indication (if any) and the speed code, the PHY asserts packet data on the D lines with receive on the CTL lines for the remainder of the receive operation.
5. Receive operation terminated. The PHY terminates the receive operation by asserting idle on the CTL lines. The PHY asserts at least one idle cycle following a receive operation.

Figure 12-5 Null Packet Reception Timing

The sequence of events for a null packet reception is as follows:

1. Receive operation initiated. The PHY indicates a receive operation by asserting receive on the CTL lines. Normally, the interface is idle when receive is asserted. However, the receive operation can interrupt a status transfer operation that is in progress so that the CTL lines can change from status to receive without an intervening idle.
2. Data-on indication. The PHY asserts the data-on indication code on the D lines for one or more cycles.
3. Receive operation terminated. The PHY terminates the receive operation by asserting idle on the CTL lines. The PHY asserts at least one idle cycle following a receive operation.