USB System Architecture

The transmitter of the transaction must not time-out before 16 bit times but must time-out by 18 bit times. This is true of both lowand full-speed transactions.

False EOPs

Problems can occur if the receiver detects the end of packet (EOP) prior to the transmitter having actually completed its transfer. This condition is known as a false EOP. If the receiver were to respond to the premature EOP by returning either a data or a handshake packet, a collision would occur on the bus. This would have the effect to corrupting two consecutive packets. Fortunately, false EOPs can be detected and the collision avoided. The false EOP having truncated the packet will very likely result in a CRC error. It is highly unlikely that the CRC will be correct for a truncated packet. The following sections describe the detection and response.

False EOP During Host Transmission

USB target devices that detect any form of packet error simply ignore the packet and do not respond to the host. Therefore, if a false EOP occurs, the resulting CRC error forces the target to simply wait for the host to deliver the next packet. This passive behavior by the target device prevents a collision from occurring. Furthermore, when the host fails to receive a response from the target, it recognizes that the packet transfer has failed and performs a retry.

Since the error condition described above results from a false EOP, the host may continue to transmit data after the target has incorrectly detected the EOP. The target device will not likely recognize the remainder of the packet as being valid and will ultimately detect the real EOP at the end of the packet. The target then waits for the next packet to be sent by the host.

False EOP During Target Transmission

If the target is transmitting data and the host receives a false EOP, the CRC error dictates that the host ignore the packet by not responding to the target. This prevents a collision, since no response is sent to the target. Since the target fails to get the handshake, it knows that the packet was not received by the host and waits for the next transaction to be initiated.

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