Universal Serial Bus InterfaceMCF5253 Reference Manual, Rev. 124-88 Freescale Semiconductor24.9.11 Ping ControlUSB 2.0 defines an addition to the protocol for high-speed devices called Ping. Ping is required for all USB2.0 High-speed bulk and control endpoints. Ping is not allowed for a split-transaction stream. Thisextension to the protocol eliminates the bad side-effects of Naking OUT endpoints. The Status field has aPing State bit, which the host controller uses to determine the next actual PID it will use in the nexttransaction to the endpoint (see Table 24-53). The Ping State bit is only managed by the host controller forqueue heads that meet all of the following criteria:• The queue head is not an interrupt• The EPS field equals High-Speed• The PIDCode field equals OUTTable 24-66 illustrates the state transition table for the host controller's responsibility for maintaining thePING protocol. Refer to Chapter 8 in the USB Specification, Revision 2.0 for detailed description on thePing protocol.The Ping State bit is described in Table 24-53. The defined ping protocol allows the host to be impreciseon the initialization of the ping protocol (that is, start in Do OUT when we don't know whether there isspace on the device or not). The host controller manages the Ping State bit. The system software sets theinitial value in the queue head when it initializes a queue head. The host controller preserves the Ping Statebit across all queue advancements. This means that when a new qTD is written into the queue head overlayarea, the previous value of the Ping State bit is preserved.Table 24-66. Ping Control State TransitionEventCurrent Host Device NextDo Ping PING Nak Do PingDo Ping PING Ack Do OUTDo Ping PING XactErr11 Transaction Error (XactErr) is any time the host misses the handshake.Do PingDo Ping PING Stall N/C22 No transition change required for the Ping State bit. The Stall handshakeresults in the endpoint being halted (for example, Active cleared and Haltset). Software intervention is required to restart queue.Do OUT OUT Nak Do PingDo OUT OUT Nyet Do Ping 33 A Nyet response to an OUT means that the device has accepted the data,but cannot receive any more at this time. Host must advance the transferstate and additionally, transition the Ping State bit to Do Ping.Do OUT OUT Ack Do OUTDo OUT OUT XactErr 1 Do PingDo OUT OUT Stall N/C 2
