UM10139

Philips SemiconductorsVolume 1UM10139Chapter 14: USB Device Controller14.5 Data FlowUSB is a host controlled protocol, i.e., irrespective of whether the data transfer is from thehost to the device or device to the host, transfer sequence is always initiated by the host.During data transfer from device to the host, the host sends an IN token to the device,following which the device responds with the data.14.5.1 Data Flow from USB Host to the DeviceThe USB device protocol engine receives the serial data from the ATX and converts it intoa parallel data stream. The parallel data is sent to the RAM interface which in turntransfers the data to the endpoint buffer. The endpoint buffer is implemented as an SRAMbased FIFO. Each realized endpoint will have a reserved space in the RAM. So the totalRAM space required depends on the number of realized endpoints, maximum packet sizeof the endpoint and whether the endpoint supports double buffering. Data is written to thebuffers with the header showing how many bytes are valid in the buffer.For non-isochronous endpoints, when a full data packet is received without any errors, theendpoint generates a request for data transfer from its FIFO by generating an interrupt tothe system.Isochronous endpoint will have one packet of data to be transferred in every frame. So thedata transfer has to be synchronized to the USB frame rather than packet arrival. So, forevery 1 ms there will be an interrupt to the system.The data transfer follows the little endian format. The first byte received from the USB buswill be available in the least significant byte of the receive data register.14.5.2 Data Flow from Device to the HostFor data transfer from an endpoint to the host, the host will send an IN token to thatendpoint. If the FIFO corresponding to the endpoint is empty, the device will return a NAKand will raise an interrupt to the system. On this interrupt the CPU fills a packet of data inthe endpoint FIFO.The next IN token that comes after filling this packet will transfer thispacket to the host.The data transfer follows the little endian format. The first byte sent on the USB bus will bethe least significant byte of the transmit data register.14.5.3 Slave Mode TransferSlave data transfer is done through the interrupt issued from the USB device to the CPU.Reception of valid (error-free) data packet in any of the OUT non-isochronous endpointbuffer generates an interrupt. Upon receiving the interrupt, the software can read the datausing receive length and data registers. When there is no empty buffer (for a given OUTnon-isochronous endpoint), any data arrival generates an interrupt only if Interrupt onNAK feature for that endpoint type is enabled and the existing interrupt is cleared. ForOUT isochronous endpoints, the data will always be written irrespective of the bufferstatus. There will be no interrupt generated specific to OUT isochronous endpoints otherthan the frame interrupt.9397 750 XXXXX © Koninklijke Philips Electronics N.V. 2005. All rights reserved.User manual Rev. 01 — 15 August 2005 197