Universal Serial Bus (USB)MCF5272 ColdFire ® Integrated Microprocessor User’s Manual, Rev. 312-28 Freescale Semiconductor12.3.2.19 USB Endpoint 0–7 Data Present Registers (EPnDPR)Figure 12-22 shows the USB endpoint 0-7 data present registers. used to coordinate user access to FIFOwith external devices to prevent data loss (overwrite) says how much free space is still available. Givesamount of data just received.Figure 12-22. USB Endpoint 0-7 Data Present Registers ( EPnDPR)Table 12-18 describes EPDPn fields.12.3.3 Configuration RAMThe USB module supports up to 1 KByte of USB descriptors. The configuration RAM begins at addressMBAR + 0x1400 and is longword accessible only. EPCTL0[CFG_RAM_VAL] must be cleared to accessthe configuration RAM, otherwise an access error results.12.3.3.1 Configuration RAM ContentThe USB host must configure a device before that device’s function can be used. The host configures thedevice by first reading the descriptors for the device. The descriptors must follow the format described inChapter 9 of the USB specification and any relevant class specification.NOTEThe USB descriptors use little endian format for word and longword fields.The MCF5272 uses big endian format for words and longwords. The usermust make sure that any word or longword fields are stored in the correctbyte order.15 9 8 0Field — DATA_PRESReset 0000_0000_0000_0000R/W ReadAddr MBAR + 0x10CE, 0x10D2, 0x10D6, 0x10DA, 0x10DE, 0x10E2, 0x10E6, 0x10EATable 12-18. EPnDPR Field DescriptionsBits Name Description15–9 — Reserved, should be cleared.8–0 DATA_PRES Endpoint n data present. This field reflects the number of bytes in the endpoint’s FIFO. This fieldis updated when the user writes to or reads from the FIFO and when the control logic accessesthe FIFO. For non-isochronous endpoints, the FIFO level is only updated when a completepacket is received or transmitted without any errors. This occurs synchronously with an EOPinterrupt. For isochronous endpoints, the FIFO level is updated by the control logic in real-timerather than only after the transfer of a complete packet.Note: for non-isochronous OUT endpoints, this field is frozen while an EOT interrupt is pendingin order to allow the user to determine the number of bytes to read for the last transfer. Forendpoint 0, EPDP0 monitors the OUT FIFO only.
