Lantronix DSTni-EX manual 1. USB Data Direction, Addressing BDT Entries, 2. 16-Bit USB Address

Table 4-1. USB Data Direction

	Rx	Tx
Device	OUT or SETUP	IN
Host	IN	OUT or SETUP

Addressing BDT Entries

Before describing how to access endpoint data via the USB or microprocessor, it is important to understand the BDT addressing mechanism. The BDT occupies up to 256 bytes of system memory. Sixteen bidirectional endpoints can be supported with a full BDT of 256 bytes. Eight bytes are needed for each USB endpoint direction. Applications with less than 16 endpoints require less Random Access Memory (RAM) to implement the BDT.

The BDT Page register points to the starting location of the BDT. The BDT must reside on a

256-byte boundary in system memory. All enabled TX and RX endpoint BD entries are indexed into the BDT for easy access via the USB or microprocessor.

When the USB receives a USB token on an enabled endpoint, it uses its integrated DMA controller to interrogate the BDT. The USB reads the corresponding endpoint BD entry to determine if it owns the BD and corresponding buffer in system memory. To compute the entry point in to the BDT, the BDT_PAGE register is concatenated with the current endpoint and the TX and ODD fields to form the following 16- bit address.

Table 4-2. 16-Bit USB Address

BIT	15	14	13	12	11	10	9	8	7	6	5	4	3		2	1			0
FIELD															ODD
			BDT_PAGE REGISTER							END_POINT			TX				///
RESET	0	0	0	0	0	0	0	0	0	0	0	0	0	0		0		0
RW	RW	RW	R	RW	RW	RW	RW	RW	RW	RW	RW	RW	RW		RW	RW			RW

		Table 4-3. 16-Bit USB Address Definitions
Bits	Field Name	Description
15:8	BDT_PAGE	Register in the Control Block
	REGISTER
7:4	END_POINT	Endpoint Field from the USB Token
3	TX	Transmit
		Shows whether the USB core is transmitting or receiving data.
		1 = USB core is transmitting data.
		0 = USB core is receiving data.
2	ODD	Bit That the USB SIE Maintains
		This bit corresponds to the buffer currently in use. Buffers are used in a ping-pong
		fashion.
1:0	///	Reserved

Buffer Descriptor Formats

Buffer Descriptors (BDs) provide endpoint buffer control information for the USB and microprocessor. BDs have different meanings based on which unit is reading the descriptor in memory.

The USB controller and microprocessor use the data stored in the BDs to determine the items in Table 4-4.

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