Fast AMBA Peripherals
12-55
GMS30C7201 Data Sheet
The DEV provides the interface between the SIE and the device
’
s endpoint FIFOs,
ROM storing the device descriptor. The DEV handles the USB protocol, interpreting the
incoming tokens and packets and collecting and sending the outgoing data packets and
handshakes.The endpoints FIFO(RX,TX) give the information of their status (full/
empty) to the AMBA interface to generate DMA request signal and AMBA I/F enable
the CPU to access the FIFO
’
s status register and the device descriptor stored in ROM.
The AMBA interface generates a FIFO read/write strobe without FIFO
’
s errors, based
on APB signal timing. Automatically it requests the DMA data handling when RX FIFO
is full. In case of data transmitting through TX FIFO (when USB generates an OUT
token, AMBA I/F generates Interrupt to CPU), the user should program the DMAC to
transmitting channel, set the transmitting enable bit in the control register. If the error of
FIFO (Rx: overrun, TX: underrun) occurs, the AMBA I/F cannot generate FIFO read/
write signals and DMA service request signals.
12.10.3Theory of Operation
The LGS USB Core enables a designer to connect virtually any device requiring
incoming or outgoing PC data to the Universal Serial Bus. As illustrated in
Figure 12-16: USBD Block Diagram
on page 12-54, the USB core comprises two
parts, the SIE and DEV. The SIE connects to the Universal Serial Bus via a bus
transceiver. The interface between the SIE and the DEV is a byte-oriented interface that
exchanges various types of data packets between two blocks.
Serial Interface Engine
The SIE converts the bit-serial, NRZI encoded and bit-stuffed data stream of the USB
into a byte and packet oriented data stream required by the DEV. As shown in
Figure 12-17: LGS Serial Interface Engine
, it comprises seven blocks: Digital Phase
Lock Loop, Input NRZI decode and bit-unstuff, Packet Decoder, Packet Encoder,
Output bit stuff and NRZI encode, Counters, and the CRC Generation & Checking
block. Each of the blocks is described in the following sections.
Figure 12-17: LGS Serial Interface Engine
Digital Phase
Lock Loop
NRZI
decode,
bit unstuff
Output
Bit stuff
NRZI
Encoder
Counter
CRC
generation
checking
Packet
Decoder
Packet
Encoder
USB
Device
Interface