Rabbit 6000 User’s Manual
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317
27. USB H
OST
27.1 Overview
Network Port D implements a USB 2.0 compliant host interface and PHY. The USB host in the Rabbit
6000 supports both full-speed (12 Mbit/s) and low-speed (1.5 Mbit/s) operation. It conforms to the Open
Host Controller Interface (OHCI) specification, and is fully interfaced with the Rabbit 6000 DMA. Both
receive and transmit have a dedicated 2048-byte FIFO.
Network Port D handles all of the DMA data transfer descriptors automatically. Two DMA channels can
be allocated for its use via the appropriate DMA Special Control Registers.
The network port requires an accurate 48 MHz clock to generate the proper USB serial rate. An external
crystal can be used to drive the internal oscillator, or an external clock signal can be injected directly.
The USB differential data pins have internal 19.5 k
pulldown resistors that may be enabled via register
settings.
The I/O interface consists of a bidirectional differential data pair, 48 MHz crystal input/output, and two
optional signals for power control and current fault detection that are shared with the Parallel Port E pins.
The interface to Network Port D is actually a 32-bit interface, so some special handling is required for all
registers except for USBWR and NDWR. See Section 27.3 for more details.
27.1.1 Block Diagram
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