Rabbit 6000 User’s Manual
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27.3 Operation
High-level support for USB is beyond the scope of this manual, but this section will describe the low-level
setup and operation of the USB host peripheral. Dynamic C has the necessary drivers.
27.3.1 32-bit Interface
Network Port D is actually a 32-bit interface, so special handling is required when reading or writing all
registers other than USBWR and NDWR.
When writing a 32-bit register, the value is written to the peripheral only when the most-significant byte
(uppermost) is written. The three lower bytes will be buffered until that final write occurs. For proper oper-
ation, all four bytes in the 32-byte register should be written every time a change to a bit is required.
When reading a 32-bit register, the read of the least-significant (lowest) byte will latch the peripheral’s reg-
ister value for all four bytes. Those values will remain until the next read of the lowest byte, so to avoid
stale data all four bytes should be read whenever a 32-bit register is accessed.
27.3.2 Setup
The following steps explain how to set up Network Port D.
1. In MSSR, select the 48 MHz clock source (crystal or external signal).
2. Write the interrupt vector for the interrupt service routine to the external interrupt table.
3. If desired, set up the USB power control and overcurrent fault signals by writing to PEDDR, PEFR,
PEALR, and USBWR.
4. Select the optional pulldown resistors on D+ and D- by writing to USBWR.
5. Select two DMA channels for USB transmit and receive by writing to the appropriate DxSCR.
6. Refer to the OHCI specification for details on loading descriptors and further setup.
7. Enable Network Port D and select the interrupt priority by writing to ENPR.
27.3.3 Transmit and Receive
Refer to the OHCI specification for transmission and reception operation.
27.3.4 Handling Interrupts
Refer to the OHCI specification for details on handling interrupts.
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