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Chapter 5 Input/Output Interfaces
5-34 Compaq Deskpro EN Series of Personal Computers
Desktop and Minitower Form Factors
Third Edition – September 1998
5.7 UNIVERSAL SERIAL BUS INTERFACE
The Universal Serial Bus (USB) interface provides up to 12 Mb/s data transfers between the host
system and peripherals designed with a compatible USB interface. This high speed interface
supports hot-plugging of compatible devices, making possible system configuration changes
without powering down or even rebooting systems. The USB interface supports both
isochronous and asynchronous communications, and integrates a 5 VDC power bus that can
eliminate the need for external powering of small remote peripherals.
5.7.1 USB KEYBOARD CONSIDERATIONS
The BIOS ROM checks the USB port, during POST, for the presence of a USB keyboard. This
allows a system with only a USB keyboard to be used during ROM-based setup and also on a
system with an OS that does not include a USB driver.
On such a system a keystroke will generate an SMI and the SMI handler will retrieve the data
from the device and convert it to PS/2 data. The data will be passed to the keyboard controller
and processed as in the PS/2 interface. Changing the delay and/or typematic rate of a USB
keyboard though BIOS function INT 16 is not supported.
The system does not support hot-plugging of a USB keyboard, nor is a keyboard attached to a
USB hub supported. A PS/2 keyboard and a USB keyboard can, however, be connected and used
simultaneously.
5.7.2 USB CONFIGURATION
The USB interface functions as a PCI device (7) within the 82371AB component (function 2) and
is configured using PCI Configuration Registers as listed in Table 5-19.
Table 5–19. USB Interface Configuration Registers
Table 5-19.
USB Interface Configuration Registers
PCI Config.
Addr. Register
Reset
Value
PCI Config.
Addr. Register
Reset
Value
00, 01h Vender ID 8086h 0Dh Latency Timer 00h
02, 03h Device ID 7112h 0Eh Header Type 80h
04, 05h PCI Command 0000h 20-23h I/O Space Base Address All 0’s
06, 07h PCI Status 0280h 3Ch Interrupt Line 00h
08h Revision ID 00h 3Dh Interrupt Pin 04h
09h Programming I/F 00h 60h Miscellaneous Control 1 10h
0Ah Sub Class Code 03h C0, C1h Miscellaneous Control 2 2000h
0Bh Base Class Code 0Ch -- -- --
NOTES:
Assume unmarked locations/gaps as reserved.
Refer to applicable Intel documentation for detailed descriptions of registers.