USB Host Controller Registers
14-45
Universal Serial Bus Host
14.3.1 USB Host Controller Reserved Registers and Reserved Bit Fields
To enhance code reusability with possible future versions of the USB host con-
troller, reads and writes to reserved USB host controller register addresses are
to be avoided. Unless otherwise specified, when writing registers that have
reserved bits, read-modify-write operations must be used so that the reserved
bits are written with their previous values.
14.3.2 Endianism and USB Host Controller Registers
The OMAP5910 USB host controller assumes that all MPU accesses to its reg-
isters are 32-bit accesses. This restriction means that the host controller driver
software may operate in either big-endian or little-endian without having to per-
form endian conversion on USB host controller register accesses. Software
that uses 16-bit or 8-bit accesses to USB host controller registers does not
work correctly, regardless of processor endianism mode.
The processor endianism does affect how the software must access the USB
data structures and USB data buffers in system memory. See Section 14.6.5,
Endianism and USB Host Controller Access to System Memory for details on
endianism, data buffers, and data structures.
14.3.3 USB Host Controller Registers, USB Reset, and USB Clocking
When the USB host controller is not clocked (because the
MOD_CONF_CTRL_0 register CONF_MOD_USB_HOST_HHC_UHOST_EN
_R bit is 0), or when the ULPD does not provide 48 MHz to the USB host con-
troller, reads from and writes to the USB host controller registers do not
occur correctly. To properly access the USB host controller registers, the USB
host controller must be clocked and must be out of reset.
The USB host controller completes its reset within about 72 clock cycles
after CONF_MOD_USB_HOST_HHC_UHOST_EN_R is active and the
ULPD begins providing clock to the USB host controller. After system software
turns on the clock to the USB host controller and removes it from reset, it is
necessary to wait until the USB host controller internal reset completes. To
ensure that the USB host controller has completely reset, system software
must wait until reads of both the HcRevision register and the HcHCCA register
return their correct reset default values.